Brl arpa doploc satellite detection complex

UNCLASSIFIED AD NUMBER AD263873

LIMITATION CHANGES TO: Approved for public release; distribution is unlimited.

FROM: Distribution authorized to U.S. Gov't. agencies and their contractors; Administrative/Operational Use; JUL 1961. Other requests shall be referred to Ballistic Research Laboratories, Aberdeen Proving Ground, MD 21005-5066.

AUTHORITY BRL notice dtd 23 Sep 1965

THIS PAGE IS UNCLASSIFIED

BRL 1136

c. 2 A

s

REPORT NO. 1136 JULY 1961 F l N A L T E C H N l C A L S U M M A R Y REPORT P e r i o d 2 0 J u n e 1958

-

30 J u n e 1961

B R L - A R P A DOPLOC S A T E L L I T E DETECT1 O N C O M P L E X ARPA Satellite Fence Series I

I;'::),.: I !:.?E 6atl

A. H. Hodge Report No. 25 i n the Series

Department of the Army Project No. 503-06-011 Ordnance Management Structure Code No. 5210.11.143

BALLISTIC RESEARCH LABORATORIES

BERDEEN PROVING GROUND, MARYLAND

ASTIA AVAILABILITY NOTICE

i

Qualified requestors may obtain'copies of this report frum ASTIA.

Foreign announcement and dissemination of this report by ASTIA is limited.

B A L L I S T I C

RESEARCH

LABORATORIES

REPORT NO. 1136

JULY 1961

FINAL TECHNICAL SUMWIY REPORT Period 20 June 1 9 9

-

30 June 1961

BRL-ARPA DOPLOC SATELLITE DETECTION COMPLM

ARPA S a t e l l i t e Fence Series

Report No. 25 i n t h e Series

Department of t h e Army Project No. 503-06-011 Ordnance Management Structure Code No. 5210.11.143 Shepherd Frogram, ARPA ORDER 8-58

ABERDEEN

PROVING

GROUND,

M A R Y L A N D

B A L L I S T I C .

.RESEARCH

LABORA'TORIES

,

,

.

REPORT

no. 1136

.

J=w8e/l~ AbarQsa Proving ((round, W. m y

1961

FINAL TECHNICAL SUMMARY REPaRT Period 20 June 1958

- 30 June 1961

BRL-ARPA DOPLCC SATEUITE 'DETECTION COMPLEX

ABSTRACT

This report consists of a technical summary of the research and development program sponsored under ARPA Order 8-58 which led t o the proposal of a system f o r detecting and tracking non-radiating orbiting s a t e l l i t e s . An interim research f a c i l i t y consisting of an illiumlnating transmitter and two receiving stations with fixed antenna arrays was f i r s t established t o determine the f e a s i b i l i t y of using the Doppler principle coupled with extremely narrow bandwidth phase-locked tracking f i l t e r s a s a sensor system. The interim system established the feasib i l i t y of using the Doppler method and resulted i n real data f o r which ccrmputational methods were developed which produced orbital parmeters fran single pass data over a single station. To meet the expected space population problem of detection and identification and t o obtain a maximum range with practical emitted power, a scanned fan beam system was proposed i n which a transmitter and a receiver antenna separated by about 1,000 miles would be synchronously scanned about the earth chord joining the two stations t o sweep a half cylinder of

.

apace volume 1,000 miles low end having a radius of 3,000 miles. An early order stopphg further developmant work on the project prevented fFeld testing of the proposed scanning system i n a scale model. The engineering study indicates t h a t the proposed DOPLoC system or s l i g h t modifications thereof may offer scrma uniqw, advatages over other systems f o r surveillance type of detection, tracking, identification and cataloging of both active and non-racUating s a t e l l i t e s .

TABLE OF CONTENTS Page

I

I1

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INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

. EVALUATION OF DOPLOC RASSIVE SYSTEM . . . . . . . . . . . . 19 6 2 . . . . . . . . . . . . . . . . . . . 19 A . The Basic ~ 0 . ~ ~stem TRACKING

.

1

. . . . . . . . . . . . . . . . . . . . .19 . . . . . . . . . . . . . . . . . . . . . . 19

DOPLOC ~ e r i v a t i o n

2 . . Tracking F i l t e r s

. S e n s i t i v i t y . . . . . . . . . . . . . . . . . . . . . . . . 20 '4. 'Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . 21 5.

Output . . . . . . . . . . . . . . . . . . . . . . .21 B . Application To Passive Tracking . . . . . . . . . . . . . . . 23 1. Transmitter power. Pt . . . . . . . . . . . . . . . . . . 24 2 . Antenna Gain. Gt and Gr . . . . . . . . . . . . . . . . . 25 3 . Operating Frequency . . . . . . . . . . . . . . . . . . . . 26 a . Radar Equation . . . . . . . . . . . . . . . . . . . . 27 b . Antenna B e w i d t h . . . . . . . . . . . . . . . . . . 28 c . Cosmic Noise . . . . . . . . . . . . . . . . d . Bandwidth . . . . . . . . . . . . . . . . . e . Mlnimum Detectable Signal . . . . . . . . . f . Conclusions Relative t o Choice of Frequency 4. Receiver Power S e n s i t i v i t y . . . . . . . . . ~ .

5. S a t e l l i t e Reflection Cross-Section .a . ~ . . .

6 . Reflections from Other Objects . . . . . . . . .

7 . Solutions of Orbit Parameters 3

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. Bsckgmund B. 1.

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Ewperlence and Developments

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44

Interim BLparlmental Fence

45

Radio Trsnemlttsr

50

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Red10 Frequency Circuits . . . . . . . . . . . . . . .

3. Automatic Search and tock-&I (A.L. 0 .) . . . . . . . . .

4 . The Phase-Lock Tracking F i l t e r . . . . . . . . . . . . .

2

5

DOPLOC Receiving Station Interkp

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59 63 70

. . . . . . . . . . . . . . . . 78 a . Digital P r i n t Gut . . . . . . . . . . . . . . . . . 78 b . S t r i p Chart Recorders . . . . . . . . . . . . . . . 81 . . . . . . . . . 81 c . MagneticTapeRecording d . Doppler Data Digitizer . . . . . . . . . . . . . . . . 82 e . Dsta Formst . . . . . . . . . . . . . . . . . . . . . 82 f . Dsta hansmission . . . . . . . . . . . . . . . . . 87 g . Data Transmission Accuracy Test . . . . . . . . . . 91 h . Dsta Samples . . . . . . . . . . . . . . . . . . . . 91 IV . TIIE FULL SCAIE.DOPLOC SYSTEM CONCEPT . . . . . . . . . . . . . . 104 104 A . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .

6

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Data Recording E q u i p n t

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B

. The

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~ n t e r i mDOPLOC System

. . . . . . . . . . . . . . . . . .104

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TABLE OF COIWERIB ( ~ o n t'd) Page .

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. . . . . . . . . . . . . . . . . ; . . . . 105 1. Detection Range . . . . . . . . . . . . . . . . . . . . . .105 2 . -Target Size . . . . . . . . . . . . . . . . . . . . . . . 105 3 . W t i p l e 0 b j e c t ~ ' a ~ a b i l i.t i . . . . . . . . . . . . . . . 105 4 Detection Probebility . . . . . . . . . . . . . . . . . . 105 5. Period Calculation8 . . . . . . . . . . . . . . . . . . .105 Design Objectives

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. . . . . . . . . . . . . . . . . . . . .105 7 . Time of Arrival . . . . . . . . . . . . . . . . . . . . . 105 8 .. Accuracy, Azimuth and Elevation . . . . . . . . . . . . . . 105 .9. Time f o r F i r s t Orbit Determination . . . . . . . . . . . . 105 10. Identification . . . . . . . . . . . . . . . . . . . . . 105 11. Adequate Data for Correlation'with Known Orbits . . . . . 106 12 . Counter-~ountermeaeure . . . . . . . . . . . . . . . . . . 106 D . Target Size . . . . . . . . . . . . . . . . . . . . . . . . 106 E . Antennae . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 F . SignalIjynsmics . . . . . . . . . . . . . . . . . . . . . . . 108 G . AntennaGain . . . . . . . . . . . . . . . . . . . . . . . . 111 H . Received Signal Power Required . . . . . . . . . . . . . . . 113 6

o r. b i. t ~ n c l i n a t i o n e

. External Noise . . . . . . . . . . . . . . . . . . . . . . 113 2 ..1ntepa.l Nolee . . . . . . . . . . . . . . . . . . . . . . 114 1

3 I

.

. Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . 114

. . . . . . . . . . . . . . . . . U.5 1. hgnsmltter Power Requirement8 . . . . . . . . . . . . . 122 Bystem Gearnatry and COM-

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TABLE;:^, comnm. (c.yt.la); Page

...;;. .;:. . .., 124. . . 125 ha.. Requests for C.ontrqct,or.Prop.os@. .: ., . . . . . ." . .

KC. s&T&-sR~,IE~.~ . .,.... . .: . . . . .. . . ., 131

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V , ~ . S c A l w I N o ~ ~ , B y 6 ~ i P R O p o s A L ' . : . e. .

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A. Scanning Ag++nne., Cm.r~ge.

0,

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131

1 . . The ~ 2, Mgi-

Data Re$q+ipg and %tp,

circulating

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gFrequency i .R* ~

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.,

Raea

qf.

Cp.

C q ' DOPLg Receivy ~ ~ c i f i c s , t i , o q e .

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143 143.

;. . . . ., . ., . ., . 143 b. F F q F n c Y S q q . . . , . ., . 144 Dynamic R q g q . . ., . . . . . . . 144 d. Receiver +in &d Val-e Lev+! . . . . . : . 144 e . ' Timing Pulses 145 ~ & i l l + r yQutputs . , . . . . . . f. Automatic Noise Level Control . . . . . . . . . . 145 Circulating Mmory F i l t e r Specifications . . . ...

1. Spectrum t o be Analyzed . . . . . ... . . . . . . . . . .

2 . Input Resis$ance . . . . . .. . . ? . : . . . : . . . .

Receiver Bandwid.+

a.

C.

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3. Effective Input Noiae V+t+ge'.

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a.

. ., ., .. . . . 139 &mow ~ i l t q. . ., . . . . . . . . . . . 139 . '., ., ., . . . 140 O q t (pR1

B. Comic Noise q@'+terference a t &59,,39m.c

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TABU OF

c

m (~ont.'d) pass

. . . . . . . . . . . . . . . . . 147 Frequency Coverage Ripple . . . . . . . . . . . . . . . 147 8electivity . . . . . . . . . . . . . . . . . . . . . . . . 148 .U near @prating

Range

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~

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. . . . . . . . . . . . . . . . . . . . 148 Spurioue Response Levels . . . . . . . . . . . . . . . . . . 148 S i g n a l Limiting AGC . . . . . . . . . . . . . . . . . . . . . . .148 VideoGutputs . . . . . . . . . . . . . . . . . . . . . . 151 Signal Enhancement .

Display Period S t a r t

..................

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Output Data Pulses . . . . . . . . . . . . . . . . . . .

18. Output Impedance Levels . . . . . . . . . . . . . . . .

Threshold Levels and S t a b i l i t y

19

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151 152 152

. External Terminals . . . . . . . . . . . . . . . . . . . . .152

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22 .

23 .

21

E

151

CMF Iaformation Available

153

Installation

............... ......................

153.

...................

153 . .

Power Requirements

. . . . . . . . . . . . 153 1. ERL Report No . 1093. he D y m d c Characteristics of Phase-Uck Receivers" . . . . . . . . . . . . . . . . . . . 153 2 . BRL Technical Note No . 1345. "The DOPLoC Receiver f o r Use with the Circulating Mamory ~ i l t e r " . . . . . . . 15.5 3 . ERL Technical Note No . 1 9 5 . "Parametric Pre-Amplifier ~ e e u l t s " . . . . . . . . . . . . . . . . . . . . . . . 155 Studies Relative t o DOPLOC R e d i v e r

'

J.

Comb Filter bnlaplaant

VII. . SATELLIZPE w m ,-TION

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FROM DOPLOC DATA . . . . . . . :,

Deecription of (hmrmd Data

D.

I n i t i a l Appraxfaationa for the Sin@e.&or solution

... ........., ........

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VIII. OF S Y S m IN ANTI-SATELUTE DEFENSE . . ..

A C r n O W L E ~ r n . . . . . . . . . . . . . . . . , . .. . . . . . . .

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0.

Conclueion

POTENTIAL

174

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B.

E. w t i p l e Paea solution

163

189 198

208

DOPLOC

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;

212 213

Fi@xa 1.

Minhm Raceivsr power as a Function of Frequency f o r Constant Beamwidth Antenna

Figure 2.

Bi-

3.

Figure 4'.

Relation Between Black Body Temperature, Comic Noise, Power Denalty and Frequency Receiver B i g n a l Input Power and T r a n m i t t e r Power versus Frequency Location of DOPLOC Tracking Station8

Figure

48..

DOPLCC ...,... ,.

FIF

5.

Tran8mitter Building . .

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Photograph af Frequency Generating and Egaauring Equipment

Figure 7.

View of Front of 50 kw Transmitter

Figure 8.

V i e w of 50 kw DOPLOC Transmitter.

Foreground

9.

Figure 10.

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DOPLCC System 50 kw Radio Transmitter and Frequency &aswing Equipment

Figunr 6.

Fig-

. .

50 kw Cubicle i s i n

Orientation of Transmitting Autennas

Block D i s g r r u ~of R.F. Section i n POPLOC Station Passive Tracking a t 108 mc . ,

Figure 11. Block D i a g r a m of R.F. Section f o r Active Tracking Figure 12.

4

Block DFagFam of Automatic Lock-On Syetem

Fl.@xe 13. Block Diagram of Phaee-Lock Tracking F i l t e r I n t e r s t a t e Model N Figure 14.

S/N Improvement by BRL Tracking F i l t e r

Figure 15.

Graph of Trsckiw F i l t e r Output Bandwidth versus Step-

Bcceleratlon Figure 16.. Phase-Lock Tracking F i l t e r Figure 17. Block Diagram of Radio Frequency Oecillator, Type 0-471

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Figure 19. .

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20.

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S%andard

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Figure 22'.

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~of Singl=Chennsl ~ ' DoPPier Ihtr Digitizer

Block M .

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:simplified N.&k DI*@

F i d 23. ..

,.

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...,, .

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f o r a Single Block Diagram of Doppler Data Reeding 8~nt.PI Channel

Figure 21.

Figure

'

Block Dia&&a q f . ' D ,. ~ ~ " b i8yn&.

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F i m 18b. Internal. . ~ i. p . .w :of . , . .&;;bncy

24. .

Systmm DataEsndling'

-'

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Data Digitigidg Equiet . . .

.,

PaperTape 90-t

~ i g t i r e25.

Punched

Figure 26.

Teledata Transmitter Receiver Unit

Figure 27.

DOPLOC Data Transmission

Figure 28.

Block Diagram of E q u i w n t f o r Frsqwncy MBasurernent Tests ,

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- C~~rmunicationNetwork ,

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140;, Forrest 'city, F ~ W ' 29.: ..DOPLOC Doppler Record :if;60 Delta ,. pev.

Arkansas Station Fi-

.30,

Fi-

3l.s.

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Typical DOPLOC Data Output

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Dopplei. Rscord of ' ~ c t l e " $ Traclri-& ~ 19% Zeta ( ~ i s c o v e r a rV I ) ,Rev. 1%a t APO, W. . :.. .. ,

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B i g w e 31b. Doppler Frequency Record 'oiActive N-8 (Macoverer =(pw

VI) Rev.

1 , a t AW,

of 1959 Zeta

MY.

Za. Doppler Record of Active S-I Track of 1959 Kappa (Discoverer VII) Rev. 7, a t AFG, Mi.

Figure 32b.

Doppler Frequency Record of Active 8-1 Track of 1959 Kapp (Discoverer V I I ) Rev. 7 a t A X , Md.

Figure 33.

Doppler Frequency Record of BFU Ianoeghere Probe (Strongam I ) Launched a t Wallops Island, Va. and Tracked a t AFG, Ma.

Figure 34.

' ~ a p p l e rFreq&ncy and Strength Records of the Launch of Explorer I. (1958 Alpha) Tracked a t AW, Ma.

6

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LIST OF F I G W . ( ~ o n t 'd) .

Figure 35.

Scanning Beam Traneverae Coverage

Fis~re36.

Area Coverage i n Plane of Fan Beme

Mgure 37.

Volume Coverege of One Transmitter Receiver Pair

Figure 38.

Tantative Tracking Station ~ o c a t i o n e

Figure 3 b .

B i - s t a t i c R a d a r Attenuation or

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p~ ve Range sum ( R +~ R2) p~

Proposed Antenna Design Cwposite S a t e l l i t e Chart f o r One 4 X 40 Degree Scanning Antanna Base Line. Aberdeen Proving Ground C q Blanding Florida.

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Crmpoeite S a t e l l i t e Chart f o r Cne 4 X 40 Degree Scanning Antenna Base Line. Shove timt In beam and number of h i t s a t scan rates of 15 and 7.5 seconde per scan.

Figure 42.

S a t e l l i t e Pasees as a Function of Altitude f o r Selected Satellites

Figure 43.

Same a s Figure 41 Bxcept f o r Different Data

Figure

a.

Figu-8 45.

F-

46.

:

Similar Data t o Figure 42 Similar Data t o Figures 4 1 and 43

An Implementation f o r the WR

Figure 47.

Typical CMF Response t o a Whum Amplitude Sinewave (with Time Weightid)

Figure 48.

Signal AGC Requirements

Figure 49.

Schem8tic Block Diagram ofReceiver

Figure

9. Optimum Noise Level f o r M%ximum Sensitivity

Fi-

51.

Sensitivity C h a r t

Fisura 52.

Dynamic Range Chart

Figure 53.

AGC Action Chart

Figure 54.

Two Crossing Signale k i t h V8rFabI.e Sweep Rates ..,

~ i & e 56.

60 Delta ~ e v .165

y.

9 Delta Re+ ': 124

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55. 60 Dalta Rev. 172 '

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Bigur~B58.. ~ d p p l e rFrequency ' TI C& m s .

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Figure 59. Problem Ceomtzy Figure 60.

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Coordinate Bystams

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:

Figure 61.

G e e t r y f o r Determining the Initial Appraxiwtione

Figure 62.

DOPLOC Frequency and R a t e of C h a w of Frequenoy a s a Fiiuction of Position i n the YZ-Plane (for 80째 inclination)

of 58 Delta Rev. 9937, Figure 63. ARPA-BRL DOPLOC ~o$~ler:,Recofd Forrest City, Arkansas ....... . . ' ~ i g u r e64. DOPLOC Observations for Rev. 30,124;: 140, 156, 165 and 172 of Discoverer XI . . . ., .. . . . ?..&

;.

k ARPA-BRL DOPLOC Record Figure 65. Doppler Frequency ~ x t r a = frcen f o r 1958 'Rev. . No. 9937 ;

::

Early in 1958 the AaVanoad Research Projects Agency (ARPA) took orer the m e p o n s i b l l i t y f o r and directian of a committee established by the Director of Gulded Missiles for the purpose of investigating the technical pr&&m8 involved i n a national s a t e l l i t e tracking network. The committee, chaired by Captain K. M. Oentry, USN, establiehed working groups i n the amas of detection and tracking, c ~ u u i c a t i o n s ,data processing, United Statee programs and fofeign progrspre. An ad hoc camittee, comprised of tha chairmen of the f i v e working groups, w s s formed when ARPA requested that immediate a t t e n t i o n and emphasis be directed toward solution of the problem of detecting non-radiating o r "dark" s a t e l l i t e s .

Following a number of meetings of the working groups, t h e ad hoc ~Wttee,and the Gentry codmuittee, a proposal f o r an intekim solution t o t8a dark satellite problem was prepared and presented t o ARPA i n a briefby .the Gentry cammitt-.

Tha committee proposed the use of s a t e l l i t e tracking techniquee developed by the Naval Research Laboratory (Minitrack) and the Army Ordnance B a l l i s t i c Research Laboratories (DOPLOC), each modified t o permit operation i n the passive (non-radiating s a t e l l i t e ) mode.

It was t e n t a t i v e l y established that a national passive detection network ahould be able t o detect objects having e f f e c t i v e radar r e f l e c t i o n cross sections of 0.1 square meter o r greater a t a l t i t u d e s betwee 100 and 2,000 miles. It was f u r t h e r deemed desirable t o be able t o determine t h e o r b i t parameters, i n a r e l a t i v e l y short time and preferably within one o r b i t period, and t o e s t a b l i s h h e d i a t e l y that the object detected was i n f a c t a new object or t o identify it as a previously detected and known s a t e l l i t e .

The b r i e f i n g t o ARPA indicated t h a t the state-of-the-art i n the tracking f i e l d would not result i n a system capable of fully meeting the desired characteristics. Specifically, it was pointed out that the techn i c a l limitation8 vould reduce system capability t o tracking a one square mtur tesgat t o 1,000 miles a l t i t u d e or less. The e f f e c t s of meteors, aircraft, e t c . , on the tracking systems could not be ccmapletely predicted.

Camputationrrl programs f o r o r b i t determination had not been developed. It was estimated that a fence spanning the United States i n East-West 15

d i a w t i o n a t about LBtitUas 32' and composed af two Minitrack and

One

DOPLGt! complex could be insfelled and placed i n initial. operation within s i x months.

On 20 June 1958 the ARPA issued Order 8-58 t o the B a l l i s t i c Research Laboratories. The order s t a t e d i n p a r t : "Pursuant t o the provisions of DOD Directive dated

7 February 1958, the Secretary of Defense has approved and you a r e requested t o proceed a t once on behalf of ARPA t o e s t a b l i s h a Doppler system complex. Thie system i s t o be i n accordance with EiRL l e t t e r t o ARPA of 8 May 1958, and w i l l , i n cooperation with the Minitrack s a t e l l i t e tracking system produce t h e capability of detecting, identifying, and o r b i t predicting of non-radiating objects i n space. I 1 Using state-of-the-art techniques and e s s e n t i a l l y available equipment BRL, with some help from the Signal Corps, USASEA, proceeded w i t h a crash program and within s i x months, by 20 December 1958 had i n s t a l l e d a threes t a t i o n complex consisting of a 50-KW illuminator radio transmitter a t Fort B i l l , Oklahoma, and receiving s t a t i o n s a t Forrest City, Arkansas and White Sands Missile Range, New Mexico. Three Minitrack type antennas were used a t each s t a t i o n . Detection w a s by r e f l e c t e d signal frm the transmitter v i a t h e s a t e l l i t e . The axes of the beams of the receiving and t r a n m i t t i n g antennas were oriented t o i n t e r s e c t i n space a t a disa t each tance of 900 miles fruu the stations. Of the three antss t a t i o n , one was directed v e r t i c a l l y above the great c i r c l e joining the etatione, one was directed 20 degrees above the horizon t o the north and the other 20 degrees above the horizon t o the south. In t h i s manner the 76 x 8 degree fan-shaped beams were designed t o detect the approach of a aatellite a s it cane over t h e horizon, intercept it again overhead, and again as it receded toward the horizon. This system was placed i n operation i n January 1959.

BRL personnel, r e d i e i n g from the beginning of tha program that the interim system would have limited range and coverage, urged ARPA t o supply funds and to authwice a raseerch grogram designed t o delralop a realistic apsclficaticm far a eyatanr capable of e d e q u ~ t e l ymaating all of the origi n a l performance specif ications. Amendment No. 3 t o ARPA order 8-58 provided funds t o extend the system and t o undertake an extensive research prcgram, investigating the problem areas which were responsible f o r the limited performance of the interim system. Other amendments established reporting procedures and provided funding. BRL was l a t e r relieved of the rasponsibility of routine tracking f o r the purpose of supplying data to

the f i l t e r center.

An extensive engineariag study of the system m q w t a w a 6

i n i t i a t e d by

&, and

taatative conclusioas were o u t a d to ARPA repre-

8antetives at a meeting at BRL an 14 A p r i l 1959 88 briefly described in BRL Rehnicsl Rote Wo. 1266, "An Apprcach to tbe Doppler Dark S a t e l l i t e Detection Problem", by L. G. deBey, July 1959. The idea of using several pairs of scanned, co-plannar, fan-shaped beams t o cover the volume above the United Statea was presented t o ARPA i n the Second Semi-Annual Summary Report for the Period 1 January t o 30 July 1959, BRL Memorandum Report No. 1220, by L. 0. &My, IV. W. Rlcbard and R. B. Pattan, under section V, entitled, "A New System Concept". By l e t t e r dated 19 November 1959, subject "Second Generation DOPLOC R & D Frogram" BRL submitted t o ARPA a proposal covering a program of research and d e v e l o p n t work designed t o validate the conclusions stated i n BRL Memorandum Report No. 1220, and t o test the perfommce of the proposed scanning beam DOPLOC system through the installation and operation of a scaled-donn model.

This proposal was not approved. On 13 April 1960 Amendment No. 5 t o ARPA order 8-58 was received. It stated "Aa a r e s u l t of technical evaluatiws of the SHEPHERD Program, it has been determined that the DOPLOC system w i l l not meet the immediate objectives of the s&tellite detection and tracking program assigned t o the Advanced Research Frojects Agency.

Accardingly, further research on the DOPLOC system i a not

The order further requested that plans f o r closing out the project be submitted t o ARPA. The cessation of work ceme juet DOPLW had demonstrated its a b i l i t y t o detect, track and cmpute r e n a b l e o r b i t s within minutes a f t e r a single passage of a non-radiating s a t e l l i t e o m a single station. It had the highest resolution o r discrimination against meteors and other extraneous signal sources of any known electronic system of s a t e l l i t e detection. W i t h the scanned fan shape beam t y ~ esystem proposed t o ARPA, acquisition would be automatic and a minimum number of low cost stations would insure s sneak-proof coverage.

Multiple t a r g e t capability would be adequate f o r any forseeable density of s a t e l l i t e s . The range and sensitivity of detection would be equival e n t t o or b e t t e r than those of any other known system. The cost of providing volumetric coverage of the space above the United States t o any given a l t i t u d e would be less then, or a t most equivalent to, the cost of a l i n e type fence having coverage t o the same a l t i t u d e . Casnputing time required t o produce an o r b i t ranged f r m f i v e t o 25 minutes on a slow computer of the ORWAC t y p . . W i t h a modern high epeed computer thase times could be reduced t o one tenth t o one hundredth of the above values. The problem of data d i g i t i s a t i o n and transmission i n r e a l time had been solved. The system would have a limited applicability t o antis a t e l l i t e and ICBM defense. required".

11. EVOLUTION OF DOPLOC PASSIVE SAlELLITE TRACKING Sy8!lIEM A.

lZIs Basic DOPZDC System

1. DOPLOC Derivation.

The DOPLOC ( D o w e r w e e LOO^) system i s a

s a t % l i t e tracking system using the phase-locked Doppler frequency tracking f i l k technique. Historically it evolved from DOVAP and PARDOP Doppler tracking systems developed by the Ballistic Research Iaboratories. DOVAP (Doppler Velocity And position) i s an "active" Doppler tracking system, i.e., one which uses a transponder in the missile t o return the signal t o * ground.. PARDOP ( P h s i v e Ranging ~ 0 ~ p l e is r ) a reflection Doppler tracking system. DOVAP and PARDOP u t i l i z e the Doppler principle f o r the accurate determination o f m l a s i l e trajectories and other f l i g h t phenomena. AEI

-

originally conceived, DOwas t o be used in conjunction w i t h an upper atmosphere ionosphere axparbent t o provl.de a high quality Doppler frequency recerd frasa a s a t e l l i t e carrgiag an ultra-stable frequency, lowJ]pwr radio transmitter. Single station, single pass operation was envisioned as a desirable goal and served as a basis f o r design and choice of system parameters. DOPLOC i s a reflection Doppler tracking system consisting of o n e or more s a t e l l i t e illuminatingradio transmitters and one or more Doppler frequency receiving radio receivers. By utilizing extremely narrow band f i l t e r s in the receiving system it i s possible t o operate with the extr-ly low signal levels that are reflected from 'the s a t e l l i t e s .

By wing proper station geometry it i s possible t o compute orbits of the s a t e l l i t e s from single passes over a single pair of stations, i.e..trsnsm i t t e r and recelver

.

Tracking Filters. The Doppler frequency phase-locked trackLng f i l t e r i s the heart of the DOPLOC system, providing a greatly m i p r o w signal-to-noise r a t i o of the output data and therefore i n c r e w d range.

The system i s designed t o take f u l l advantage of the narrow bandwidth i n signal-to-noise impromment. Redetection f l l w perPomima3 i s realized, .taking f u l l advantage of product detection prewded by l i n e a r amplifiers. The tracking f i l t e r is a phaae-locked, audio frequency, electronic bandpass filter that continually follows or tracks the Doppler 2.

Dtveloped f o r the Ballistic Research Laboratories by the Ralgb M. Parsons company, Pasadena, California, under contract no. DA-04-li95-CWD-483.

frequency automatice;lly. Its function i n the D(mL0C recelviag s t a t i o n is t o BE&@ possible the datection and accurate frequency measurement of a i g m l ~which are much weakar then the noise a t the output of the receiver.

of the Doppler frequency FB a c e ~ i e h e da u t m t i c a l l y by means of a tight, phase-locked fe& back loop, thereby enabling the uae of an w-ly narrow filter bandwidth t o obtain a large signal-to-noise imgroYe* m n t . A uniqw feature of t b Ballistic R e s e a r c h Laboratories1 Interstab tracging Wtsr Fs the succaesful employment of a third order servo control circuit. This c l r c u l t gives the filter an e f f e c t i v e and "aided tracglng" type of operation which maintains good lock-on t o the ai@ under widely varying dynamfc flight condltiona. Thus the w r o w e s t pos8ible bandwidth consistent w i t h the radial ~ ~ c e l 8 r a t i ocomponent n of lnotion can be uaad in curdar t o achieve a maximum signal-to-nolee h-t. available i n Bandwidths manually adjuetsble f r a n 1.0 t o 50 cgs tb(spre88nt models. The narrowest bandwidth gives tha DOPLOC receiving etation a capability of dstectingand xa%intaininglock-on t o a e i @ which is 38 db below the noiee a t the receiver output, i . 6 . the noise powm. w i t h i n the receiver output bandwidth is 6300 timee greater than the aimel mr. This is possible w i t h the 1-cps f i l t u r bandwidth and 16 kc m i m r bsndvidth. A 16 kc bandwidth is used i n ordsr t o p e e a D o p p l e r frepwncy a h i f t of 32 dc and leave a 2 kc eafety =gin a t each edge of the geesband.

The DOPLOC receiver s y a t m uaed vacuum tube mamplifiers with a noise figure (m)of 2 t o 3 db a t 108 mc. !tke i n t e r n a l phase noiee i n the receiver local o s c i l l a t o r s was low enough t o permit opemtion down t o 1 cps bandwidth (B). The tracking filter requires a minimum of 4 db output signal-to-noise r a t i o (s/N) t o maintain lock-on.

Based .an these paminetere t h e overall received p o r n s e n s i t i v i t y ( P ~ ) 18 camputed as follows: Pr = X ? l ? x K T x B x 9 / ~ = 2 x.4 x x 2.5 = 2 x watts where K i s Balt&mannls constant, and T i s temperature.

3. Beneitivity.

*

This tracking f i l t e r was designed f o r BRL by the I n t e r s t a t e Engineering Corporation, Anaheim, California, on contract DA-04-495-ORD-822. '

20

.

With a steady, unmodulated signal of 2 x watts into the receiver, the signal-to-noise ratio of the output of the tracking filter corresponds to an output phase jitter of 3 6' rme or 0.1 cycle. A signal about 3 db stronger is required to lock on initially with the automatic eyetern. When signal freqwncy and amplitude dynamics require a larger bandwidth, the threshold received pwer level to maintain lock-on increases linearly with the bandwiath used. The exact bandwidth required to maintain lock-on is a function of the received signal strength, the rate-of-change of frequency and the type of modulation that may be &liberately or indirectly imposed on the signal. Ballistic Research Laboratories Menorandun Report No. 1173, "DOPLOC Tracking Filter" by Victor W. Richard, contains a discussion of the bandwidth selection problem and gives the basic equations and graphs of the relation between bandwidth, rate-of-change of frequency and signal-to-noise ratio.

4.

Antennas. With the extreme sensitivities realized by the optimum use of very narrow bandwidths and the low noise receivers it is generally possible for tracking active satellites to use antennas that have sufficiently broad radiation patterns, that the antennas need be oriented only in the general direction of arrival of the expected signal.

The direction in which the antennas are pointed need not be known accurately. Accurately known and controlled radiation phase patterns are not neceseary. The only requirement on the antennas is that they deliver sufficient signal, preferably as nearly constant as possible, irrespective of the state of polarization of the incoming signal. As further discussion will show, the success of the passive DOPLOC system is dependent upon the most effective utilization of the best antennas known to the state-of-the-art. In addition to the three Minitrack type receiving antennas available at each receiving station, flat top, corner reflector and dipole antennas were also available.

2. Data Output. The basic data output of the receiving station is Doppler frequency correlated with time. Since very stable beat frequency sources can be used, the Doppler frequency record can be interpreted to

be a very accurate record of received carrier frequency.

Tha Doppler

frequency output of ths traEking filter is s u f f i c i m t Q Clean, if filw i s locked on a t a l l , t o permit d i r e c t e l e c t r o n i c ~0untiXXand reed-out or print-out i n any desired form such a s arabic numerals, punched tape i n binary code, IBM cards, wide paper analog frequency, m8gnetic tape, e t c . These data can be used with appropriate cmput s t i o n b l program t o obtain t h e o r b i t a l parameters desired. The 1

accuracy of the solution i s determined l a r g e l y by s t a t i o n geometry with respect t o t h e s a t e l l i t e o r b i t and not by electronic e q u i p n t l i m i t a t i o n s and e r r o r s . Propagation e f f e c t s must be taken i n t o account f o r maximum accuracy. When locked, the tracking filter output i s within or 1/4 cycle of the input, thus i n d i c ~ t i n ga. c a p a t i l i t y of providing a Doppler frequency accuracy of b e t t e r than one cycle. An accuracy of one p a r t i n ten thousand i s obtained with multistation missile tracking Doppler systems such a s DOVAP. Because of the lack of any r e a l l y dependable system t o c a l i b r a t e the accuracy of the DOPLOC system, t h e o r b i t determination e r r o r i s not known with a high order of precision.

However, based on the agreement between the o r b i t a l parameters determined

9 0 '

by DOPLOC data and Space Track data, the DOPLOC data appear t o be as good a s the Space Track data.

A prime feature of the DOPLOC system is the t r a n s l a t i o n o r hetero7

dyning of the input radio frequency s i g n a l down t o a frequency spectrum low enough i n frequency t o be recorded on magnetic tape. This transl a t i o n i s done without degradation of t h e signal-to-noise r a t i o o r l o s s of i n t e l l i g e n c e from the o r i g i n a l s i g n a l out of t h e antenna. Thus t h e Doppler frequency from any pass can be stored and played back through t h e tracking filter and e n t i r e d a t a processing system i n the event t h a t any of the equipment items f a i l e d during r e a l time.

A discussion followe of the problems of modifying t h e basic DOPLOC

system J u s t described t o provide a system with the anticipated c a p a b i l i t y of detecting, identifying and o r b i t predicting of nan-radiating objects i n space.

A paasivi tracking eyattm w i t h the capabiSity specified by AWA mqulres the very beet of a+lable technique6 and demands components of the hlgtiest performance attainable within econcaaic f e a s i b i l i t y . The pftrfonaance characteristics and parmeter values proposed f o r the DOPLOC p a s i r e s a t e l l i t e tracking system, baaed on the state-of-the-art w i l l be treated in t h i s section.

The baeic relation between ran& and the other pareaeters of a

reflection system can be expressed aa r

Pt = tmusuiitter pouer, Pr = receiving p o w , Gt and a,, are transmitter and receiving antenna power gain, A = w a v e Length and

where:

R = ->

a = efYective reflection ecias section.

It ie of interest to salve for the range of a system using all high performance, state-of-the-art ccmponents f o r which known techniques of fabrication e x b t even though they may not be e c o n d c a l l y feasible or attractive a t this time. Opt& parameters w i l l be used in order t o obtain a theoretical msxinrum perfonaance f r a which perfonaance i n practice can be predicted by applyingexpected degradation factors. Under these conditions a transmitter of one megawatt i s possible; transmitting and receiving antenna gains of 20 db each are reasonable since they are associated with moderately wide coverage, shaped radiation patterns of watts about 6' x 60'.

A threshold received power requirement of 2 x w i l l be assumed, although seldm possible t o attain in practice because of ex-al noise ~OUPOBB, vier bandwidth requiremate and F n i t i a l lock-on requirements. NaxUnum range w i l l be computed with three target sizes, 0.1, 1.0 and 10 6quare meters, covering most of the exgected target siees.

For a ~'0.1 sq. meter

=

For

a

2330 miles

= 1.00 sq. meter

R =

'fl x 2330

=

4140 miles

Bor o = 10 sq. aeters

R = =.

4fix 4140 7370 miles.

As previously stated these ranges are the maximum ones theoretically possible under optimum conditions. In practice sevaral econamic and technical. factors enter to modify the parameter valuas attainable. A diecussion follows on the nature and megnitude of some of these factors as they apply to the DOPLOC Passive Rgcking System.

1. Transmitter Power

-

Pt. The use of one-megawatt transmitters at 108 to 150 mc at this time appears to be well within the state-of-the-art and limited only by economic considerations. However, such transmitters have to be specially built and require up to 24 months lead time at a cost of at least $1,000,000 each, for the basic transmitter without the antennas, shelter, installation and power facilities. Since the fence was to be installed on a six-month crash program it was necessary to procure an existing transmitter. The largest available unit located was a 50-KW f.m. broadcast transmitter. Since the reduction in power from one megawatt to 50 KFI reduces the range by only a factor of 2.1, the 50-KW transmitter was considered to be adequate to demonstrate the feasibility of the DOPLOC system in an interim installation.

2.

Antenna Gain

-

In the o r i g i n a l planning it appeared

Gt and Or.

t o be technically and economically f e a s i b l e t o utilizeaatennas with about 20 db gain and 6' x 60' fan-shaped besms. The closest thing immediately a v a i l a b l e was the Minitrack type antenna. It produced approximately the deeired beam shape and was r a t e d by the manufacturer a t 18-db gain. The reduction f r o m 20 t o 18-db gain reduced t h e range by a f a c t o r of 1.25.

Unfortunately, i n p r a c t i c e in the f i e l d , the realieed antenna gain was

s t i l l somewhat l e s s , apparently'nearer 16 db.

A f a c t o r which must be

considered i s t h a t t h e r a t e d gain of the antennas could n o t be realized i n practice due t o p o h r i z a t i o n r o t a t i o n and misalignment'between t h e

An average l o s s of 4.3 db has been estimated f o r t h i s f a c t o r , reducing the range by 1.27. It i s a l s o impossible t o antennas and the t a r g e t .

o r i e n t t h e transmitting and receiving antennas and t o shape t h e i r patterns t o overlap a t optimum gain over a l a r g e volume of space.

This condition

i s caused by the geometry involved, which i s due i n p a r t t o the curvature of t h e e a r t h .

The antenna o r i e n t a t i o n i s t r e a t e d i n d e t a i l i n BRL Report

No. 1055 October 1958, e n t i t l e d "Doppler Signals and Antenna Orientation f o r a DOPLOC System" by L. P. Bolgiano.

a g r e a t c i r c l e were chosen.

Three geographic locations along

Fort S i l l , Oklahcrma, was chosen f o r the

transmitter, and White Sands Missile Range and Forrest City, Arkansas, f o r t h e receiving s t a t i o n s .

The distance between Fort S i l l and WSMR i s

480 miles and f r m Fort S i l l t o Forrest City, 433 miles.

The site

locations are a s follows:

Latitude Longitude

WSMR

FORT SILT.,

33'42 9 1 8 "

34O39'20"

35째00t54"

98'24 920"

90'45

84'32 129"

89'01 '23"

106'44

Great Circle Bearing

16"

79째51'16"

Three antennas were located at each s t a t i o n .

FORREST C I T Y

50"

I

They a r e designated a s

The center antenna a t Fort S i l l

t h e center, north and south antennas.

had I t s ground plane horizontal and i t s long a x i s i n an approximate northsouth direction.

This placed the broad dimension of t h e antenna beam i n

an approximate east-west direction.

The north and south antennas were 25

oriented t o d i r e c t t h e fan-shaped beams 20' above We horizon.

A t the

z?eceiving s t a t i o n s , the three antennae were directed t o have the axee of ths bums i n t e r s e c t a t a distance of 900 miles inmn the s t a t i o n s . The receiving antennae were therefore t i l t e d toward the transmitting anteXUIW. The above mentioned report (by ~ o l g i a n o )contains a d e t a i l e d computation of the antenna orientation angles and reduces the i n s t a l l a t i o n i n e t r w t i o n s t o a s e r i e s of rotetione about e a s i l y defined axes. The types of "9" curves t o be expected from s a t e l l i t e s flying various courses r e l a t i v e t o t h e detect i o n system are plotted. Doppler s h i f t and rate-of-change of Doppler s h i f t a r e computed and graphed f o r 108 megacycles per second signal r e f l e c t e d from a passive s a t e l l i t e . Orientation angles t o converge the beam patterns of a transmitting and receiving antenna, each with a 76' x 8' beam, a r e computed and l i s t e d i n tabular form. Computational procedures a r e described i n d e t a i l t o f a c i l i t a t e f u r t h e r investigation. It i s shown that a one squaw meter cross section s a t e l l i t e within the antenna b e m patterns w i l l be detectable t o a range of about 1,000 miles. In f i e l d practice t h i s Value of 1,000 miles range was found t o be too optimistic f o r emall s i z e s a t e l l i t e s . The a c t u s l achieved range was nearer 500 miles. Bolgiano a l s o shows t h a t tracking f i l t e r bandwidths g r e a t e r than 10 cps a r e required only f o r the c l o s e s t approach of very close s a t e l l i t e s Operating Frequency. The choice of an optimum frequency f o r a DOPLW passive s a t e l l i t e tracking system required considerable study. The frequency of 108 mc w a s available and had been chosen f o r the e a r l y U. S. s a t e l l i t e work. Equipment was readily available i n t h i s frequency range.

Thus it appeared expedient t o s e t up the interim fence on a frequency of 108 mc. However t h i s was not believed t o be an optimum frequency or r e a l l y even a good choice. The cosmic noise a t 108 mc had been measured t o be l a r g e ena t c e r t a i n times t o reduce the receiver system sensitivi t y by a s much a s 6 db, i . e . by a f a c t o r of 4 i n power of 1.4 i n range.

The average cosmic noise l e v e l has been measured t o be about 2 t o 3 db above receiver noise with some periods of very l i t t l e or no cosmic noise.

A t about the time of the i n i t i a t i o n of work on the fence, the opinion of many radio engineers appeared t o favor a much higher frequency than 108 mc.

3.

This opinion waa 'based on the f a c t that as the frequency of operation is increased the e x t e r n a l noise, such as cosmic noise decreases. A more intensive investigation of t h i s subject showed that, of the several f a c t o r s having parameters which vary with frequency i n the 100 t o 1,000 mc range, the 8 d w t a g e s and disadvantages a s one increases frequency almost exactly balance each other out t o such an extent that it was found t h a t the b e s t frequency range f o r a r e f l e c t i o n DOPLW system was between 100 and 150 ~ c / s .

A s applied t o the s a t e l l i t e fence program, the optimization of the frequency

problem is one of choosing a frequency which w i l l give a maximum probability, within technical and economic 1Mtaticols of detecting non-radiating s a t e l -

lites. The maximum amount of transmitter power available f o r radiation is assumed t o be fixed. The principal f a c t o r s which a f f e c t the selection of the optimum frequency include the following: 1. R a d a r equation.

2.

Antenna beam width requirements.

3.

Cosmic and g a l a c t i c noises.

4.

Bandwidth

5. Minimum detectable signal.

The way in which these f a c t o r s a f f e c t the choice of frequency t o be used is discussed b r i e f l y in the following paragraphs. a. Radar Equation. The radar equation f o r the r e f l e c t i o n type Doppler signal i s the combination of two equations, each of which presents the inverse-sqimre power l o s s i n one l e g of the path from transmitter t o s a t e l l i t e t o receiver.

It may be written i n the form:

where Pr i s the receiver power, K i s an a r b i t r a r y constant, Gt i s the gain of' transmitting anterina, Pt i s the transmitted power, As i s the e f f e c t i v e

s c a t t e r a r e a of the satellite, Ar i s the e f f e c t i v e a r e a of the receiving antenna, and rl and r2 a r e the distances fmm the satellite t o the trans-

m i t t e r and t o the receiver, respectively.

27

The value of As i s lFmited i n its maximum value by the physical

2

projected area of the s a t e l l i t e . Consequently the rrrtio of As t o rl ha8 a limiting value s e t by the physical size of the s a t e l l i t e and by the range. S h i l a r l y , on the receiving path, the eolid angle within which energy i s received from tha s a t e l l i t e is determined by the r a t i o 2 oi tbe effective antenna area t o the square of the elant range, r2 The larger the effective area of the receiving antenna, therefore, the larger i s the magnitude of the signal received a t the receiving s i t e f o r eny given magnitude of transmitted power and mtenna gain a t the transmitter and receiver. The effective capture area of a receiving antenna, of fixed gain, decreases as the frequency increases. Therefore, aa the operating frequency is increased, the received power w i l l decrease, based on capture area considerations.

.

b. Antenna Beemwidth. The beam angular dimnsione for both the trcmclmitting and the receiving antennae for a s a t e l l i t e detection system euch as DOPLOC must be established on the baeie of the orbital paremetere of the s a t e l l i t e and the minimum data requirements for an orbital solution. The eolid angle within which the system i e t o receive energy from a s a t e l l i t e must intereect the eolid angle through which energy frcrm the tranemitter i s radiated, and it must be of such dimneione that the probability of recognition of a s a t e l l i t e pseeege i s high. The numerical e ~ c i f i c a t i o n eof a-r dimsneione explicitly determine the gain snd the dimensions of tha antanna in wava lengthe. To obtain a maximum capture area from euch an antenna, it 10wain deeirable t o uee tha l m e t poeeible frequency. Otherwise, maximum energy c a p t w i s not achieved, and the objective of maximum range i e defeatma. Increasing the antenna area in square wave lengthe and using higher frequencies (keeping the physical area approximately consfant) produces narrower beem widths than those required t o permit the t o t a l required mount of data, with the r e s u l t that the received signal w i l l not have eufficient duration. Beamwidth considerations therefore also favor low frequencies.

c.

Cosmic NcrLse.

Cosmic and g a l a c t i c noise i s made up of electro-

magnetic disturbances o r signals having c h a r a c t e r i s t i c s similar t o white When, however, these signal6 are examined over a wide frequency range they l o s e some of the s i m i l a r i t y t o white noise. Consequently, the

noise.

equivalent black body temperature of the radiation v a r i e s with the frequency of measurement, decreasing from between 2,000 t o 20,000 degrees Kelvin a t 64 mc/s, depending on the sky area, t o l e s s than 100 degrees a t 910 mc/s.

(True white noise c h a r a c t e r i s t i c s would r e s u l t i n a constant

temperature a s a function of frequency.)

The e f f e c t i v e sky temperature

has been reported by W o n 1" t o change a t d i f f e r e n t r a t e s with respect t o frequency, depending on the area of the sky which i s being observed.

The

frequency and the temperature have an inverse exponential r e l a t i o n , the exact value of the exponent depending on whether the measurement i s made i n the direction of the galactic plane or not.

The equation applying

when looking toward the center of the galaxy i s given as:

where Tw Is the equivalent black body temperature, K

8

i s the constant of

proportionality f o r the galaxy, and f is the frequency i n mc.

A similar

equation i s given f o r outer cosmic areas:

where Kc i s the constant of proportionality of cosmic space. A s i n g l e 2 exponent of -2.3 is given by Cottony of the National Bureau of Standards which gives an overall average relationship of

Superficially, the rapid reduction of Tw with frequency would make it appear that the higher t h e frequency used, the b e t t e r the results.

This

i s not necessarily the case, since it has been shown that the desired s i g n a l w i l l a l s o decrease as the frequency increases. Since the received . . signal f a l l s off as the inverse square of the frequency, the new increase i n signal-to-noise ratio v a r i e s exponentially w i t h frequency between -6.1

9 Superscripts r e f e r t o references at the end of the report.

29

and -0.7 based on the cosmic noise consideration alone.

There i s a l s o

f i l t e r bandwidth e f f e c t t o be considered which requires an increase i n bandwidth by the squere m o t of the frequency increase which tends t o b a I w e out the cosmic noise f a l l - o f f , depending upon which exponent is used. Figure 1 shows the variation of minimum detectable s a t e l l i t e

(SIN

1 ) as a function of frequency, aasuming s value o f 100 mc ss a reference, both f o r areas of the sky toward the g a l a c t i c center (tovard s a g l t t a r i u s ) and toward the pole of galaxy (extragalectic). The minimum l e v e l i s shown f o r two cases, constent signal bandwidth and Signal

0

bandwidth proportional t o the c a r r i e r frequency.

"..

The minimum signal requirement i n the presence of cosmic noise i s derived by converting the cosmic noise l e v e l , which i s given i n equiva>. l e n t sky brightness o r black body temperature i n degrees Kelvin, t o power density i n watts per square meter. This conversion, based on the Rayleigh-Jeans law, i s a s follows:

where p i s i n watts per square meter, K i s the Boltzman constant, f i s i n cps, B i s bandwidth i n cps, Tw i s the black body temperature i n degrees Kelvin, and C i s the velocity of propagation. Black body radiation is randomly polarized, with equal power i n each plane, therefore a l i n e a r l y polarized receiving antenna w i l l intercept only one half of the power given by the above expression. It i s s i g n i f i c a n t t o note with regard t o the above equation t h a t the noise power density i s proportional t o the square of the frequency. The net e f f e c t of increasing the operating frequency i s the combination of t h i s f a c t o r with the f a l l - o f f of cosmic noise w i t h frequency a s follows:

These relationships between black body temperature, power density and frequency are shown i n Figure 2 using measured values a t 108 mc a s

COSMIC NOISE VS. FREQUENCY FIGURE

. 32

2

r,eference points.

These p l o t s show t h a t t h e r e i s a q u i t e small decrease

i n cosmic noise power density with frequency.

A ten-to-one increase i n

frequency reduces t h e cosmic noise power density by only a f a c t o r or two.

I n summary, t h e exact manner i n which cosmic noise v a r i e s a t t h e output of t h e antenna or t h e receiver a f t e r passing through t h e bandwidth l i m i t i n g c i r c u i t s , i s strongly dependent upon t h e b a s i c type of t r a c k i n g system under consideration.

The interim DOPLOC system required a fixed

antenna beam width and increasing bandwidthwith frequency, therefore t h e optimum frequency based on cosmic noise consideration i s a low frequency.

Figure 3 s u m r i z e s t h e e f f e c t of cosmic noise with a p l o t of t h e r e l a t i o n between r e c e i v e r s i g n a l power required, t r a n s m i t t e r power, and c a r r i e r m e saving i n t r a n s m i t t e r power by ope?ating a t low frequency

frequency.

i s shown i n t h i s graph.

.

The major opposing f a c t o r not shown on the graph

i s t h e increasing height of the antenna s t r u c t u r e a s the frequency i s

decreased.

This can be a c r i t i c a l f a c t o r since t h e c o s t of unusually high

s t r u c t u r e s increases roughly a s t h e cube of t h e height. d.

Bandwidth.

k

The information bandwidth required f o r tracking t h e

Doppler frequency s i g n a l r e f l e c t e d from a s a t e l l i t e i s proportional t o t h e square r o o t of t h e c a r r i e r frequency.

Since the signal-to-noise r a t i o

i s t h e c r i t i c a l f a c t o r i n s a t e l l i t e detection, both a minimum bandwidth and minimum operating frequency c o n s i s t e n t w i t h ' o t h e r conditions must b e accepted f o r optimum r e s u l t s .

Fundamentally, t h e f i l t e r s f o r a l l t r a c k i n g

systems must have s u f f i c i e n t bandwidth t o encompass t h e random phase noise and higher d e r i v a t i v e s i g n a l components.

For t h i s reason, t h e lowest

frequency which has n e g l i g i b l e ionospheric noise should be used. .

2

e.

Minimum Detectable Signal.

The introduction of narrow band

tracking f i l t e r s and of new types of low noise r e c e i v e r s has stimulated an i n t e r e s t i n determining what physical parameters r e a l l y l i m i t t h e d e t e c t i o n of a coherent s i g n a l .

Qualitative arguments had been advanced suggesting

t h a t quantum l i m i t a t i o n s might e x i s t a t power l e v e l s within t h e capabili t i e s of modern r a d i o receivers.

Accordingly it was considered of i n t e r e s t

t o i n v e s t i g a t e whether quantum mechanical considerations s e t l i m i t a t i o n s t o

33

RECEIVER SIGNAL INPUT POWER AND TRANSMITTER POWER VS. FREQUENCY FIGURE

34'

3

achievable sensitivities and to evaluate their engineering importance. A emall contract designed to support an investigation of the probable quantum limitation of detectable signals was awarded to the Electrical Engineering Department of the University of Delaware, Newark, Delaware, with Dr. L. P. Bolgiano, Jr. and Prof. W. M. Gottschalk as the principal investigators.

This work was reported in "Quantum Mchanical Analysis of Radio Frequency Radiation" by L. P. Balgiano, Jr. and W. M. Gottschalk, Report No. U in the BRL DOPLW Satellite Fence Series, June 15, 1960, Electrical Engineering Deparbwnt, University of Delaware, Newark, Delaware. In this report an intuitive account of thermal radiation, making complementary use of classical wave and particle descriptions, is used to give a qualitative introduction to the nature and magnitude of the effect which might be expected. Quantum mechanical computations of the phenomena obtained in specific detectors and preamplifiers are reviewed and found to support the obsenrability of quantum fluctuations. It is found that the random signals observed in wide band radiometers are sufficiently weak to make these quantum fluctuations of comparable magnitude to classical wave fluctuations if receiver noise is ignored. It is also shown that the specific power level at which quantum levels becaane important may depend strongly on the information to be extracted from the signal. For the purpose of this discussion relative to frequency selection for DOPLW, it should suffice to say that if the quantum mechanical limitation should prove to be a limiting factor, it can be expected that the minimum detectable signal power will be proportional to the square of the frequency, and will thus mitigate in favor of a low DOPLOC carrier frequency. f. Conclusions Relative to Choice of Frequency. The considerations presented above indicate that the optimum frequency for the DOPLOC system should be as low as is consistent with the bandwidth required to accammodate ionospheric noise. Experience with the analysis of DOVAP (Doppler) and satellite tracking records at BRL indicates that, became of the ionospheric backgmund noise modulation on the signal, the probable minimum bandwidth setting on a tracking filter operated at 76 mc is approximately 5 cps, where aa for nipale at 108 mc it is approximately 2.5 cps. Since the maxFmum

35

bandwidth, from consideration of missile signal dynamics, which might be required to lock on to the satellite at an altitude of 500 miles is 4.5 Cps, and at 2500 miles, 1.4 cps, and after lock on, a m one or two cycles would be required to track, it is evident that operation at 76 mc shows the presence of an excessive amount of noise, whereas operation at 108 mc is perhaps an excellent compromise. Actually, as indicated above, a reasonable broad band of frequencies may be considered essentially optimum since the loss of sensitivity with frequency is shown by the Figure 3, page 34, to vary at a rate of foa3 to f0'7 relation. On the basis of the data shown, the range within which the frequency should be selected extends from 100 to 150 mc with 175 mc as the highest permissible value. Thus 108 mc was quite suitable for the interim fence system. For the ultimate DOPMC system which was proposed but never instrumented, a frequency of 150 mc was chosen.

4. Receiver Power Sensitivity. An effort was made to design the receiver circuits to provide a signal sensitivity of 2 x 1 0 ' ~ ' watts when used with a 1 cps bandwidth. Under controlled conditions and with the antenna pointed in low noise directions, this sensitivity was actually obtained. However it is not practical to attain such high sensitivity and nesrar bandwidth in tracking earth satellites at 100 to 1,000 miles altitude and at 108 mc frequency. As previously noted, cosmic noise parer will on tho average double the required power for reception. Also passive satellites at 100 1,000 miles altitude require more bandwidth than 1 cps to acccmmodate only the dynamics of the problem. The highest sensitivity is needed when searching at low elevation angles to detect satellites as they emerge over the horizon. This ie compatible with the information bandwidth required since the rate-of-change of frequency is then smallest, permitting the use of the narrowest bandwidth. Wider bandwidths are required to maintain lock-on as the satellite passes by the station, particularly at the lower altitudes. This again is compatible since the distances are shortest and sensitivity requirements least at closest approach.

-

Any automatic iock-on system for use with the tracking filter requires cisignal about three db greaterfor capture or lock-on than is required to hold lock and track once the signal is locked on. This is because of the 36

f a s t r a t e s a t which the lock-on device must sweep in frequency i n order t o cover the spectrwn t o be searched.

Thus a t o t a l of approximately

6 db

increase i n signal may be required i n practice above t h a t assumed i n previous range calculations. imately 1.4.

This can cause a further range reduction of apprax-

Zn summary, the received power required f o r locking onto a

signal, under d i f f e r e n t conaitions of bandwidth m d cosmic noise is tabulated a s follows: Received 8ignal Power Required watts (-190 dbw) 2 x

lo-19

lom1'

watts (-187 dbw)

watts (-180dbw)

2 x

watts (-177 dbw)

Bandwidth, cps

Cosmic Noise Level

2.5

Negligible

2.5

3 db above receiver noise

25

Negligible

25

3 db above receiver noise.

5. S a t e l l i t e Reflection Cross Section

-a.

Since the DOPLW system

had been i n i t i a l l y designed f o r use with radiating (active) s a t e l l i t e s , there was r e a l l y no problem i n obtaining the required signal strength from very small missile borne transmitters over the a l t i t u d e range' and from horizon t o horizon.

To adapt DOPLOC t o the small signals which would be reflected from

small bodies of 0 . 1 t o 10 square meters effective reflection cross section required a careful study of signal levels t o be expected from practical transmitter powers and realizable s a t e l l i t e s .

Body configurations, dimen-

sions, ,spin, tumble, and extent of cmouflage a l l a f f e c t the reflection cross section of a body.

The t e r m "effective" cross section i s used t o

mean the value of a i n the radar equation that muet be used t o determine

the actual range of propagation with the particular s a t e l l i t e target under coisideration.

The effective reflection cross-section area i s strongly

affected by the alignment of the t a r g e t ' s reflection amplitude pattern and polarization with respect t o the illuminating and receiving s t a t i o n antennas.

By using the equations available i n the l i t e r a t u r e f o r the monostatic case,

i.e., transmitter and receiver coincident, the effective dLmensions of

simple spheres and cylinders were calculated.

The values thus obtained

were f o r peak reflection cross section with optimum orientation of the target on the maximum of the reflection pattern lobe. Calculated values f o r spheres and cylinders are as follows: Max. Cross Section

Sphere Dia.

Cylinder Length

0.1 sq meters

1.5 f e e t

2 1/4 f e e t

1.0

4.2

3 112 f e e t

10.0

11.7

14 t o 50 f e e t .

The areas for the cylinders were computed f o r a length-to-diemeter r a t i o of 15. A range of 14 t o 50 f e e t was estimated f o r a cross section of 10

aquare meters because of the effects of resonance.

If the effective

length of the target happens t o be an exact multiple of a half wave length, it w i l l have a much larger effective area than i f it i s some random length.

For the b i s t a t i c case, where the transmitter and receiver are widely separated compared with the range of tracking, the angle between the incident and the reflected rays further modifies the effective cross section area.

Parer and effective croso section area calculations s r e treated i n more d e t a i l i n BRL Report No. 1330, e n t i t l e d "DOPLOC Observations of Reflection Cross Sections of Satellites" by H. Lootens. Using the DOPLOC data and and the basic radar equation, effective cross the equation = -nde2 A

section areas were computed f o r a number of s a t e l l i t e s that were tracked.

The computations are based on actusl, measured data. To give an idea of the magnitude and ranges of effective cross section area, the data below a r e taken from Lootens1 report. In the original data effective cross section areas are camputed f o r the center, north, and south antennas when sufficient data &re available. Slnce the principal i n t e r e s t here i s t o show the extent of the variation i n reflection cross section, data from the center antenna only are given.

TABU I1

-

1

Effective Cross Section Area for Selected Satellites and Revolution Numbera Revolution Ao.

SATEUITE

'58 Delta.

.

,

(cross Section Area (sq. ft.))

Meaeured in Center Antenna

SATELLITE

' 5 9 zeta

Revolution No.

(Croas Bection Area (sq. f t . ) ) Meaaured i n Center Antenna

TABLE 11

SAT!EUI'l!E

- 1 (Cont. 'd) (cross Section Area (sq. f t . ) ) Measured i n Center Antenna

Revolution No.

'60 Epsilon 1

165 386 522

'60 Epsilon 2

6. Reflections from other Objects.

It was i n i t i a l l y expected that

both airplanes and meteors would produce spurious reflections which might be troublesame t o eliminate from the DOPLCC data.

Since it was known t h a t

Stanford Research I n s t i t u t e personnel had made an extended study of reflections frcrm meteors and meteor t r a i l s , advantage was taken of an existing contract with SRI t o have t h i s problem studied.

The effects of meteors and

meteor t r a i l s are discussed i n d e t a i l in a report "DOPLCC System Studies"

by W. E . Scharfman, H. Rathman, H. Guthart, and T. .Morita (Final Report, Part B, Report No. 10 i n the BRL DOPLOC S a t e l l i t e Fence Series), Stanford Research I n s t i t u t e , e n l o Pmk, California.

In practice, meteors enter the atmosphere with velocities between 10 and 75 kilometers per second.

Although some meteors are decelerated

noticeably i n the atmosphere, most of them evaporate and deposit their ionization along t h e i r path a t a nearly constant velocity.

While a meteor

i s coming in, an echo i s received from the elemental scattering lengths of ionization formed along the s t r a i g h t - l i n e path of the t r a i l . This "Doppler echo" has the range and velocity of the meteor a t the start of the ionization t r a i l , and shows a considerable Doppler s h i f t which depends on i t s velocity and on i t s orientation with respect t o the transmitter and receiver.

:

The'echo from the meteor i t s e l f i s normally toosmall t o be

directly detected, so t h a t the detected signal i s due t o the reflection from a trail which i s increasing i n length with time. The mplitude of the echo r i s e s t o a large value a t the point ( i f any) where the angle formed by e ray from the transmitter and the normal t o the trail axis is equal t o the angle formed by a ray from the receiver and the normal, t o the t r a i l axis.

After the meteor has passed t h i s point, referred t o as

the apecular point, the received signal i s composed of a Doppler echo superimposed on a continuing epecular echo from the body of the t r a i l near tills point. The "body echo" decays while remaining a t a more or l e s s fixed range, and shows only a s l i g h t Doppler s h u t caused by ionospheric winds.

Thus the head echoa ware at very high Dopglsr frequenciee an8 wsre unable t o pass tha tracking f i l t e r , w h l h the echo6 fian the trails showed 1ittI.a or no Dopplar elope. Tham coubÂś easily be i&ntrtiied se trail echo0 and presant8d no data d u e t i o n probhm.

During the e n t i r e o b m r v ~ t i a nm o d apprcuhately 1 0 so call& unidentified bodies were indicated i n tha DOPLOC data. These data had the proper Doppler frequency change for s a t e l l i t e s , but did not correspond t o any known o r b i t s . They may have been caused by meteors or could have resulted frm equipment, malfunctions, or possibly even unidentified bodies.

No data are available t o indicate the seriousness of the problem of diecriminating against meteor signals when the scanned beam antennas are used a s proposed f o r the f u l l scale system. However it appears t h a t t h i s problem can be e a s i l y solved by limiting the range of Doppler frequency which can be locked i n on the tracking or circulating memory f i l t e r

7.

Solutions f o r Orbit Parameters. When the interim DOPLOC fence was f i r s t conceived, the only problem which had not already been essentially e i t h e r solved or f o r the solutlon of which known engineering techniques were available, was t h a t of determining the o r b i t a l parameters from DOPLCC data. The three or four s t a t i o n DOVAP problem had been solved with consistency and accuracy for years. But i n the DOVAP problem the missile carried a transponder and a minimum of three stations with reasonably good s t a t i o n location geometry were available.

42 .I'

L

The goal s e t f o r the DOPLCC

system was t o determine an o r b i t with s u f f i c i e n t accuracy t o e i t h e r i d e n t i f y a satellite as a known one or a s a new one from data obtained from a single pass over a single receiving s t a t i o n and i n a time a f t e r detection of l e s s than one pass o r o r b i t period. The f i r s t e f f o r t s seemed t o be r a t h e r hopeless, b u t by the end of t h e second six-month

reporting period a method was s u f f i c i e n t l y well developed t o be included i n the second technical summary report. More recently, an extensive e f f o r t , both i n house and by contract, has resulted i n t h e development of s a t i s f a c t o r y methoda of solution. detail l a t e r i n t h i s report.

These methods w i l l be t r e a t e d i n smne

111. DOPLOC EXPERDENTAL FACILI'PY A. Background Experience and Developments Since 1945, when BRL first established th? DOVAP instrumentation at White Sanaa Missile Range, New Mexico, the Ballistic Measununent Laborrrtcuy

had been actively working t m d extending the r u e and trajectory data gathering equipnt. Through a contract DORAN e q u i p n t was developed in an effort to remove the Doppler data. By contract with Ralph M. Parsons Company

dependability of with C o n ~ i rthe ambiguity from the a reflectam Doppler

syetem was developed for measuring the velocity of missiles, specifically artillery shell, at short ranges. This was a strictly passive system and had inherently many of the problems that were later to be encountered in DOPLOC. Another trajectory data gathering system which was developed and

demonstrated, but which has never been used extensively, was called SFTSfRZDOP. In addition to the system develo1;rment work, BRL had devoted considerable effort to the development of swcial narrow band tracking filtars, high gain rsdio receivers and stable oscillatore or constant frequency eourcee. The problem of receiving low energy signals in the presence of high noise levels had been studied extensively. Such problems as attempting to eliminate the effects of spin and attitude change from the DOVAP data had led to special techniques in signal reception.

Within a few hours after the Russian6 announced the launching of the first Sputnik satellite, BRL had established a tracking station and wae supplying data to other interested agencies. These data were of course Only active data from the satellite's own radio transmitter. However narrow band techniques and many of the features later built'into the DOPLOC system were employed. Thus, when ARPA asked for a satellite tracking facility, BRL had already developed a considerable competence in the detection of low energy signale in the presence of noise.

In b y 1958 BRL proposed to establish, in co-operation with the Naval Research Laboratory, a satellite detection and tracking facility. This

total facility would extend across the southern United States in an east to west direction from coast to coast at about latitude 32' N. The Naval

44

.

Research Laboratory would provide sections of what was later called a fence, extending from the east coast to about the Mississippi River, and from the west coast eastward to near White Sands Missile Range, New Mexico. BRL would provide the center section extending from White Sands Missile Range to Memphis, Tennessee. This proposal was approved by ARPA and resulted in ARPA Order 8-58 dated 20 June 1958. This order directed BRL in co-operation with the Minitrack satellite tracking system to produce the capability of detecting, identifying and orbit predicting of non-radiating objects in space. The only specific direction contained in the order relative to the nature of the facility was that BRL would establish a Doppler system complex.

The method of reporting to ARPA and an indication of the funding to be expected were contained in the order.

BRL personnel realized that no facility or developed technique.within the United States existed which was capable of meeting the proposed performance specifications suggested by the Gentry committee. Briefly this specification called for detecting and tracking all objects having effective reflection cross section areas greater than 0.1 square meter to altitudes of 2,000 miles. The plan of attack was therefore to establish an experimental facility with a minimum range capability and to establish engineering studies and research projects as required to develop a specification for a full scale DOPLOC system. Following this plan BRL launched a crash program to install in the field an interim facility within a period of six months beginning approximately 1 July 1958. Innnediate action was taken to collect together the applicable equipment that was available in BRL and to place orders for such equipment aa could be bought directly "off-the-shelf". As fast as the problems could be analyzed sufficiently to prepare scopes of work, negotiations were started toward contracts with other research facilities.

B. Interim Erparimental Fence In making plans for the interim fence, the choice of an ogerating frequency was one of the first actions required. Since 108 mc was available for the Minitrack equipment under the Geophysical Year Program, and

since a q u i a study showed this f~equencyt o be clom t o optimum, and with that equipment aLteady designed, 108 mc wae chosen f o r the interim fence. !l!hwr&ical and prsctical considerations leading t o and confinnine the wiedam of this choice of frequency were e~nnmarizeda t sane length i n section I1 of t h i s report.

Another early problem concerned the choice of station s i t e s .

The upper

end of White Sands Missile' Range and Fort S i l l , Oklehom, were found t o be close t o the &wiredgreat circle path and about the proper distance apart t o covar t h e distance between W8MR and h a p h i s with o m trcmaplitter f o r illumination and two lacoivers f o r detection. Since land aud many other facilities w e r e laadily available on the Dovarnment w e d and operated faili t i e s , it was decidad t o uss WSMR and Fort Sf11 as two of the station6 e d t o locata a thiz-3 near Mmphis, Tennessee. A great c i r c l e from WSMR t o near Mmphia through Fart S i l l

ran through F m s t City, Arkansas, and just south

af Xau~phis. Since most of the land just south and southwest of Memphis i s e i t h e r heavily populated o r e l s e very l o w r i v e r bottom land which is subject t o flooding, the Forrest City area w s s chosen as a probable, good location f o r one receiving station. A high section of land known a s Crawly's Ridge runs just east of Forrest City, Arkansas. A h i l l t o p one m i l e from the town proved t o be an excellent location. It was close t o power and water, yet f a r enough from develowd areas and traveled roads t o be quiet from the point of view of electromagnetic noise. The radio transmitter station was then located a t Fort 9111 and the other receiving station near Stallion S i t e near the upper end of WSMR. A s it turned out, the Fort S i l l and Forrest City s i t e choices were excellent. The White Sands Missile Range s i t e l e f t much t o be desired. This location was too isolated, being twelve miles from Stallion over undeveloped desert t r a i l s . Stallion was i t s e l f 35 miles from the nearest villsge where housing could be obtained. Although the s i t e was probably quiet in so far as man-made e l e c t r i c a l interference was concerned, it was plagued with sand storms, sand s t a t i c and power failures. Long power and signal l i n e s were required v i a the attendant l i n e failures. Sand leakage i n the building also caused am? damsge t o moving e q u i w n t , tape recorders, relays, e t c . Sand also tended t o foul

up the cooling system and prevent proper cooling of electronic e q u i p n t .

-. .

46

As a research station and a quick and ready place to try out ideas experimentally, BRL retained and improved the active satellite tracking station at Aberdeen Proving Ground. This station was moved from a temporary shack to a permanent type building located on Spesutie Island.

This facility has proven to be very useful both to BRL personnel working on the DOPLOC system and to the missile firing agencies working at Wallops Island and at Cape Canaveral. Figure 4 is a rough outline of the United States and indicates the positions of the DOPLCC stations.

The design of the station buildings and antenna fields was turned over to the Little Rock Office of the U. S. Army Engineers. Using drawings from Little Rock, the Albuquerque and Tulsa Offices of the U. S. Army Engineers contracted for and supervised the construction of the WSMR and Fort Sill stations. The Little Rock Engineers contracted for and supervised the construction of the Forrest City station. The two receiving stations were housed in prefabricated metal buildings. These buildings were 24 x 100 feet single story structures, set on concrete slabs. The transmitter occupied a ],refabricated metal building which was 20 x 80 feet, also on a concrete slab.

IA addition a 20 x 12 foot garage building of sheet metal was provided for an auxiliary generator at the transmitter building. Since the generator was never procured, the garage was utilized for a storage facility. Figure 4a shows an exterior view of the transmitter building.

Since it was contrary to BRL policy to assume routine operational , :. responsibility for a project such as the fence, the Signal Corps was requested to plan to assume responsibility for the operational phase when and if a suitable installation became available. In addition the U. S. Army Signal Engineering Agency was requested to install the radio transmitter at Fort Sill.

After installation by USASEA the operational phase was to be turned over to the U. S. Army Communications Agency. This agency assumed control of the transmitter station and furnished a station supervisor and teletype operators.

At the receiving stations USACA furnished station supervisors and teletype operators. since the equipment at the stations later became a research facility, USACA withdrew from the operation when it became apparent that no routine operational phase would develop. Except for the station supervisors and teletype operators the stations'weremanned by Philco Technical Representative contract employees.

47

P

DOPLOC STATION COORDINATES

APQ YdJlYUID IO(MILST CITY, ARKANSAS

We 45' 50. VEST A W 36- 00' 54. Y n T Y O I " 24' 20. WEST MiD M0 8s' 90. m u

m. S ~ L ,O K U ~ W

-

IMITL M O B , NEW @Elm

8 0 0 ~44' 40.

WEST

UD SSO 4s' 18.

mt((

D I S T M E E f s f f E W STAfDYS

L P . 0 . To C-ST

su:

w mus 4os r l u s

.yoe:

*amlLLs

aw:

uw

JD roar

mw s1u m earn

FIGURE 4,

.

L

*

LOCATION OF DOPLOC TRACKtNG STATIONS

Figure 4a DOPLOC Transmitter Building /

.

1. Radio Transmitter. In the DOPLOC satellite detection and tracking system, a high power radio transmitter is uscd to excite a ~ v r w beam antenna which in turn illurninstas a satellite to cause radio frequency energy to be reflected back to a receiving antenna. TO maat the six month installation period, it w e neceseary to utilize existing equipfent, since the lead time for fabrication of any reasonably high power transmitter would exceed six months. A used cmmercial 50-KW transmitter, manufactured by Westinghouse Corporation wae locatad in storage. Since it had been built several years previously, a contract was arranged with the Gates Radio Corporation to remove the f .m. modulation equipnt and to completely refurbish the transmitter. Since the DOPLOC system requiree a very stable frequency source for determining the transmitter frequency, the original frequency standard or oscillstor and the low power fregwncy multiplier and amplifier stages were replaced with a high stability frequency standard, phase stable multiplier and low power amplifiers. This exciter equipent was engineered and fabricated by the U. S. Army Signal Engineering Laboratory, Fort Monmouth, New Jersey.

Since the frequency standard which served as the basic frequency source for the transmitter had an instability of one part in 109 or less over a period of thirty minutes to one day, a precise method of frequency measurement and monitoring was required. The frequency measuring equimnt at W e transmitter station consisted of a second precision frequency standard, the radio receiving and frequency comparison equipment required to compare the standard's frequency with that of the National Bunau of Standard$' standard frequency transmissions, a time code generator, a real-time counter and a ten-megacycle-per-secondcounter. Figure 5 is a bloak diagram of the transmitter and frequency measuring system. Figure 6 is a photograph of this equipnent. Figure 7 and Figure 8 are two views of the transmitter installation.

The code generator and real-time counter served as an electronic clock indicating hours, minutes and seconds in digital form with the accuracy of the basic frequency standard. Using code pulses and a calibrated oecillographic sweep, the time counter was synchronized with WWV signals to plus .,

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DOPLOC SYSTEM 5 0 KW RADIO TRANSMITTER AND FREQUENCY MEASURING EOUIPMEWT

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Figure 6 Photograph of Frequency Generating and Measuring Equipment

Figure 7 View of Front of 50 KW Transmitter

e counter18 output war pinout on tho or mlnu on0 acllliucond. m first rir v W r of a Emwlett-Fackard, d l 36-A d i g i t a l printer onco uch srcond, or oiteilar, a t tho choice of th. 0pMtm. Tb output of tha froquamy rt.adacd, a b l i b n t d with WLN, n r m d u r tim bua f o r the ton-mgscyclr-per-wmd o m t a r , H.P. aodal 7219.

An r.f. mob. frua tho output amplifier of the t r ~ r m i t t o rlupplied a 10%- rignal which wu fod t o t h i s cycle countor t o obtain a frequency .muuranent. Tb. output of t h e first fivu digit8 of tho cycle counter wu fed to tho remaining five digit w h e e l 8 of the printer w b n it w a s printad out on c w by control pulses frm the tLp. c o b gonorator. Thus, tho printed record contained the transmitter froqwncy t o an accuracy of p l u or minua one cycle, w i t h real time data, a t int.rva.8 an ravrll a s o m second. A hall-duplex, leered, cammercisl taletype line connected the tM.mitting station t o a l l the receiving stations and t o the canputation centor a t the B a i ~ i s t i cResearch Laboratories, allowing either manual or tape transmission of the transmitter frequency data.

The output of the transmitter was fed t o a system of motor driven, remota1.y controlled, coaxial cable switchor which providrd for switching

the transmitter power t o any of the throe high gain antennas. The antenna8 were located 200 f e e t fran the transmitter and fed through 6 1/8" d i m t o r coaxial cable. Each antenna radiated a fan shaped beam having a solid angle coverage of 76 x 8 degrha. Orientstion of the antennas i s shown i n Figure 9.

The beam frm the center antenna waa directed vertically with i t s 76-degree dimension i n the east-west direction. The center of the north antenna beam was rotated t o 20 de@-ees above the horizon with the 76-degree dimension in the north-east t o north-wet sector, w h i l e the south antenna was similarly inclined t o the south.

This arrangement was designed t o p o w i t acquioition and tracking of a ~ a t e l l i t esoon siter it appeared above the horizon, Md then switching t o t h e other two antennas in euccession f o r tracking through t h e i r respective bet o obtain three segnvnts of the typical Doppler "S" curve. Tiltiw end rotational adjurtmnts wore provided i n both the transmitting and n c e i v i n g antennas t o provide f o r aligning the bow8 to,view and illuminate the s m e space.

55

Figure 9 Orientation of Transmitting Antennas

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In order t o provide complete test f a c i l i t i e s and a l s o t o g e t t h e transmitter i n t o operation as soon as possible, a s i n g l e dipole antenna was mounted over a concrete s e r v i c e p i t on a copper sheet ground plane.

This antenna was w e d f o r a few weeks before t h e high gain antennas were delivered. However, it had l i m i t e d power handling capacity of about 25 KW because of an improperly designed s e c t i o n . t h e high gain antennas became a v a i l a b l e .

The dipole was never used a f t e r Extra precaution was taken t o

provide an adequate ground plane system and good e a r t h grounds i n both t h e t r a n s m i t t e r b u i l d i n g and t h e antenna f i e l d .

A s p e c i a l duct and blower

system was provided t o cool t h e t r a n s m i t t e r i n summer and t o conserve some of t h e t r a n s m i t t e r heat t o h e a t t h e building i n winter.

2

DOPLOC Receiving S t a t i o n Interim

-

Radio Frequency C i r c u i t s . When

t h e interim DOPLOC fence equipment was being planned, r a d i o r e c e i v e r s which operated d i r e c t l y a t 108 mc were not immediately a v a i l a b l e .

A system of

preamplifiers and converters was t h e r e f o r e used t o reduce the frequency t o t h e tuning range of a standard type R-390A r a d i o r e c e i v e r .

A BRL b u i l t

preamplifier operating a t 108 mc was connected between the antennas and a Tapetone Model TC-108 converter.

Early i n t h e program t r a n s i s t o r i z e d ,

s t a b i l i z e d , c r y s t a l o s c i l l a t o r s were used a t 61.2 mc and t h e frequency doubled t o 122.4 mc t o convert t o 14.4 mc a t the input t o the R-390A receiver.

M o t h e r standard frequency source was used f o r t h e conversion frequency i n t h e receiver.

The frequency biased Doppler s i g n a l was then fed t o t h e

tracking f i l t e r and t o the magnetic tape recorder.

Later i n t h e program,

Borg frequency standards became a v a i l a b l e a s frequency sources.

Rohde and Schwarz frequency synthesizers and decade frequency m u l t i p l i e r s became available

f o r conversion purposes.

These were very convenient since any

frequency over extremely wide ranges could be obtained t o an accuracy of 1 cycle per second and an i n s t a b i l i t y of l e s s than one p a r t i n 1 09

.

Figure 1 0 i s a b a s i c block diagram of t h e r. f . s e c t i o n used i n t h e interim DOPLOC s t a t i o n .

A 108-mc crystal-controlled s i g n a l generator was loosely

coupled t o the antenna or connected t o a dipole antenna f o r a b u i l t - i n sensitivity calibration.

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BLOCK DIAGRAM OF R.F. SECTION IN W U K : STATION PASSIVE TRACKING AT I O B Y C

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Figure 11 chows ...- . a .. ..block disgrarP of tha equiprunt wed f o r active tmcklng, (Radiating s a t a l l l t u s ) . F& channel6 of -iving equipment were available i n each station. Cme channel could work directly into tha e n c a r s and teletype equipment thence be.ck t o the computing laboratmy. Othar mcorde which occurred during record tranamlssion were aagnetically recorded and transmitted sequentially. Each receiving station wss a l s o provided with receiving equigrment capable of receiving. frm active s a t e l l i t e e over a wide range of frequencies, actually from 20 mc t o 960 me. In general just one channel of e q u i p e n t was available a t these special frequencies. mL Report No. 1123 e n t i t l e d "DOPLOC Instrumentation System f o r S a t e l l i t e hacking" by C. L. Adem describes i n d e t a i l the mechanics of the instrumentation.

For extending the frequency range from 55 t o 9 0 mc, a Nems-Clarke

radio reciever and a range extension unit, model REU-300-B were used.

3. Automatic Search and Lock-on.

One of the most ingenious develop-

ments of the DOPLOC system was an automatic signal search and lock-on &*ice which automatically placed the tracking f i l t e r on any signal occurring within the search range of the equipment. The energy reflected from a s a t e l l i t e is weak and uaually imbedded i n noise. For a passive system t o perform defense surveillance requires 24-hour-a-day operation with maximum detection sensitivity and the shortest possible reaction time. Manual searching requiring constant operatar attention would c a w excessive operator fatigue and greatly reduce the dettrction capability of the system. The short duration and low signal strength of the signals would l i m i t the operation of the tracking f i l t e r s .

Signals f a r down i n

the noise would require excessive time t o identify and lock on manually.

Therefore an automatic search and lock-on system was found t o be necessary.

The automatic lock-on (ALo) described below overcame the limitations of

manual operation and provided a f u l l y automatic search f a c i l i t y . following basic requirements were eetablished for the ALO: a.

The

The chosen audio frequency band, i n t h i s case 12 kc must be searched i n a minimum time.

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b. The system must be capable of finding the smallest theoretical signal determined by the search bandwidth. c.

The time (known as acquisition time) required t o recognize and

use the signal .%hatis present i n noise must be held t o a minimum. d.

The system must position the tracking f i l t e r oscillator t o the

desired frequency and place the f i l t e r i n a lock condition.

A block diagram of the ALO system i s shown i n Figure 12.

To meet the requirements of minimum search time, ten f i l t e r s of fixed bandwidth were placed i n the desired audio frequency band and the signal frequency swept across the f i l t e r s .

The ten fixed f i l t e r s were each spaced 100 cycles per second apart t o cover a one-kc band, compatable w i t h tracking f i l t e r bandwidth characteristics. These f i l t e r s were then switched twelve times t o cover the f u l l 1 2 - k c band. The r a t e of switching determined the time available t o find a signal a s well as the t o t a l sweep time. Three switching r a t e s were used, 2.5, 5 and 10 cps. When the 10 cycles per second rate was used, the time f o r each 1-kc band was 100 milliseconds and f o r the whole band 1.2 seconds. The mixing and switching techniques employed i n the ALO removed the requirement f o r 120 individual oscillators or ten switchable oscillators normally needed f o r coverage a t 100-cps bandwidth from 2 t o 14 kc.

Each f i l t e r consisted of a mixer (which had the output of an oscillator f o r one input and the input signal plus noise f o r the other), a low pass filter and a control relay. The low pass f i l t e r s were variable i n fixed steps fran 0.5 t o 50 cps. !The p a r t of the frequency spectrum viewed by each f i l t e r was the oscillator frequency plus or minus the bandwidth of the lowpass f i l t e r . Shifting the oscillator frequency allowed swltching of the f i l t e r t o cover another frequency band. The oscillators were placed 100 cps apart and covered a 1-kc band. The oscillator frequencies were switched i n groups of ten from two t o three kc, three t o four kc and so on t o U t o 14 kc.

Mri.83. pulses necessary f o r gating and switching were derived f r m a counter with a count cycle of 12. A frequency generator chassis using 1 kc and

100 cps inputs, obtained from the stations timing system, provided the multiplication and mixing t o produce 10 simultaneous output frequencies.

Each pulse from the counter changed the eignals being mixed and shifted the range by 1 kc.

If a signal was present in the noise i n the frequency

range from 2 t o 1 4 kc, of s u f f i c i e n t amplitude, and within the bandwidth of one of the o s c i l l a t o r s , it activated one of the f i l t e r s .

After the presence and the frequency of a signal was indicated by the ALO, the phase-lock tracking f i l t e r had t o be correctly positioned.

In

the track position, an internal o s c i l l a t o r was locked t o the input signal by the use of a feed back loop.

The automatic lock-on c i r c u i t s used an

external loop around the tracking f i l t e r i n the s e t position t o lock the o s c i l l a t o r near the input signal.

The fixed f i l t e r , upon receiving a

signal, closed i t s control relay w i t h one s e t of its contacts, putting the o s c i l l a t o r frequency of the fixed f i l t e r into a c i r c u i t termed the " s e t frequency" control. This c i r c u i t was p a r t of the external loop and compared the fixed f i l t e r o s c i l l a t o r frequency with the tracking f i l t e r o s c i l l a t o r frequency and generated an e r r o r voltsge which was applied t o the tracking

filter and controlled its o s c i l l a t o r t o produce a phase-lock between the two o s c i l l a t o r s .

In the automatic system, the " s e t track" switch i n a standard tracking f i l t e r was replaced by a rotary solenoid. A t the time the fixed filter o s c i l l a t o r was applied t o the " s e t frequency" control, voltage was a l s o applied t o the solenoid.

The tracking filter was positioned t o the correct

frequency before the solenoid moved t o the track position.

In the'track

position, the tracking f i l t e r loop uas closed and a phase lock was obtained between the internal osciUator.and the true input signal.

4. The Phase-lock Tracking F i l t e r . The phase-lock tracking f i l t e r used i n the interim DOPLOC system was the Model N as supplied by the I n t e r s t a t e Electronics Corporation, Anaheim, California.

This tracking

f i l t e r was the r e s u l t of several BRL developmental contracts s t a r t i n g with the development of PARDOP by the Ralph M. Parsons Company, Pasadena, California.

It was an electronic bandpass f i l t e r whose center frequency

was made t o t r a c k automatically the frequency of the input signal.

This . .

was accomplished with a v e r y t i g h t , phase-locked, servo-controlled c i r c u i t .

A very large signal-to-noise improvement was obtained by the use of an, =-trem-py narp~.; filterban&jidth relatpreto the iw2t c(rcai+_h a & v i & + _ h ;

Figure 13 i s a block diagram of the $racking f i l t e r system with the basic closed loop system s h a m i n heavy l i n e s .

The input Doppler s i g n a l was fed t o the input mixer where it was subtracted from the VCO output frequency t o r e s u l t i n a 25-kc s i g n a l .

was amplified i n a tuned 1 . f . stage of about 250-cps bandwidth.

This

The ampli-

f i e d s i g n a l was then fed t o the main phase detector where it was compared i n phase with the o u t g t of a 25-kc fixed frequency c r y s t a l o s c i l l a t o r .

U t i l i z i n g a cross-correlation type detector and equalizer network, the o s c i l l a t o r was controlled t o follow the v a r i a t i o n s of frequency and phase of the input signal.

This technique allowed smooth tracking and extrapolation of

t h e output phase i n the event of s h o r t periods of s i g n a l drop-out by including an e f f e c t i v e acceleration memory.

Noise pulses o r other s i g n a l i n t e r r u p t i o n s

could r e s u l t i n t h e l o s s of s i g n a l s f o r a s h o r t time during tracking missions.

The memory f e a t u r e allowed the filter t o operate through these i n t e r v a l s witho u t losing lock o r introducing e r r o r s i n the data. Output s i g n a l s from the voltage controlled o s c i l l a t o r were t r a n s l a t e d down t o the audio frequency

In t h e model IV filter, this range was 100 cps t o 20 lsc. The output Doppler s i g n a l from the mixer was i n phase with the input Doppler s i g n a l t o the tracking f i l t e r because the inherent 90째 phase

r m g e i n the output mixer.

s h i f t i n the basic tracking loop was campensated f o r by t h e phase-shift networks i n the c r y s t a l o s c i l l a t o r c i r c u i t .

The output mixer f e d an output

8 . c . amplifier through a low pass filter which removed modulation products

a t the output terminals.

I n addition t o the main tracking phase-lock loop, a closed loop auto-

matic gain control (A.G.c.)

s e c t i o n was b u i l t i n t o the f i l t e r .

This included

tin a u x i l i a r y phase detector, d.c. reference signal, d.c. amplifier and 1.f. amplifier. The measured amplitude of the a u x i l i a r y phase detector was sub-

. t k c t e d from the d.c. reference signal and any difference was amplified and applied as bias t o the 1 . f . amplifier.

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In addition t o the deai&d f i l t e r e d Doppler a*gnal, other o u t p u t ~ ~ w e r e made available both ' t b f r o n t . pan.1 .

indicating metera and t o output . . terIDirial8.

These were the c o r r e l a t i o n output, the.din pM.6~aetector output a n d t h e . . imgortant i n the operation -log frequency output. Two factors:which were and application dynamics.

of

t h e tracking filter were iiignal-to-nofse r a t i o and si&nal

The signal-to-noiae r a t i o improvement b y , t h e tracking -filter was t h a t . , of t h e r a t i o of input noiss,.bsndwidthto f i L t e r bandwidth. Line A o f Figure

1 4 shows the r e l a t i o n o f the input noise-to-signal (N/s) i-atio i n db t o the bandwidth f o r which the outgut S/N w i . 1 1 be unity, wh6n We input bandwidth is 1 0 kc. The input N/8 conditian was convenientlj expressed i n %enus of r a t i o s i n db above unity N/S since input S)N r a t i o s ,Bo niuch knialler than one .are. o r d i n a r i l y used. Figure 14 a l s o shows the i i e a g ~ k drn&ik.m input N/S t o maintain lock-on f o r filter bandwidth s e t t i n g s between 0.5 and 50 cps. . . the tracking filter t o lock on t o a aignal Tests indicated the c a p a b i l i t y of 'which i s buried i n nbise 6300 t o 1 (38.db) when the n o i s i input bandwidth was 16 kc and the' f i l t e r bandwidth was one cycle per second. For a receiver .with a 3-db noise f i g u r e , the.38-db value i s equivalent t o an input power of'2 x watts. In t e r n s of receiver performance with conUb6nly usid u n i t s , t h i s i s a s e n s i t i v i t y o i -197 . . . .d. b. i o i 0 . 0 0 1 uiicl'ovolts across 50 ohms.

This gives & '4-db output S/N, correspo~diiigt o about 36' o r 0 ; i djrcle per ... , . second rms phase J i t t e r .

3

The r a t e of change of received s i g n a l frequency due t o r a d i a l components of velocity, acceleration and r a t e of change of acceleration defines the s i g n a l dynamics. For optimum information reception, the proper bandwidth can be selected t o match the s i g n a l dynamics a s well a s t h e S/N r a t i o of the s i g n a l received. Figure 15 ahowe the r e l a t i o n of the tracking f i l t e r bandwidth t o t h e maximum r a t e of frequency change and t o the equivalent accelera t i o n i n g's. Additional technical specifications f o r the tracking f i l t e r a r e contained i n BRL Report No. 1123. Figure 16 i s a photograph of the f r o n t panel of t h e tracking filter.

.

,

5. Timing. Any system of satellite tracking is dependent u p m having bq+ a universal time standard and a locel: time or &B~encY standard. For the DOPIAX iiiterfm system the National Bureau Of Stanstandard frequency emissions constituted the standard of time. LOC~. oscillators of the stabilized quartz frequency controlled type served as the 1 source of frequency.

The local oscillator or frequency standard in each station was a type 0-471manufactured by the Borg Corporation. It had outputs of 100,000 and 1,000,000 cpa. This equipment was temperature controlled and kept in constant operation. The basic crystal frequency was actually five mgacycles per second. Stand-by batteries were built into the oscillator cabinet and sen& to supply power over brief periods of power makns failure.

Figure 17 is a block diagram of the radio frequency oscillator, 0-471(~~-1)/~.

Figures 18 a and b are photographs of the frequency standard. This standard provided a frequency which had an instability of less than 1 part in 109 over periods of 30 seconds to 1 day.

\

Various frequencies are derived from the Borg oscillator 100 KC/S output by divider and multiplier circuits. One type of divider circuit is a commercially available Time Code Generator, Model ZA1935 manufactured by Electronic Engineering Company.

This circuit served a dual purpose. It produced outputs of 10 kc, 1 kc, one per second pulses,. one per minute pulses and one per 10 minute pulses.

In addition to these outputs the unit generated a time code suitable for recording on magnetic tape, and displayed this code on front panel decimal indicators in increments of hours, minutes and seconds. These decimal indicators, appearing as two vertical columns for each unit of time, changed as the code was generated, and the visual display could b6 pre-set to any desired time in a 24-hour priod, allowing the coded time to correspond with the time of day. In most operations it was convenient to set the clock type indicators to Greenwich Mean Tine. Figure 19 is a block diagram of the DOPLOC timing system. :I:

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Harmonics of 100 kc and one per second pulse output were used to the time output with signals received from the National Bureau of Standards station WWV. Provision was made on the rear of the time code generator chassis t o couple loosely the harmonics of the basic 100-kc frequency t o the antenna of a W receiver such as Specific Products Canpany I s type W C . This receiver .provided several bands f o r reception of WUV time signale. In this locality 5 mc provided the best signal. Repeated

tusts conducted over a 24 hour geriod indicated that the instabiltty in the basic 100-kc frequency was approximately 2 parts i n 108

.

Since the one-per-second output from the time code generator could be shifted i n small time increments, t h i s signal could be accurately synchronized with the one per second tone bursts f r m WWV. This WWV signal was dieplayed on the ~ r t i c a curis l of a monitor scope whose horizontal sweep was triggered from the station-generated one-per-second pulse. Proper manipulation of either the "Advance" or "Retard" button enabled the operator t o establish coinci&nce of the start of the tone burst w i t h the start of the scope sweep t o an accuracy of one millisecond. Sychronization of the station.'s timing system occurred a t l e a s t once each day and was always checked just prior t o each tracking operation. Clock time of day was encoded on the basis of a 24-hour clock w i t h sFx code groups neceesary t o display a 24-hour interval. The code was generated in a 4-2-2-1 binary coded decimal form, w i t h each time indication identified by a 20-digit code.

The output was a 100-cycle carrier, modulated a t a 25-pulse-per-second rate, upon which a 1000-cycle signal was superimposed. Although not ueed for

seakch purposes, the code was suitable f o r magnetic tape search.

Figure 20 indicates how the outputs derived Pram the ttnre co&e &urera t o r c i r c u i t were distributed via switch c i r c u i t s atad cathode followers t o the various timing and control c i r c u i t s required i n the system. 8tsnderd pulse shapers and inverters produced the desired polarity, duration, and amplitude of signals for the timing applications.

Timing mar& were producad on one edge of the stripchart record on both the Varian and Banbarn recorders at the rates of ane per second and one p r minute. In addition, one-per-tan eecond markers could be super-

imposed directly on the date tracp of the Varian recorder. The 1000-cycle output af tha tims coda generator was ueed to produce calibration freqwncies in 3 kc steps o m r the range of 3 to 15 kc for calibrating the Varian recorders.

By front panel switching for each of four reoording channels, a printout selector circuit allowed proper data and timing inputs to be switched to the pre-set and display counters for either period or frequency measurements. A low frequency pulse generator operating from the one-per-second input also allowed switching the print rate of the digital recorders from one-per-second to once per 2, 4, 8 or 10 seconds.

The digital clock and the real time counters were driven f r m the basic one-per-second pulee derived from the time code generator. The real time counters operated in a manner similar ,to that of the decade counters in the time code generator described above. Time wae advanced in one-persecond increments and displayed in hours, mbutes and seconds, with the outputs being a discrete voltage level for each digit displayed. A Hewlett Packard type 560A digital printer was used to print out this information.

The digital clock also advanced in one-second increments, and time was displayed in hours, minutes and seconds with the output in the form of contact relay closures for each digit indication. Modification of the 56W digital printer was necessary before it would accept these data. The data digitizing also required time indications and these were produced in a binary time code generator by counting the one-per-second time pulses in a straight binary form and displaying the output with front panel indicators.

Command pulees, generated external to the data digitizer and synchronieed to the time pulsee, shifted the time data out of the code generator into the data digitieer for read-out.

tk

.. .

.

.

A &precorder d o n W c i r c u i t. was y e d t o. .begin time p u l s e. .. . . . . . recording a t a one-per-second rate t o the n e. e. s. t. $en-second real g e e . . indication. This tin% was manually recorded and served as a known start time f o r s e t t i n g up indications f o r play-back of tape recorded data-. The :

,

recorded one-per-second pulses were then used ae the timing source f o r all play-back requirements.

6. Data Recording ~ ~ u i g r n e n t .The basic Doppler informetion, available a t the output of the phase-locked tracking filter, was a constant amplitude, varying frequency s i n e wave. Other data available from the f i l t e r were the correlation signal, the phase e r r o r output, analog frequency i n terms of a d.c. voltage and the autamatic gain control voltage.

Recording was acccsn-

plished in both s n u g and d i g i t a l form u t i l i e i n g magnetic tape recorders, s t r i p chsrt recorders, d i g i t a l p r i n t e r s and =per tape punches. Figure 21

i s a block diagram of the =cording section used in the DOPLOC receiving s t a t i o n s . Each t;ype of recording i s b r i e f l y *scribed i n the following: D i g i t a l P r i n t Gut. Pre-set counters and display counters provided readings of either period o r frequency of the tracking s i g n a l fr& the outa.

put Of the tracking f i l t e r . The type of recording u t i l i e e d , period o r frequency, was determined by the required accuracy of a s p e c i f i c tracking operation. When frequency output was desired, a known number of cycles of the external timing frequency obtained from the precision frequency standard, was counted by a pre-set counter a f t e r proper s e t t i n g of the multiplier d i a l s .

A start pulse derived a t t h e beginning of t h i s count and a s t o p pulse derived a t the end of the count, were used as gate controls f o r a second counter c a l l e d the display counter. The signal of unknown frequency was fed t o the gate of t h e display counter, where it was gated by the control gate pulses, t o the counter decades f o r counting and displaying. For e x m p l e if a 10-kc time base was used and t h e m u l t i p l i e r d i a l s of the pre-set counter were set t o read 10,000 then the gate would be open f o r one second, and a 5-kc fre.*

A

quency would be d i r e c t l y read out as the number 5000.

.

.i

-

When period output was desired, the signrrl frequency wa8 fed t o the input of t h e p r e - s e t counter, and the multiplier d i a l s were set t o count the"dee1red number of cycles of t h i s frequency.

The display counter then

counted the timing pulses f o r the interval of a k n m number of cycles of the signal frequency and displeyed the count. For oxauiple, assuming the Doppler frequency t o be 2 kc/s, the multiplier dials of the pre-set counter might be s e t t o 1000 to allow the display counter gete t o be open f o r one

half second.

---.. w u u u 3s

--->

rrou

LL-

I f the timfng frequency were 100 kc/s, the dlsplqy counter -.._L--

-a

A-

--- &-.---- us - a1w

m e rrwu- w g u r u a +n w c nnu

1

mccsmu

~n

AM

uuuuro.

mbaa * r u m

thus produced a period measurament which i s the inverse of t h e previouely &scribed f'requency measurcrmant.

The period measurement obviously provided the highest accuracy.

For

improved accuracy of measurement a 10-mc counter was used t o obtain the period count. The recorded data were printed out on the Iiewlett Packard model %0A printer. This printer operated with counters using the binary decimal counting system and produced a specific w l t a g e l e v e l f o r each displayed number. Thie voltage l e v e l was fed t o the corrr~pondingd i g i t a l place i n t h e printer which sensed t h e voltage an8 printed the number a s played on the counter. The printer accepted data entereb i n p a r a l l e l snd a l l columns were printed simultaneously on command. The capacity of the printer was eleven 6 i g i t a i piaces. For fiequeiicy, or imer resOiu%iOii period measurements, the f i r s t f i v e d i g i t s were used f o r the Doppler d a t a , and the l a s t s i x were f o r time indication. For period measurements Of higher resolution, using the 10-mc counter, the first seven digits were used f o r the Doppler data, and the remaining four f a r tima indications.

The time data presented t o the appropriate p r i n t wheels could be one of two kinds, depending on the c i r c u i t r y used t o generate them. I f the data were generated from modified counter decades with a specific voltage level produced f o r each decimal number i n the decade, then no internal modification of the printer was required. Barever t h i s requireu a separate r e a l. ..time counter f o r each d i g i t a l printer used. I f aeveral channels of data were being printed, the equivalent number of r e a l time counters required became cumbersome and d i f f i c u l t t o synchronize with WWV time sigqals.

The second kind of r e a l time data was generated from a d i g i t a l clock, Chrono-log model 2600-4, the output of which was i n relay contact closures.

-:? ' ~ o d i f i c a t i o n st o the d i g i t a l p r i n t e r were required for those positions where 80 g

-

.

.%

'

* i

tima was recorded. The advantages of using this clock were the ease of s e t t i n g up the real time information each day, the ease of synchronization w i t h WHV time signals and the f a c t that one clock could drive many p r i n t e r s .

A l l of the printed time data were i d e n t i c a l and properly correlated.

Tne disadvantage was the modification of the p r i n t e r t o accept the time informt i o n frm the d i g i t a l clock. The p r i n t e r became a hybrid instrument with p a r t of the p r i n t wheels operating from voltage l e v e l s and p a r t from contact closures. Once the modification had been made the r e l i a b i l i t y and ease of operation were greatly improved. b.

S t e i p Chart Recorders. Two types of s t r i p chart recorders were a l s o used t o record the output of the tracking f i l t e r . The f i r s t was a single channel Varian recorder, model 0-11-A, u t i l i z i n g a f i v e inch wide recording paper. Timing marks were placed on the edge of the paper by an independent s t y l u s and a l s o superimposed on the data stylus.

The Doppler

data w e r e f e d t o an integrator where the frequency was converted t o a d.c. voltage signal. Frequent frequency versus deflection calibrations were made.

The autamatic gain control and correlation outputs of t h e tracking f i l t e r were recorded on a two-chamel Sanborn model 152-100B, normally a t 1 mm per second chart speed. Timing marks were recorded on t h e edge of the paper. Calibrations were applied after a s a t e l l i t e pass. c. Magnetic Tape Recording. Since provision was made f o r handling only one channel of data through the r e a l time encoder f o r transmission over the teletype l i n e t o the computer, some provision had t o be made f o r storing the data from the other three channels. Back up data were a l s o desirable i n case of other equipment f a i l u r e o r e r r o r s i n data transmission. Therefore, a l l data from the tracking filters were recorded on magnetic tape. An Ampex FR-114 fourteen-channel recorder u t i l i z i n g oneinch tape was used a t each receiving s t a t i o n . Tape speeds from 1 7/8 inches per second t o 60 inches per second were available. On most tracking operations tape recordings w&e made of receiver output, tracking filter output, s t a t i o n t h i n g s i g n a l s and WWV time signala. Spare channels were tbsn available f o r recording any other data desired. In the reproduce mob four channels were available f o r simultaneous playback of any four of tbr switch selected fourteen channel6 of data.

81

An Ampex model FR-1100 magnetic tape recorder, using 112" tap, and

recording a t speeds of 3 314 t o 30 inches per second was used t o recard the binary output data i n d i g i t a l form a t t h e output of t h e serial shift

register. d.

Doppler Data Digitizer.

To permit rapid data handling and real

time transmission back t o the computer it was necessary t o d i g i t i z e and ehcode t h e Doppler data a t the s t a t i o n s . Two b a s i c functions of the d i g i t i z a t i o n system w e e t o produce in binary form real time data a t the beginning of each recorded output and period data of t h e Doppler signal.

The f i r s t function was accomplished by the binary time code generator a t each tracking s t a t i o n and the second by the Doppler Data ~ i g i t i z e r(DDD) f o r each recording channel i n the s t a t i o n . Standard t r a n s i s t o r i z e d building block l o g i c was used i n both equipments. The DDD was b u i l t t o BRL specifications by t h e Hoover Electronics Company of Baltimore, Mryland.

This equipment i s described i n d e t a i l i n BRL Report No. 1123 by C. L. Adams.

Figure 22 i s a block diagram of a single channel Doppler Data Digitizer.

Figure 23 i s a simplifies block diagram of the interim DOPLOC Data handling System. Figure 24 is a photograph of t h e data Digitizer. The d i g i t i z e r operated a tape punch d r l y e r which i n turn operated a Friden paper punch Model 2 t o produce the binary data i n the proper format. e.

Data Format.

The punched-paper-tape type of output data were

produced i n s t r a i g h t binary form because of the input requirement of the BRL ORDVAC computer used i n the calculation of the s a t e l l i t e o r b i t a l parameters. Each recording channel i n the s t a t i o n produced data i n stand& f i v e l e v e l teletype tape i n the format shown in Figure 25.

A camplete data block, or word, was contained in seven rows of the

five-channel tape with t h e and Doppler data in sFx r o w s of the f F r s t three2,channels. A t the beginning of each w a r d , in p a r a l l e l with the binary data i n row one, a block o r t i m e marker b i t was produced in channel four. This occurred a t a one-per-second r a t e and w a s correlated with the d i g i t a l p r i n t e r readout s i g n a l . Modification i n t h e d i g i t i z e r c i r c u i t s w a s mde t o allow a one-per-minute b i t to b e punched in row two of c h a ~ e l f o u r t o . f u r t h e r c o r r e l a t e timing data. An end of word marker, acting as d

a .caiiputer control, w e s prOauced i n a l l five :channels of row seven. ,. .3:. ...& 82

.

GATED DOPPLER INPUT FROMPRE-SET COUNTER

BINARY COUNTER 2 lo

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BUFFER ( ~ a CONTROL

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DOPPLER GATE

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FIGURE.22

T4YE INPUT IPER S,FCWO

TO 3 OTHER ~ I K ECHANNELS

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SERIAL REGISTER

RECORDER

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MAGNETIC TAPE RECORDER

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1

SHIFT M I S T E R

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FOR E S l W cwwul

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MAGNETIC TAPE

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FRIOEN PAPER TAPE PUNCH

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OOPPLER GATE

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BLOCK DIAGRAM OF SINGLE CHANNEL DOPPLER DATA DIGITIZER

-

I COWUTING DIGITAL INDICATOR (DECIMAL)

DATA

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MQITIZER

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TAPE RECORDER (7 CHANNEL)

RECORDER #2

PAPER TAPE

(7 CHANNEL)

REAL TIME COUNTER (SYNCHRONIZED

TIME CODE. GENERATOR (BINARY)

wlrn wwv)

PLAYBACK REGISTER (BINARY)

i RECEIVING SITE i COMPU~NGSITE&

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PUNCHED TAR! TRANSMITTER RECEIVER

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PAPER TAPE PUNCH

TIM~~NG

PULSES

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PUNCHED TAPE TRANSMITTER

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OUTPUT DATA

SIMPLIFIED BLOCK DIAGRAM

- DOPLOC

S Y S T E M DATA HANDLING., . .

84

FIGME 8,-

3.y

Figure 24

Data Digitizing Equipment 85

Manual operation of a start switch at the console, or automatic start switching from the autcmnatic lock-on circuit6 when the tracking f i l t e r locks on a received signal, began the tape punchi* sequence. The first data word contained universal time data and a l l subsequent blocks contained Doppler data. The d a t a were punched i n parallel, one row a t a time a t a r a t e of 60 lnillieeconds per row. After each end of row marker i n row seven, the tape

was at rest f o r appraxlrnstely 0.5 second while the counting process i n the The om-per-second timing signal triggered the readout systeia and the count j u s t accumulated was punched out.

d i g i t i z e r took place.

f.

Data Wansmission.

A retry irportant part of any satellite tracking

If data are to be transmitted i n real time and & b i t s camputed i n minutes a f t e r a s a t e l l i t e pass, the transmission system m u s t be reasonably f a s t j accurate and dependable. In designing the DOPLOC interim e q u i s n t it was determined that satisfactory tx-ansmieriion could be obtained using a 60-word-per-minute ccanmercial telenet is the data tr'ansmisaion equipment.

graph channel, i f connected t o terminal equipmint that contained a system of parity checking of transmitted data. A commercial paper tape t r a n s m i t t e ~ receiver called Teledata, manufactured by Friden, Incorporated, was chosen.

The model 7-B Teledata shown i n Figure 26 was used with five channel tape and consisted of a reader-punch chassis and a transmitter receiver chassis.

One Teledata unit was required a t each f i e l d station location and one f o r each s t a t i o n ' s output a t the computer location, connected by a full duplex telegraph line. This type of connection allowed simultaneous transmission i n both directions.

Any type of f i v e b i t code read f r m the transmitting tape was trans-

mitted a s a seven b i t code by adding a redundant b i t t o each of two groups

of the f i v e tape code b i t s t o farm two parity groups, one group containing four b i t s and the other group containing three b i t s . The sequential transmission cycle was so arranged that b i t s were sent alternately from the two parity groups. The received code was checked for accuracy i n the tape punch section by independently checking of each group of b i t s . Operation of an even number of punch pins i n either group was registered as an error.

The pins associated with the redundant b i t s do not punch the tape so that

the received tape was i n the identical f i v e channel form a s the transmitted tape.

In order t o u t i l i z e the f u l l capabilities of the f u l l duplex line,

provision was made t o switch the lines a t the ends of the links from the Teledata t o cammercial teletype machines, thereby providing simultaneous two-way teletype communication during periods when the data tapes were not being transmitted.

In addition t o t h i s combination communication-data link

between the receiving s t a t i o s s and the BRL computer, a standard half duplex teletype link t i e d a l l stations together i n parallel.

A separate 100-word-

per-minute teletype l i n e linked the Laboratories1 canputing center w i t h the National Space Surveillance Control Center, Spacetrack, New Bedford, Wssachusetts.

During the Vanguard project, another half-duplex teletype link existed between the computer center and the Naval Reaearch Laboratory in Washington D. C . The tracking station at AFG and the BRL computing center each had a commercial half-duplex teletype machine which could be connected on c a l l t o other similar machines throughout the nation. It was necessary t o operate w i t h t h i s variety of communication e q u i p e n t because of the large number of agencies involved i n launching the various types of s a t e l l i t e s and probes which were t o be tracked. communication network.

Figure 27 shows the data transmission and

A series of t e s t s were run on the data links t o determine the accuracy

of bata transmission. Both Teledata and teletype equipments were adjusted t o optimum performance and a punched tape with a known code format containing 4000 data b i t s was transmitted from BRL t o each receiving station.

Then, over a period of several days, t h i s t e s t tape was transmitted back t o BRL from the stations a t selected times. A s t a t i s t i c a l record was made of errors. The same tape was transmitted over the links without the Teledata parity check. This resulted i n roughly twice a s many errors a s were made by the Teledata machine. The teletype equipment also showed no indication that an error had been made. With the Teledata equipment, an

LIKE MACHINES BALLISTIC RESEARCH LABS. COMMUNICATION-COMPUTER

CENTER

Fy WHITE SANDS

NOTES

.-

.,.. .

0 6, 0 1

-

INDICATES TELETYPE MACHINE ON FULL IDUPLEXI LINE

2 INDICATES TELEOATA TRANSCEIVER UNIT. SWITCHED TO FULL DUPLEX LINE 3 INDICATES MULTI-POINT TELETYPE MACHINE ON HALF DUPLEX LINE

-

@INDICATES

ON-CALL

@INDICATES

FULL-TIME LEASE TELETYPE MACHINE

'TRANSMITTING SITE

TELETYPE MACHINE

A L L LINE!$ AND MACHINES ARE FOR 60 W.P.M. TRANSMISSION,EXCEPT AS NOTED. f ' F : FIGURE 27 DOPLOC DATA' TRANSMISSION

- COMMUNICATION

NETWORK

e r r o r caused the machine t o stop transmitting and thus permitted e r r o r location and correction. Data transmitted over a 2000-mile l i n k using 60-word-per-minute telegraph l i n e s showed an e r r o r r a t e of only one p a r t i n 60,000. This m e quite adequate f o r the interim DOPLCC system data handling requirements. A more rapid r a t e of data handling was planned f o r the f i n a l equipment. g.

Data Transmission Accuracy Tests.

An analysis of data received

from tracking both active and passive s a t e l l i t e s showed an occasional R b S s c a t t e r of as much a s three cycles per second. This resulted i n some suspicion of the performance of the phase-locked tracking f i l t e r . A s e r i e s of tests were made a t the Laboratories' tracking s t a t i o n on data simulating those obtained on a c t u a l satellite passes. The test results showed a random s c a t t e r i n the data corresponding as closely as could be determined t o the precision of measurement, thus remwing the suspicion from the track* filter. The random s c a t t e r was thus a t t r i b u t e d largely t o propagation phencrmena. The satellite spin and i n s t a b i l i t y of the satellite transmitter i n active s a t e l l i t e s w i l l introduce a limited mount of data s c a t t e r also. E f f o r t s t o analyze the data f o r systematic e r r o r s as w e l l

as random e r r o r s reeulted in an increase i n the precision of the period measurements by using a ten megacycle counting r a t e instead of the one hundred kc rate. After an extended s e r i e s of tests using simulated data mhed with noise and various tracking filter bandwidths it was determined that the peak frequency s c a t t e r was 0.16 cycles per second and the nus frequency e n o r was 0.052 cycles per second. Figure 28 is a block diagram of t h e equipment used f o r the frequency measurement tests. h. Data Samples. Examples of the types of recorded data a r e shown i n F i g u r e s 29 34. Figure 29 i s a dual channel, s t r i p chart record of data obtained on pass 140 of s a t e l l i t e 1960 ~ e l t aisco coverer XI) a t the Forrest City, Arkansas, s t a t i o n . Operation was i n the passive mode. The upper record of t h e chart indicates frequency as a function of time and shows three d i s t i n c t segments of received Doppler frequency. These segments correspond t o the times of passage o f ' t h e s a t e l l i t e through the beems

-

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FREWEWCY svwTnESIZER

DHIITAL RDKAK'R

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FIGURE

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28

BLOCK DIAGRAM OF EQUIPMENT FOR FREQUENCY MEASUREMENT TESTS

-

ONIVlAMVYY ' O N ~ O M S 9NlAOUd N 3 3 O M 3 8 V 1 V +GI 'A3M M 3 M 3 A 0 3 S l a ) V 1 3 Z 61461 J O M 3 V t l l S - N 3A113V J O Otl033M A3N3nO3MJ M 3 l d d O O

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Figure 32b

DOPPLER FREQIUENCY RECORD Of' ACTIVE S-N TRACK OF 195!9 KAPPA (DISCOVEIRER PII) R E V 7 AT ABERDEEN PROVING GROUND , MARYLAND

of the three receiving antennas.

The s t e p wave forms show the functioning

of the automatic search and lock-on system used f o r acquisition of the r e f l e c t e d signal. The frequency range of the automatic search equipment

was changed a f t e r s a t e l l i t e passage through one antenna beam, and search over a higher frequency range continued before s a t e l l i t e a r r i v a l a t the next antenna be&. The lower channel indicates signal strength data received a t the time of s a t e l l i t e passage a t each antenna position. C a l i bration data f o r each channel are niarked a t the r i g h t of the record, and timing marks are indicated on the lower edge of the record.

Two types of recordings of Doppler data output are shown in Figure 30.

The first is the Doppler period output recorded i n binary code on standard five-level punched paper teletype tape.

The f i r s t data block contains

Greenwich Mean Time a t the beginning of the recording run.

Subsequent data

blocks, recorded a t the rate of one per second, contain the Doppler period measurement.

The second type of recording i s the printed record obtained

from a Hewlett Packard, Model 560A d i g i t a l p r i n t e r . shown.

Two examples are

Each printed l i n e contains Universal Time data and the Doppler

data a t p r i n t out time.

When frequency i s recarded the f i r s t slx d i g i t s

indicate time and the remaining f i v e d i g i t s indicate Doppler frequency.

When high r e s o l u t i ~period measurements are desired, only four d i g i t s of time, minutes and seconds, a r e printed, allowing the remaining d i g i t s t o indicate the period measurements.

Figures 31 and 32 a and b a r e typical examples of single and dual channel s t r i p - c h a r t records obtained a t the DOPLOC s t a t i o n s during a tracking operation.

The dual channel records contain signal strength data

and Doppler frequency data, each indicated a s a function of t h e . T h e marks are on the lower edge of the chart. Calibration data f o r each channel are recorded a t the beginning of each record j u s t p r i o r t o the run.

In these Figures the calibration data were transcribed from the o r i g i n a l record and placed near the pertinent section of t h e record f o r c l a r i t y .

The single channel records a l s o contain t h e Doppler frequency as a f u n c t i w of time but allow higher frequency resolution because of the increased chart width.

101

The two similar s e t s of d a t a a r e included t o show t h e d i s t i n c t differences i n the records f o r two s a t e l l i t e s .

Figures 31 a and b show

t h e data obtained during t h e tracking of 1959 Zeta

isco coverer VI) which

psssed almost d i r e c t l y over t h e receiving s t a t i o n , indicating a r e l a t i v e l y s t e e p frequency vs time curve (usually c a l l e d the "St' curve). Figures 32 a and b show similar data recorded during tracking of (Discoverer VII)

1959 Kappa

indicating a lower slope of t h e "S" curve because t h e

s a t e l l i t e passage was a t some distance from the receiving s t a t i o n .

Figure 33 shows the s t r i p c h a r t data recorded during t h e e a r l y portion of f l i g h t of the BRL ionosphere probe (strongarm No. 1 ) on 10 November 1959.

Data were obtained f o r approximately 24 minutes and included information over nearly the complete t r s j e c t o r y .

Figure 34 i s an example of s t r i p c,hart data recorded during a rocket launch.

The Doppler data record ok'tained on 1958 Alpha ( ~ x p l o r e rI ) indi-

c a t e s s t e p l i k e s h i f t s i n frequency which correspond t o rocket acceleration due t o f i r i n g of successi.ve stages.

This type of data i s very useful i n

determining the success of a rocket launching and t h e subsequent o r b i t i n g

-.

of a s a t e l l i t e .

Sample o r b i t a l parameters from the DOPLOC system a r e shown below.

The f i r s t s e t i s from passive tracking of revolution 124 of 1960 Delta and the second i s from a c t i v e tracking revolution 34 of 1960 Garmns 2. d a t a a r e compared with the published r e s u l t s of Space Track, Bedford, Mrtssachusetts.

1.

1960 Delta (Discoverer X I ) Rev. 124 (passive)

DOPLOC ( S t a t u t e Miles )

SPACETRACK

DL-WRENCE

Semi maj. a x i s

4115

4121

-6

Eccentricity

.0198

.0177

.0021

Inclination

80. jiO

60. ioo

2i0

:

.Right Ascension of .

Ascending Node

207 .'joO

207 .2T0

.23O

'Psgument of Perigee

97-49

125.22

-27.73'

.'Mean :

Anomaly a t Epoch

40.17~ 102

The

2 . 1960 (hama 2 ( m e i t D)~ e v .34 (Active)

m m mlse

BPACEW

DIFFEI($RCE

(atetuta

- ,0181

Eccentricity

.02U

0394

Inclination

51.40'

51.22'

.18'

Right Asoansion of Ascending Node

267.21'

266.75'

0.46'

Argument of Perigee

321.17'

277.15O

43.62O

Mean Anamaly a t Epoch

lbi.07'

.

. .

BRL R e p & the BRL DO-

a.L123

0f the tracking 8ctivitieS of etetioas f o r both active and psseive type tracking. ~0ntsine8 -8

IV. THE FULL SCALE DOPLOC SYSmM CONCEPT A.

Introduction Amendment No. 3 to ARPA order 8-58 provided funds for a supporting

of

arriving at reccomnendations and specifications by July 1960, for an "ultimate" full scale Doppler type satellite detection and tracking system. The tentative conclusions of Systems

research program with a goal

engineering studies were outlined to ARPA representatives at a IEeetinI3 at BRL on 14 April 1959 and briefly described in BRL Technical Note NO. 1266,

"An Approach to the Doppler Dark Satellite Detection ~roblem",by L. G. deBey. This desciption did not attempt to justify the concept presented nor to derive quantitative parameter specifications. In the interest of publishing the general concept at an early date, the result of a number of studies which had been under way for some time were largely omitted. This section of the technical summary report presents the proposed "ultimate" DOFT.0.2 fence in somewhat greater detail and includes changes in design parameters which resulted from continuing studies.

B.

The Interim DOPLOC Complex

The interdm fence, which was designed and installed before the R & D program was initiated, made use of essentially off-the-shelf items of technical equipment and was intended to provide a capability of detecting one square meter targets at msximum altitudes of 500 to 1000 miles. This system employed antennas having east-west fan beams Of 8 x 76 degrees (16 db gain) directed to produce one vertical beam and two beams at elevation angles of 20째, one toward the north hind the other toward the south.

It can be shown that to meet the full ARPA design objectives the transmitted power required with these antennas would be about 30 megawatts, assuming parametric receiver front ends (1 db NF), cosmic noise level 3 db above .t 7 ,

receiver noise, 10-cps bandwidth, 10-db output signal-to-noise and an operating frequency of 108 mc. Such power levels in a single,illuminator installation would be difficult to obtain. Also several such installations would be required to cover the space volume above the United States. This situation led to an attempt to find a mesns of improving-the.gainof the " ~ t e x h aand s of using them more efficiently.

-I.-<

ff:

104

C.

Design Objectives

Finn requirements of system performance or specifications were not issued by ARPA during t h i s design period. Instead, a number of design objectives were informally stated in various technical conferences dealing with the dark s a t e l l i t e fence problem. These objectives, when viewed from the atandpoint of a system employing Doppler techniques, gave r i s e t o certain specific system requirements which more nearly represent specifications. The following tabulations contain lists of these objectives and 8pecFfic Doppler system requirements around which the f u l l scale Doppler s a t e l l i t e passive detection and tracking system was planned.

1. Detection Range.

A l l objects i n orbit between altitudes of 100

and 2000 statute miles. It is now believed t h a t a l l objects having effect i v e reflection areas greater than 1 square meter can be tracked over the range of 100 t o 3000 8. miles.

2.

Target Size.

A l l objects below 2000 s. miles whose effective radar

cross section i s 0.1 square meter or larger.

3.

Multiple Object Capability. System should be capable of handling multiple objects. Maximum number not specified. Design objective i s capacity f o r the order of 100 1000 objects i n orbit over the United States a t one the.

-

4. Detection Probability. The system should not permit "sneak" satellites t o pass; the fence should be t i g h t .

5.

Period Calculations.

6.

Orbit Inclinations.

Accurate t o within 1

-

5 seconds.

All.

-

7. Time of Arrival. 1 5 seconds.

8. Accuracy, Aximuth and Elevation.

500

- 5000 f e e t .

0.1'

t o 0.5'.

9. The Required for F i r s t Orbit Determination.

10.

Identification.

monitor eJniaslone

Resolution

Less than 20 minutes.

Indication of she and direction of mtion. Would

snd fndlcate tumbling or 8tabFlity of f l i g h t .

iog

11. Adequate Data for correlation with G o w n Orbits. ..

12. No counter-countermeasure proyisions except for extreme narrow banding of receiving equipment.

Based on the use of Doppler techniques and the mathematical solution required to derive orbital parameters from such data, the following Specific system requirements are postulated: 1. A minimum of twelve Doppler frequency measurements must be obtained

for low altitude satellites and twenty-five or more frequency measurements for medium to high altitude satellites.

2.. The system must track the satellite w e r a segment; of its orbit

carresponding to at least 150 seconds of flight time for lar altitude satellites and for 600 seconds of flight t h e for maximum altitude satellites.

This tracking need not be continuous, but may consist of a number of short segments.

I).

Target Size

The objectives outlined above werenot based on a f o m l investigation of & potential threat created'bythe technical feasibility of orbiting.. . .&k saieliiies. Eather they constitute good guesses as io ine pro-r order of magnitude. Doubt exists that the value of the minimum detectable target size is realistic. It is difficult to imagine that a satellite about 20 inches in diemeter, which would have an effective reflection cross section area of 0.1 square meters at 100 mc if uncamouflaged, would'pose a serious threat, from the reconnaissance point of view, at an altitude of 2000 miles. It is conceivable, however, that satellites of larger physical size could be camouflaged to make them appear as 0.1 aquare meter targets, the degree to which this can be accomplished being dependent upon size, camouflage weight, frequency-bandwidth considerations, effect on flight performance and economic factors. Sincearbitrary specifications of a target %wo?or three times s&llep in effective cross section than that which can.'b& expected to perform. a useful reconnaissance function would require i,: 222

antenna,gains or transmitter pover l a r g e r by these same f a c t o r s (involving cost increases of millions of d o l l a r s ) an econolnically sound proposal f o r an "ultimate" system could not be assumed. Thus an investigation t o a i d i n establishing a r e a l i s t i c t a r g e t size was deemed t o be required. Such a study was i n i t i a t e d within the B a l l i s t i c Research Laboratories but d i d not progress s u f f i c i e n t l y t o j u s t i f y a report a t the closing of t h e project.

E. Antennas A number of beam configurations and combinations were studied and

discarded because they did not appear t o meet a l l of the system requirements.

Fixed f a n beams spaced every f e w degrees would provide enough data points but would require l a r g e numbers of antenna arrays and independent high powered driving transmitters w i t h attendant high cost. High gain pencil beams, scanning to give adequate volume coverage, would reduce the power of individual transmitters, but would require large numbers of c o s t l y arrays and transmitters in order t o provide a t i g h t fence (each pencil beam transm i t t e r receiver would have t o t r a c k a given s a t e l l i t e t o obtain enough data, thus leaving the fence "open" f o r other s a t e l l i t e s ) . Furthermore, w i t h a b i - s t a t i c system i n which transmitter and receiver pencil beams emanate f r o m d i f f e r e n t surface locations, the beams i n t e r s e c t only a t a small volume coverage, o r c e l l i n space a t any i n s t a n t . To provide the required c-lege one beam would be required t o acan the length of the other w h F l e it waa scanning, and thus would use t o o much time, permitting other areas to go unmapped, or s a t e l l i t e s t o cross other areas undetected.

A s a result of the above considerations it was decided that a fan

shaped beam was required. The dimensions of the beam were then t o be determined. Because of the curvature of the e a r t h it i s not possible t o have more than one transmitter and one receiver scan a given area i n a coplanar manner such that t h e receiver w i l l always see all objects illuminated by the transmitter. To scan even one p a i r of antennas together requires t h a t the scan a x i s be the chord of the e a r t h c i r c l e between the two antenna s t a t i o n s . The wide dimension of the fan i s determined primar i l y by %he distance between t h e two s t a t i o n s , which i n . t u r n i s dictated

by the minimum radiated power required t o detect a minimum size target a t 2000 t o 3000 miles a l t i t u d e .

The dimensions of the United States,

indicated t h a t a base l i n e of approximately 1000 miles was about correct.

A t greater distances, low a l t i t u d e s a t e l l i t e s would not be seen a t a l l

points between the stations, while f o r smaller separations of stations, f u l l a d ~ t a g ewould not be taken of available power. Early e0timateS indicated t h a t the beam dimensions should be about 60' by lo. Further study resulted i n the adoption of a 60 x 1.4 degree fan beam, scanned from horizon t o horizon.

F. Signal Dynamics The maximum Doppler frequency, using 108 mc f o r a s a t e l l i t e i n o r b i t a t 100 miles a l t i t u d e approaching a DOPLOC complex i n a plane of a great The m i m u m first derivative 2 For a circular o r b i t f d of the f r e q u e n c y - t h curve i s about 100 cps 2 The minimum a t an a l t i t u d e of 2000 miles fd w i l l be approximately 9 cps c i r c l e through the stations i s 6000 cps.

.

.

information bandwidth required t o acccmodate Doppler signals w i t h these r a t e s of change of frequency is given by:

For h = 100 miles, Bi = 10 cps; while f o r h = 2000 miles, Bi = 3 Cps. .

.

Since the received s i b a l is expected .to be less a t the higher altitude,

it i s fortunate t h a t a smaller information bandwidth is tolerable since it i s extremely important t o u t i l i z e the smallest practical bandwidth.

BRL experience has shown 2.5 cps t o be about the minimum useful bandwidth

for 108 mc.

This lower lhit i s due t o phase pertubations introduced by

the antennas and the propagating medium.

The maxFmum signal processing or integrating time i s given by:

.

i '-

Ti ranges from 0.1 second when when a 3 cps f i l t e r i s used.

8

10 cpe f i l t e r i s used t o 0.33 seconds

3

Gabor

4

and Woodward derive an expression

f o r the lincertainty of frequency measurement.

where s is the power signal-to-noise r a t i o .

For a phase locked tracking

filter of the type w e d in the DOPLCC fence the minimum usable signal-tonoise r a t i o has been experimentally shown t o be about 4 db. Since other types of narrow band frequency measuring equipment6 might require a greater signal-to-noise r a t i o , a s a f e t y margin was provided by assuming a required output S/N = 7 db. The frequency uncertainty (Af) i s thus dependent upon integration time assumed and w i l l be e i t h e r 1.4 cps or 4.5 cps f o r integrat i o n times of 0.33 and 0.1 seconds, respectively. The r e s u l t of o r b i t computations containing random e r r o r s of frequency measurement of these same general magnitudes show t h a t the accuracy of the solution i s not seriously degraded by such e r r o r s .

Shorter integration times result i n higher noise-to-signal r a t i o s , d e t e r i o r a t e the accuracy of frequency measurement and reduce detection range. A heavy penalty i s paid i n terms of t o t a l transmitter power required by even a twofold increase i n bandwidth. The minimum signal duration (integration time) cannot be appreciably l e s s than 0.1 seconds. Conversely an increase i n the integration time above the value of 0.33 seconds would reduce the bandwidth below 3 cps.

For the scanning antenna system proposed, the maximum signal processing time i s equivalent t o the time required f o r the narrow dimension of the scanning f a n beam t o pass over the t a r g e t , the time on t a r g e t ( t T ) and i s equal to: Beamwidth (Q

t~

= Scan Angle Veloci!y

(wl

Conversely f o r a given tT, the beemwidth i s :

The angular scan velocity i s determined by the number of data points required

and the total time the target is within tracking range. These factors vary as functions of target altitude. At low altitude the slope of the Doppler =?&we is high P Q ~ fe~er&t= pojnts a_-* .reguireB. Fewer than six points On he Doppler curve will not yield a solution, even in theory. Experience has shown 12

-

15 points to be adequate. With close approach satellites at high

altitudes, the slope is low and the "S" curve has much less character, requiring more data points to obtain a solution.

The time the target is within tracking range, T, is summarized in Table I, and ranges from 192 to 1000 seconds. The angular scan rate, W, is given by

where ND = number of data points required. @ = scan angular coverage.

The lowest value of total time for tracking, T, must be used since this will determine the maximum scan rate. Thus for: = 1 60'

.w.

=

l2 160 192

=

- -0 second

LLJ

I

*

One scan period is thus Ts = @ W = 10 = 16 seconds.

The number of data points which can be obtained for targets at higher t altitudes may be obtained through the use of the expression ND = Ts Representative values of ND for selected altitudes are given in table IV

-.

-

1

MINIMUM MJMBER OF DATA POINTS VERSU8 ALTITUDE* Altitude Miles

V

WS~C.

S Miles

T Sec

.

Number Data Points

*For circular polar o ~ b i t

0.

V

= S a t e l l i t e velocity a t a l t i t u d e specified.

S

= Length of observed s a t e l l i t e path i n antenna coverage volume.

T

=

Total time during which s a t e l l i t e may be detected.

Antenna Gain

The minimum effective n a r r w dimension of the composite fan beam eyetem, camprised of two coplana~fan beams, can now be determined by the relation: 0

= W t T

The minimum value of 0 w i l l be obtained when tT is smallest. The time on target, tT, was shown previously t o have values between 0.1 and 0.33 seconds depending on the beamwidth required by signal dynamics.

Thus the minimum effective beamwidth i s : 0 = 1 0 ~ 0 . 1= l d e g r e e Although larger values of tTwould allow the use of greater beamwidths the resultant lower gain of the antenna system would force the uee of higher transmitter' powers or would reduce the output S/N r a t i o . On the other hand, the f a c t t h a t the reciprocal of the narrowest bandwidth

(3 c p e ) ' a l l w s an integration time as great a s 0.33 seconds does not

mean that t h i s f u l l time must be used unless the lowest value of frequency uncertainty must be achieved.

It has been shown t h a t the uncertainty

equivalent t o integration times of 0 . 1 seconds i s adequate f o r the dark

It i s thus advantapeous i n establishing the remaining system parameters t o assume 0 = 1 degree ( e f f e c t i v e ) . The e f f e c t i v e value of Q i s stressed intentionally. Each of the two coplanar beam introduces s a t e l l i t e probIem.

a 3 db p a t t e r n f a c t o r a t the design beamwidth and the t o t a l l o s s due t o p a t t e r n f a c t o r i s 6 db.

In order t o achieve an e f f e c t i v e beamwidth of one

degree a t the 3 db point the individual beams must each introduce a pattern l o s s of only 1.5 db a t a beamwidth of 1.0 degree.

A n t e m s having 1.4

degree beamwidth at the three db points w i l l y i e l d an e f f e c t i v e system beamwidth of 1.0 degree. The fan beams required would thus have cross sections of about 1.4 x 60 degrees a t the 3-db

The gain of an antenna i n

terms of i t s beam dimensions i s given conservatively by:

It should be noted t h a t economic signal dynamic, and data quantity

considerations determine the minimum tolerable beam dimensions and hence t h e maximum antenna gain which may be used. The proposed DOPLOC s a t e l l i t e fence was thus an antenna gain limited syatem.

This f a c t o r had an impor-

t a n t bearing on the choice of optimum operating frequeccy.

Having determined the anten,m beamwidths and gain, the next problem was t o f i n d a design which would be economically and physically p r a c t i c a l i n s i z e and cost, and which would pro-?ide f o r the necessary angular scan without serious degradation of the beam ohape. This problem resulted i n an extensive engineering stud'j both by BRL and by prospective contractors who were requested t o submit bids on a scaled down model of the scanning antennas and transmitter f o r the "ultimate" DOPLOC system. On f i r s t consideration it appearea t h a t a Wullenweber type antenna design mounted with i t s normal diameter i n a v e r t i c a l plane would provide f o r the necessary scan:operation.

The f i r s t idea was t o feed the various trans.ndtting elements

.?"i'l. ?

1

112

of the WuXIenmbar antem fram cammutated low power circuits in such a manner that the r e ~ u i r e dnumber of antenna ehmente was fed eegwntially h o w that uhen the around tba rim of tho aataana. Further etudy ah& bsclpre from tha Wullenweber antanne. are tilted off the exis of the antenna eyeand than ecanned, the beam breaks down into a conical shape and will not perform the required scan at low angles. This subject will be treated in more detail in connection with the proposal submitted to ARPA. Suffice it to say here that satisfactory antenna designa were derived.

B. Received Signal Power Required.

.

.

The minimum usable received signal power which will provide suitable DOPLOC data is determined by the following factors: 1) External noise, 2) Internal noise, 3) Bandwidth, 4) Output signal-to-noise ratio. These factors are discussed in relation to the "ultimate" DOPLOC system.

1. External Noise.

5

Noise sources external to the DOPLOC system are expected to provide the major noise contributions for a carrier frequency in the range of 100 to 150 mc. These sources are extraterrestrial such as cosmic and galactic as well as of local origin such as automobile ignition, diathermy or other man made interference. Cosmic noises from the galactic plane and extragalactic regions are the most continuous sources of extraterrestrial noise, with occasional periods of solar noise when the sun is excited. High intensity, discrete radio star cosmic noise sources, most of which have angular dimensions less than lo will produce a strong pulse of noise when the high gain receiving fan beam sweeps past them. These noises did not prove to be serious since they could at worst only interfere with a few data points.

The amount of extraterrestrial noise available at the input to the receiver'is a function of the antenna pattern width and the region of the sky in the beam at the moment. As the beam sweeps across the galactic sector which is about 15 to 30 degrees wide it crosses the region of highest cosmic noise density. Therefore it was necessary to design the system to operate in the presence of this noise. The relation between the amount of noise picked up and the antenna gain and beemwidth ie a camplex function of

the orientation of the antenna and its gain level. When an antenna is oriented toward the galactic plane, its noise output will initially incream as its beam width is decreased. However, a limit ia resched where a further declrctaee in antenna beamwidth (corresponding to an increaee in gain with a

,point source) will not appreciably increase the comic noise received. mi8 limit is between 10 and 15-db antenna gain. Whan an antenna is directed of? the galactic plane and the noise is uniformly distributed in space, the comic noise received is independent of antenna beamwidth or gain. The proposed 1.4 x 60 degree antenna with 25-db gain is considerably over the 15 db gain limit for increasing cosmic noise. The noise measurements made with the interim DOPMC antennas, which had 16-17 db gain, should apply directly. Tlr cosmic noise measurements made with the intertm DOPLOC center antennas shoved a maximum noise temperature of less than 1160' K, 99 per cent of the time.

This 'temperature corresponds to a noise power of 1.6 x watts per cps bandwidth.

Atmospheric and local noise sources that have been observed include lightning, ignition, power lines and electrical equipment. The noise from these sources is generally of the impulse type, consisting of vary high intensity, short duration pulses that contain very little energy and are not expected to seriously affect tracking for an appreciable portion of time.

Sand static can also be a problem in desert areas where the antennas are exposed to blowing sand.

2. Internal Noise. A preamplifier with a 2-db noise figure is attainable in the 100 to 150 mc region with vacuum tubes. The internal noise for a 2-db noise figure is 2.4 x watts per cps bandwidth. This does not include the source resistance noise which for the DOPLOC system is the antenna with its associated noise temperature. The botal of the antenna noise plus the receiver noise is, 1.6 x = 1.84 x + 0.24 x watts per cps bandwidth. . .

3. Bandwidth. The bandwidth required in the receiving equigaent is dependent on both the orbital parameters of the satellite and on the design of thel'receiver. The method and amount of bandwidth limitation prior to

f i n a l detection is also &pen&nt on the characteri.8tice of tbe detection be eyetam wed i n the receivar. Ths RF sectian of a WPMC raeaiver relatively conventional. Tho ruepame ehwld be Unsrv 111thinput signal f o r the range of signals aad noise which must be received. !the receiver should be free of regemratica and oscillation tendencies, sud its phacre characteristics should be relatively unaPfected by variation of amplification. The output bandwidth of the IF amplifier should pass the maximum Doppler frequency, and should preferably be adjustable t o quite narrow values when it i s possible t o eliminate the noise by narrow banding techniques. The interim DOPLCC receivers used about 15-kc bandwidth i n the i.f. amplifier section, but through the uee of phase-locked tracking f i l t e r s i n tbe output circuits obtained the effect of pre-detection bandwidth reduction. The trackisg f i l t e r operated a t bandwidthe of 2.5 to 10 cps.

I.

Sya-

Geometry and Coverage

The purpose of Wm original DOPUX prop-

was t o provide s a t e l l i t e

detection coverage t o the volume of space above the United States. No counter measures were t o be initiated. The principal goal was to detect and identify every object passing over the protected area w i t h a known orbit, or t o compute a new o r b i t i f the pass were the f i r s t one for a given s a t e l l i t e . The spacing between the transmitter antenna and the receiver antenna was determined primarily by the altitude limits over which the system vae t o openrte. Ae indicated above the two antennas were t o either physically rotate or t o cause their beams t o sweep around an axis formed by an earth chord by drawing a straight l i n e between the two stations i n the plane of the great circle containing the two stations. m e range along the earth's surface t o the sub-target point, for a target a t 100 miles altitude i s 420 miles when the angle between the horizon and the line of sight t o the target i s 10 degrees. Based on these data it would seem t o be necessary t o place the stations every 420 miles or less apsrt. Greater spacing would permit volumes of space i n which 100 mile targets would not be seen even though they were between the stations. However, because of tbe beem broadening and side lobe effects, and also because of the proximity Of 100-mile objects t o the system, the distance between the stations can be axfended t o 1000 miles without leaving serious dark areas for low flying objects.

115

It i s of course not sufficient marsly-to & t e a t t h ~object. It be tracked over a large enough s e m n t of i t s p t h t o pannit tbs orbit parameters t o be determined. The time the target i#within tracking raw3 i s a function of its altitude and the volume covarsge of the tracking system. The volume coverage, conversely, can be determined by the minimum permissible time the target must be tracked a t a given altitude. For example a t an a l t i t u d e of 2000 miles t h i s minimum tim,e f o r s a t e l l i t e s in circular orbits is 610 seconds while a t 100 miles this time is about 150 seconds. A t the higher a l t i t u d e the s a t e l l i t e velocity i s about 4 miles per second and the distance travelled along the orbit during the 610 seconds i s 2440 miles. The b i - s t a t i c DOPLOC system m u s t then have a capability Of tracking the target along t h i s segment of its p t h .

Figure 35 shows the transverse coverage of the nodding o r scanning antenna. The coverage radius (2237 miles) intersects the 2000-mile altitude surface a t points 2440 miles apart thus providing the capebility of tracking the target along the required segment of the i r b i t . Table IV 1, page 111, shows the length of orbit segment f o r various values of altitudes between

-

100 and 2000 miles and the times required f o r s a t e l l i t e s i n circular orbits t o traverse these diatances.

The proposed patterns and coverage of two antennas of the nodding type i n a plane containing the base line, are shown i n Figure 36. The area of coverage i s approximately bounded by the earth's r a d i i extended through the two stations, these r a d i i lying i n the plane of coverage shown in the

previous figure. One range, transmitter t o target, has been determined above as being 2237 miles maximum. The other range, target t o receiver, i s approximately 2450 miles maximum.

The t o t a l volume of space swept out by one of these nodding antenna systems, and thus the volume coverage of one transmitter receiver pair, i s shorn in Figure 37. Total coverage would depend ugon the number of tramm i t t e r receiver pairs employed and upon t h e i r deployment. .

.

Figure 38 i l l u s t r a t e s a possible location of stations within the United S'tetes. A pair of transmitters, TI and T2 would be located a t about 40 i:" r degrees north l a t i t u d e and 98 degrees west longitude. Receivers R1 and R2

VOLUME COVERAGE OF ONE f RANSMITTER-RECEIVER PAIR' FIG. 37

TENTATIVE TRACKING STATION LOCATIOF(S

FIG

38

81 -STATIC ATTENUATION OR

VS

RADAR RANGE' SUM (RI+ R2 )

would be located about 1000 miles e a s t andwest along the 40 degree l a t i t u d e circle.

In principle, these s t a t i o n s would y i e l d s u f f i c i e n t cwerage t o

permit obtaining enough data from which o r b i t s could be computed.

In general

only data from one base l i n e would be received f o r each s a t e l l i t e pass f o r s a t e l l i t e s i n near polar o r b i t s and from two base l i n e s for satellites i n low i n c l i n a t i o n o r b i t s .

The improvement i n strength of the o r b i t solution

and the additional course angle data which could be obtained through the use of additional s t a t i o n p a i r s with similar nodding antennas, makes it appear desirable t o add two (or possibly more) base l i n e s t o the network.

Error

propagation considerations make it desirable t o i n s t a l l these base l i n e s a t r i g h t angles t o the o r i g i n a l base l i n e . and R4

-

T

m i t t e r s T1

4

Figure 38 shows base l i n e s R

3

- =3

located about 125 miles e a s t and west of t h e location of trans-

- T2.

This i s not necessarily the optimum location i n t h e east-

west d i r e c t i o n but i s approximately correct.

Targets passing through t h e

coverage volume of t h e system would be intercepted by the nodding beams of one or more traiismitter-receiver p a i r s .

During the time on t a r g e t i n t e r v a l , tT = 0.1 sec., the system must Locate the Doppler frequency i n the range of frequencies from 2 kc t o 1 4 kc and measure the frequency t o an accuracy determined by the system output The interim DOPLCC automatic lock-on and T' tracking f i l t e r required a minimum of 1.6 seconds t o accomplish t h i s sequence signal-to-noise r a t i o and t of operations.

It i s thus not adequate f o r the scanned beam system.

The

c i r c u l a t i n g memory f i l t e r (CMF) developed f o r the ORDIR radar by Columbia University i s well s u i t e d t o t h i s application.

The CMF w i l l be discussed i n

more d e t a i l under the section of t h i s report devoted t o the research program.

Other approaches using comb f i l t e r s were investigated within BRL and an experimental comb f i l t e r was developed.

This w i l l a l s o be discussed

i n more

d e t a i l elsewhere.

1. Trailsmitter Power Requirements.

f o r the b i - s t a t i c radar case i s given by:

The t o t a l f r e e space attenuation

r

r

Attenuation (db )

This function is giMn in Figure 58 a f o r the case where R1 and

R;! are

the variables and: Qr = Ot a 25ab (1.4' x 60' beauwidth) x = 9.12 ft (108 mc) a =1

BQ.

ft. (0.1

2)

In the figure f o r the case of R1 = 2237 miles and r a t i o : V p t = 1.7 x or -247db = F(A).

The transmitter power required i s given by:

R;!

= 2450 miles the

r'

Pt

=

i q ~

The received power required has previously been determined as being -

2.8 x 10-l9 watts.

Hence P

=

2.8 x 10'19

1.7

6

= 1.65 x 10 watts.

lo-*?

Although t h i s i s a high CW power level it can be achieved w i t h a t e - o f the-art components e i t h e r by using a multiplicity of power amplifiers, each feeding one o r more elements i n the transmitter array, or by paralleling several high power transmitter tubes.

V. SCAI8 MODEL G-S

REAM SYSTEM PROPOSAL

The full s c a l e o r improved DOPLOC system as proposed f i r s t i n BFU b o r a n d u m Report No. 1220 i s based on the use of the b i - s t a t i c , continuous wave r e f l e c t i o n principle and u t i l i z e s f o r the basic detection and tracking u n i t a high power radio transmitter and a high gain, narrow-band radio receiver, each coupled t o high gain antennas driven through synchronizing equipment.

Thin, fan shaped, coplanar beams a r e scanned about the a x i s

formed by a s t r a i g h t l i n e between the s t a t i o n s t o cover a space volume consisting of a half cylinder whose length i s equal t o the distance between t h e s t a t i o n s and whose radius i s 2000 miles.

Computed parameters, based on

t h e radar equation and v e r i f i e d by experience with the interim fence equip-

2

ment, indicate that t h i s system would detect and track objects having 0.1 m e f f e c t i v e cross sectional area over an a l t i t u d e range of approximately 100

t o 2000 miles when a two-minion-watt output power i s used a t the trsnsmitting antenna.

Engineering considerations indicated t h a t such a system was f e a s i b l e with e x i s t i n g state-of-the-art techniques and equipment. The basic cost of auch a system, while reasonable i n comparison t o the coat of other systems of similar proportions, represented a large investment in both funds and engineering e f f o r t .

Therefore since one of the goals of the reoriented DOPLOC

program was the demonstration of the f e a s i b i l i t y of the concept of determining o r b i t parameters by passive continuous wave Doppler techniques, it was proposed t o instrument two 700-mile base l i n e s with scaled-down models of the proposed ultimate equipment.

One of the base l i n e s was chosen t o u t i l i z e the Aberdeen Proving Ground s t a t i o n as the principal research receiving s t a t i o n , using e x i s t i n g equipment augmented with equipment from White Sands M s s i l e Range receiving s t a t i o n .

A transmitting s t a t i o n having two 100-KW transmitters and ecanning antennae

was t o be located a t Camp Blanding near Jacksonville, Florida.

With one

antenna beamed toward Aberdeen, Maryland, and one toward Forrest City, Arkaneas, two base l i n e s a t approximately r i g h t angles t o each other would provide geometrically strong data f o r o r b i t computation and would have permitted u t i l i z i n g most e f f e c t i v e l y the technical s k i l l of the BRL persmnel without extensive travel and l o s t time i n s e t t i n g up experiments a t remote f i e l d r-~.L

rzt?

uti &&;a..t

124

stations. As an alternate location it was proposed that the scale mcdel systam of two base lines could be located to fill the gap in the fence in the southeastern section of the United States between Forrest ~ity,"&kansas and WSMR.

The scaled d m system would have employed 100-KW radio transmitters, 4 x 40 degree scanning beam antennas, and base lines of approximately 700 miles. Low altitude coverage would have extended down to approximately 125 miles above the surface of the earth, while the high altitude 1 M t would have been about 650 miles. Such a system would have provided adequate detection capability to thoroughly explore the computational and data handling techniques and would have permitted the test and development of any improved techniques found necessary to adopt the DOPLCC concept to the scanning beam methods.

A scanning system of W e type would have offered significant improvement over the flxed beam system in detection capability. Instead of relying on having all factors affecting detection suitable during a short period of time that a satellite falls within a severely restricted angular segment of btection volume, this system would have continuourtly scanned a 160' sector of space volume and have made repeated observations of every object in the scanned volume. With the a priori knowledge that satellites must follow closely the classical laws of orbital motion, the multiplicity of DOPLCC observations on each satellite pass greatly simplifies the problem of distinguishing between satellites and other sources of reflected signals such as meteor trails, aircraft, ionospheric clouds, etc. In addition, it is this multiplicity of observations over a length of arc during a single p a r which gives the DOPLOC system the capability of determining orbital pammters vithout waiting several hours for the satellite to complete one or more revolutions along its orbital path.

A. Request for Contractor Proposals

BRL ~pecificitionNo. 16-9-59,entitled "Supplemental Information to Oral Briefing Pertaining to Requirements for Scsnning Antenna ~ y s t k " ,

was prepared and an open briefing was held by BRL and the local procurement

o f l i c e on 16 September 1959 t o acquaint prospective bidders with the requiremanta of the propoaed scaled down DO-

a

system and to s o l i c i t bid9

covering the furnishing and i n s t a l l a t i o n of one and two baseline systema 8 s . had been proposed t o ARPA.

-

Tb basic b i d covered the supplying Of the

CWQ

Blanding Aberdeen base l i n e , less the receiving equipment. !This included a prefabricated instrument s h e l t e r a t Camp Blending, a t r a n m i t t e r , two scanning beam antennae, and the antenna synchro~lieingequipwnt. As an

alternate bid, proposals were a l s o requested covering the b a s i c bid, and i n addition, a second transmitter housed in the common instrument s h e l t e r and two scanning antennas with synchronizing equipment f o r the Camp Blanding F o r r e s t City base l i n e .

Approximately 25 contractor f a c i l i t i e s were repre-

sented a t the briefing.

!i%elve proposals, some of which were joint e f f o r t s ,

-

were received.

Prices ranged fram l e s s than a million d o l l a r s per base l i n e t o approximately f i v e million d o l l a r s per base l i n e . In general the lowest priced systems proposed using mechanically r o t a t i n g l i n e a r antenna arrays.

Other system proposals were based on r e f l e c t i n g type, horn f e d antennas

scanned by organ pipe scanners, o r modified Wullenweber type antennas fed with organ pipe scanners or electronic scanning systems. Several i n t e r e s t i n g r e s u l t s came from t h e analysis of the bids received.

S i x of the proposals contained antenna designs based on scanning the fan beam by r o t a t i n g the antenna a r r a y s t r u c t u r e , Seven proposals contained antenna designs based on stationary antenna s t r u c t u r e s with the beam scanned by e l e c t r o n i c methods. The proposals were requested f o r two frequencies, 150 mc and 216 mc since these two frequencies had been suggested a s possibleassignments by the FCC. It was a l s o considered of value t o know the c o s t of the system f o r what might be considered the lowest and highest p r a c t i c a l operational frequencies. A t the lower frequency (150 mc) the tracking range i s greater f o r a given antenna power but the antenna s i z e i s l a r g e r . A t t h e higher frequency (2i6 mc) the antenna s i z e i s reduced by one t h i r d but the tracking range i s reduced and twice the transmitting power i s required compared with t h a t required a t 150 mc t o maintain a 2000-mile a l t i t u d e tracking capability. Soon a f t e r the contractor b r i e f i n g the FCC authorized the use of 150.79 mc f o r DOPLOC dark s a t e l l i t e tracking. Therefore, eubsequant

plaMing was based on the use of the 150.79-mc frequency.

to

It is of i n t e r e s t

note that the 216-mc system antenna and 100-KW transmitter cost estimates

were only between f i v e and e i g h t per cent lower thaa f o r the 150-mc systems.

Considering that increased transmitter power would be requlred a t the higher frequency t o obtain the desired range, the higher frequency system would actually be more expensive t o i n s t a l l .

During the analysis of t h e proposals, an important disclosure was made regarding w h a t appears t o be a t h e o r e t i c a l l i m i t t o the use of a c i r c u l a r a r r a y o r Wullenweber type antenna where the wide dimension of the f a n beam muet be t i l t e d away from the plane of t h e array. The e f f e c t i s that the t i l t e d swept beam degenerates a t low elevation angles. When this e f f e c t

was c a l l e d t o the a t t e n t i o n of the contractors who had proposed using a circular scanned a r r a y it stimulated a r a t h e r extensive t h e o r e t i c a l study of t h i s problem. The general conclusion i s t h a t f o r a 4 x 40 degree beam such as was required f o r the scaled down model the pattern degeneration could be tolerated. However f o r 1 x 60 degree beams an extreme amount of compensation would be required or the beam would degenerate from a l i n e i n t o 8 "bow t i e " e f f e c t which would destroy its low angle coverage.

Since one of t h e conditions of the specification required that the design be capable of scaling up t o the two megawatt 1 x 60 degree, 1000-mile base l i n e system, the c i r c u l a r a r r a y antenna proposals were excluded from the f i n a l considera t i o n . In general these designs were chwacterized by considerable complexi t y , high cost and some new component development requirements.

Ultimately, t h e cost and available funds r e a l l y determined the choice of antenna system proposed f o r the scaled down model. The cheapest antenna consisted of a linear array of Yagi antennas mounted on a r o t a t i n g boom 73 feet i n length. Sixteen Y a g i s , each having seven elements, were used t o form the proposed 4 x 40 degree beam. Two arrays, pointed 120 degrees a p a r t , were mounted on the boom t o provide continuous scanning of the fan beam with continuous rotation of the boom. One array was t o be switched t o the feed l i n e when i t s beam was ten degrees above the horizon, then, a f t e r scanning t o within 10 degrees of the opposite horizon, it was switched off

and the other array switched on. Four scans par minute, all in the same direction, were to be made by a rotational rate of two revolutions per minute. A l l polarizations (circular right or left hand and linear vertical or horizontal) were to be Drovided by the use of crossed Yagi elements.

The desired polarization was to be selectable by changing feed connections to the Yagi's.

Synchronization of the transmitting and receiving antenna beams was to be accomplished by a servo motor controlled drive system which was synchronized to a precision frequency standard at each station. Synchronization was to be maintained under winds up to 80 m/hr. There was some question as to whether a rotating boom type of antenna could be scaled up to a 1 x 60 degree beam capable of handling two megawatts of power. One contractor did submit a mechanical design for such an antenna.

A more expensive antenna, but one which seemed more m i l y adaptable

to scaling to the ultimate system requirements consisted of two complex curved surfaces arranged back to back. Each curved surface was illuminated by a set of horns to produce an eighty-degree scan from ten degrees above the horizon to vertical or just beyond vertical. The horns were fed in sequence from an organ plpe scanner. h e set of horns and its reflector scanned from 10 degrees above the horizon up to vertical. The power would then be fed to the other set of horns and the scan continued from the vertical down to 10 degrees above the opposite horizon. This constituted a basically new approach where instead of sequentially feeding a large number of radiating elements located on the periphery of a circle, only a relatively small number of elements are fed and the c m d reflecting surfaces form the narrow beam.

The resultant reduction in complexity is considerable and a much desired degree of versatility is gained in'control of beam pattern shape and polarization. Figure 39 shows a drawing of the propoeed 8ntenna design.

This reflecting type antenna appears to be the best design for meeting the DOPLCC requirements. The scanner provides accurate angular information relative to the position of the beam once the system has been calibrated.

It also has the ability to be scaled to the 1 x 60 degree beam with the two .+

"THE INFORMATION HEREON IS PROPRIETARY AND THE FACT THAT A PERSON MAY BE GIVEN ACCESS THERETO UNDER GOVERNMENT PROCEDURES IS NOT DEEMED A LICENSE TO USE SAME WITHOUT THE EXPRESS WRITTEN APPROVAL OF MELPAR."

Figure 39 Proposed Antenna Design Melpar, Inc.

megawatts parer handling capabilities. The organ pipe scanner and horn feed system have been designed and tested out in other installations.

Therefore the engineering information is available. In view of the larger cost of the horn fed reflecting antenna and the desire to obtain as much information as was practical f r m the scale model installation, one pair of organ pipe scanned antennas was recommended for the Aberdeen Camp Blanding base line and one pair of mechanically rotated Yagi arrays was recommended for the Camp Blanding Forrest City base line. In fact, when

-

-

the future of the program was in doubt,several research programs ranging in amount of effort from the installation of a single base line in accordance with the lowest priced proposal to two base lines of the scanned reflector antenna system at several times the minimum cost were proposed to ARPA. All of these proposed programs were directed toward arriving at a set of specifications for an ultimate or improved DOPLOC system. As a byproduct of these programs, the application of DOPLOC to the anti-satellite defense problem was to be studied. Although tentative plans were made to install the scaled down base lines and a site was obtained at Camp manding, Florida, ARPA did not approve this pnrt of the program. The project was therefore deactivated wlthout field testing the scanning beam concept.

VI.

A.

.

SuPPClRm 8mEB

Soenukrg Antsm Corn-'

canduatod t o btarmlne the number of r a t e l l i t e paeeee .crorr orch of the two proposed bare l i m e , Clrprp Bl8ndln&t Aberdeen and of tlrm that tha s a t e l l i t e Can@ BlandForreet City, atad th6 would be i n tho antanas be- during each psre. ! b i r r was accaupliehed by wing s mp of th. Mitad 8ta-8 a d mpremntiag tho 4 x 40 , O e antenna pettome a s two space volumer, each having a width of 800 mlles, a height of b00 miles and a length equal t o the distance bstwwn the stations. Thls distance for the Aberdsen C a u g B l a n d i n g base l i n e is 730 miles, while the Farcast City Cemg Blanding baee l i n e 16 610 milee.

A study

WM

-

-

-

-

Ue-

eatel$ihe predictlone furnished by S p c e Track Control Center, each satellite w e examined dvrLag the period indicatud and the t o t a l number. of w e e e croeeing sach base. l i n e .vae recorded. 'The length of timein-bsam for each pees WEW calculated u t i l i t i n g the sub-satellite trace and sssllming a nominsl e a t a l l i t e velocity of five rmilee per second. A tabulat i o n of reeulte 1 8 ehown i n Table VI 1 and a graphicel representation i e dieplayed i n Figuree 40 t h r o w &5.

-

..

'5

'"

u,

'03 H31L131C

",HI ,O"N

3h31nP

itldvd

Udva3

W l d

OlXOL

ti231131U

01.

01C

"*i

J

,.,

-9

i::.;.j: :1

. . . . . . . . - .. .

-

I : : . : ...:

........

.....

I . . . . . . . . . . . . .

:

. J . . . .

-

Abe-n Proving Clround, lvsryland Camp BLsnbbg, B l o r i d 8 ( A W ) Forrest City, Arkansas Cemp Elanding, Florida (FCY)

-

Over lap-Area encompassed by both of ebove p a t b m e .

2.

S a t e l l i t e s and Period Examfned

-

1959 1 Dectmbsr 1959 1958 Epsilon, 1 Ma3r 1959 23 October 1959 (Dew) 1959 Epsilon, 13 A m s t 1959 (Lsunch) 23 Beptamber 1959 (Decay) 1959 Zeta, 19 Auguet 1959 (Launch) 20 October 1959 ( ~ e c a y ) 1959 Iota, U October 1959 1 December 1959 1959 Happa, 7 November 1959 (Launch) 26 November 1959 (Decay) 1958 Delta, 1 M a y

-

-

-

-

-

3. Altitude Distribution Total Passes Passes Below 401 Miles Percentage of Total Below 401 Miles

4.

APG

814

FCY 750

OVERLAP

400

377

49.1

50.3

221 49.4

447

Time-In-Beam Distribution ..;. !.

For APG base l i n e 53.0 $ of passes.are i n the beam from 150 For FCY base l i n e 54.6 $ of passes are i n the beam from 120 For overlap pattern 39.8 $ of pa'saee are i n the beam from 120

- 179 sece.

- 164 sece.

- 164 secs.

It can be seen from these data that, of the two proposed base lines, the Aberdeen Camp Blanding base l i n e was the most promising, both fram the standpoint of t o t a l number of passes and from the tims-in-beam of these passes. The long base l i n e accounts f o r the larger t o t a l number of passes, while the geographical location of Aberdeen and Camp Blanding i s such t h a t

-

a s a t e l l i t e having an orbital inclination of fram 50 t o 90 degrees crosses the pattern nearly diagonally and therefore, is within the beam a greater length of time. The data also ahow that it w i l l be necessary t o decrease the antenna scan time f r a 15 seconds t o 7 1/2 seconds per scan t o provide 10

- 20 DOPLOC h i t s per pass f o r the majority of passes crossing the baee l i n e . ,

,

. ..

5 r.

'.<.:

138.

B.

Cosmic Noise and Interference a t 150.79 mc

Following the assignment by the Federal Communication Commiesion of 150.79 mc f o r t h e DOPLCC frequency a study was made of the r a d i o i n t e r f e r ence occurring over the frequency band of 148 153 me. A standard dipole

-

antenna, a preamplifier, a s e n s i t i v e receiver and i n the study. Graphic recordings of signal l e v e l service dispatch s i g n a l s were observed a t 149 and and receiver noise the band from 150 151 mc was

-

a decibel meter were used were a l s o made. Taxi 153 mc. Except f o r cosmic c l e a r . Calibrating the

db meter t o an a r b i t r a r y scale reading of 10 when the preamplifier was terminated with 50 ohms showed a cosmic noise ranging from 0.5 t o 1.75 db above the reference point.

S t a r t i n g a t 0800 hours l o c a l time the noise

gradually increased t o a peak a t around 1200 hours and then decreased toward evening. Using the same equipment and reference p o i n t but tuned t o 108 mc the noise l e v e l ranged from 1.5 t o 3 db above the reference w i t h a similar amplitude d i s t r i b u t i o n as a function of time between 0800 and 1630 hours.

No l o c a l interference was observed except when vehicles passed within about 200 yards of the antenna.

C.

Circulating Memory F i l t e r In order t o obtainthe maximum b e n e f i t of narrow band technique8 i n the

interim system, phase-locked tracking f i l t e r s were ueed. These f i l t e r s were supplied by the I n t e r s t a t e Electronic Corporation, Anaheim, California.

The f i l t e r s were designated Phase Locked Tracking F i l t e r M d e l IV, and had been developed by I n t e r a t a t e f a r BRL under a aeries of research contracts.

When t h e scanningbeam antenna concept w a s developed it became apparent t h a t the time on target would be q u i t e &ort and that a Large amount o f d a t a could be generated by multiple targets. Only appraximately 0.1 second of time would be available t o lock onto a signal and t o measure the frequency.

This type of performance seemed t o be beyond the c a p a b i l i t y of any type r i l t e r which operated on the principle of the I n t e r s t a t e u n i t .

The Electronics Research Laboratories of Columbia University were known t o have developed a frequency estimator using coherent integration detection methods f a r the CEDR radar. Contract DA-30-069-509--2748

U9

was therefore negotiated with Columbia University to cover a study of the applicability of the ORDIR system techniques.to the tracking of passive satellites. Final Report No. ~1157,No. 14 of the BRL DOPLOC Satellite Fence Series entitled "Summary of The Prelimimry Study of the Applicability of the ORDIR System Techniques to the Tracking of Passive ~atellites", Columbia University, School .of ~ n ~ i n e e r i nresulted ~, from this study. !Phis report contains the results of a study into the problems of coherent integration and frequency estimation in the DOPLOC CW Doppler earth satellite tracking system.

Specifically the report discusses the incorporation of the coherent signal proceeser known as the Circulating Memory Filter (cMF) into the satellite tracking system. After the parameters of the system are reviewed the theory of operation of the CMF is presented. The coherent integration of signals and the resultant improvement in the signal-to-noise ratio are discussed and the problems involved in distinguishing signals from noise by means of a threshold test are investigated, all with particular reference to the CMF. A technique for estimating the frequency of a sinusoidal input to the CMF and an implementation of the technique are also investigated.

Finally the requirements imposed by the CMF on the remainder of the satellite tracking system are discussed. These include requirements for the receiver which furnishes the CMF input and for the data recording, transmitting and processing equipment which makes use,ofthe CMF output.

The detailed specifications for the CMF recommended for use in the satellite tracking system are extracted from the Columbia University report inciuded are specifications for a frequency estiinto this report. mator to accept the CMF output and furnish digital data to a computer and for a receiver which would present acceptable input'algnalsto the CMF. .. . ~ ,,., See.'Section VI - D.

so

.-

1. The Digital Frequency Readout (DFR~. The basic video signal pr&c$uced in the CMF represents, in analog form, a frequency analysis of the CMF input signal. The time positions of the peaks in the video wave

form are measures of the discrete frequencies present in the CMF input The use of a threshold crossing device in the CMF results in a ai&XL.

convumion of the video analog data into a d i s c r e t e form.

More s p e c i f i c a l l y ,

a pulse w i l l be generated a t the instant the video increases above the approp r i a t e threshold and another pulse w i l l be generated a t the i n s t a n t the video decreases below this threshold.

Such a p a i r of pulses w i l l occur f o r

each freqwncy component present a t the CMF input and of s u f f i c i e n t smplituie t o exceed %be h w c t i o n threshoid.

The object of tha DFR is

to convert the data (frequency) pulses a t the

output of the CMF u n i t i n t o binary numbers which r e p r e e t a unknown frequencies.

One method of accomplishing t h i s i s shown i n Figure 46.

ing on the l e f t of t h i s f i g u r e i s the output of the CM!? u n i t .

Enter-

For the

purpose of t h i s explanation, it i s assumed t h a t t h r e e unknown frequencies

are present and aU..of these w i l l be read out.

The f i r s t pulse of a pair of c a l i b r a t e pulses causes the t r a i n of clock pulses t o appear a t l i n e s la, 28 and 38 a t t h e respective outputs of the l o g i c blocks.

These pulse t r a i n s are applied through binary stages ( f l i p -

f l o p s ) t o t h e counters i n sections 1, 2 and 3.

The second c a l i b r a t e pulse

causes t h e t r a i n of clock pulses a t l i n e s l a , 28 and 3a t o be s h i f t e d t o l i n e s l b , 2b and

Jb,

respectively, thereby bypassing the binary stages.

The e f f e c t of t h i s i s t o divide the number of clock pulses e x i s t i n g between the pair of c a l i b r a t e pulses by two.

Hence any count r e g i s t e r e d later on

a counter w i l l indicate the number of clock pulses occurring from the

average time i n s t a n t between t h e c a l i b r a t e pulses t o the i n s t a n t when the counter was stopped.

The average time between threshold crossings i s a

good measure of the applied frequency since t h i s method e s t h e t e s the time of occurrence of the peak i n the video signal.

After the second c a l i b r a t e

pulse has occurred, then, the clock pulses a r e entering all three counters along l i n e s l b , 2b and 3bJ respectively.

When the f i r s t data pulse of the

f i r s t p a i r of d a t a pulses occurs, the clock pulses a r e switched s o t h a t the counter i s counting a t half its previous rate.

The second pulse of the

f i r s t p a i r of data pulse6 causes the counter of section 1 t o stop.

The

count thereby r e g i s t e r e d i s the number of clock pulses between the center of the c a l i b r a t i o n pulses and t h e center of the f i r s t p a i r of data pulses.

When the counter of section 1 i s stopped, the l o g i c u n i t of section 1 sends a s i g n a l t o the l o g i c u n i t of section 2 i n order t o prepare it f o r responding t o the next p a i r of data pulses.

The second p a i r of data

pulses then causes a count t o be r e g i s t e r e d on the counter of section 2 i n a manner s M l a r t o that described above.

The t h i r d pair of data

pulses causes the counter of section 3 t o stop.

A clock pulse r a t e of about 1 0 mc i s reasonable since t h i s has a

period of 0 . 1 ps, which corresponds t o a frequency quantization of about 2 CpS (using the conversion f a c t o r of 20 cps/ps).

If a 10-mc clock r a t e

were used, t h e maximum count possible i n 1 m s i s 10,000.

14 b i t register.

This requires a

The implementation presented here i s only one of a

number of p o s s i b i l i t i e s and i s given a s an i l l u s t r a t i o n of t h e general nature of the implementation problem.

2.

Digital Data Recording and Data Rates f o r CMF.

In t h i s study it

was assumed t h a t t h e following data would be recorded whenever a "frequency readout" occurred: a.

Time (of occurrence of a frequency readout).

b.

Antenna beam elevation angle.

c.

Echo frequency count (up t o three counts).

To determine the average data transmission r a t e it was assumed t h a t : 1. On every scan of the antenna beam, one t a r g e t readout occurs.

2.

3.

On every scan of the antenna beam, one meteor echo i s observed.

he

meteor and t a r g e t readouts d o not occur simultaneously.

Based on a reasonable f a l s e alarm probability, the data r a t e derived has an average r a t e of about 18 h i t s per second. A s an indication of c h a m e l capacity, a 7-channel teletype operating a t 65 wards per minute has a capacity of about 18 h i t s per second.

Thus t h e recording rate required

was not found t o be excessive.

3. CMF DOPLOC Receiver Specifications. a.

Receiver Bandwidths.

The receiver pre-detection bandwidth should

be large enough t o accommodate the 24 kc Doppler frequency range, but not so large a s t o enable extraneous echoes t o be processed.

Although t h e CMT

provides large rejections for signals outside of its 12-kc input fI'equencY range, such signals might produce undesirable inter-modulations or cause temporary saturation of the CMF input circuitry. A 1 db pre-detection bandwidth which is no larger than 20 kc and no smaller than 15 kc was spcified for the 1.f. stage which feeds the CMF. The receiver pre-detection band pa88 characteristics must be maintained flat over the entire 24-kc Doppler frequency range. Since, the CMF rejection characteristics are specified to be flat to + 1 db (maximum) withln its 12-kc frequency corerage, the receiver characteristics apould be roughly + .1 db to prevent further deterioration.

-

..

b. Frequency Scan. The receiver will scan the twenty-four kc Doppler frequency range in two steps (12 kc per stage). Each frequency scan will be actuated by a command pulse which is generated by the CW at the termination of its integration period. c. Dynamic Range. The receiver must meet the following dynamic range specifications: 1. For a noise free, sinusoidal input voltage of amplitude 55 dbYrms atany frequency in the 24-kc frequency coverage, all receiver output spurious responses shall be more than 40 db (with respect to the input) below the output peak response.

For a noise free input signal consisting of two slnusoids each of amplitude, 55 db nus and at different frequencies within the 24-kc coverage all receiver output spurious responses shall be more than 40 db (with respect to the input) below the output peak responae. Spurious responses generated in the receiver outside the 1 kc channel containing the signal must be kept 55 db below the signal peak.

2

d. Receiver Gain and Voltage Levels. As a compramise between CMF and ~QC,.,quirements and lose in detectability due to the addition of CMF and receiver noise, the receiver gain was set at such a level that the sum of the CW and receiver noise would attain a level of about lldb. This . . carresponde to a loss in &etectability of about 0.4 db and to a maximum CMF'AGC requirement of about 15 db, that is, to a maximum signal of about 70 db.

4

,

.

I

<

,-?;-...;;1,The db values quoted are relative to the usual reference level of CMF internal noise referred to the CMF input.

T % ? 3'

e. Timing Pulses and Auxiliary Outputs. The receiver must also supply (in binary form) all remaining information not supplied by the CW that is either necessary for computing purposes (receiver oscillator

settings, absolute time, etc) or desirable to record (gain settings, antenna positions etc )

.

f. Autamatic Noise Level Control. During the antema.scanperiod, cosmic noise fluctuations may cause the receiver noise level to vary by several db. Such fluctuatians could cause a large increase in the system false alarm probability and might therefore result in a temporary saturation of the data transmission equipment. To control these fluctuations an automatic level control technique which changes the gain setting only when changes in the noise level have occurred is required. A parallel controlschene with the following design factors is recaamnended: 1. An accurately calibrated receiver gain bias characteristic of sufficient dynamic range to allow correction for anticipated input noise variations.

2. A gain stability in both the receiver and estimator cha~els sufficient to maintain the above calibration.

The overall response of the DOPLOC receiver shall be such as to supply the following to the input of the Circulating Memory Filter: 1. Carrier Frequency 2.

3

- 11 mc .

Bandwidth (1 db points). No -largerthan 20 kc and no smaller than 15 kc.

F S Voltage Levels. 11 mv -+ 1 mv minimum detectable signal, 560 mv + 6 mv maximum processed signal to 4 volts maximum output signal.

Receiver Gain. The receiver shall have a gain such that the rms value of its output noise (after AGC) shall be nominally 3.6 mv + 0.5 mv i n 8 10-cps band.

-

4.

-

5. Receiver Noise

The receiver shall have a noise actuated AQC such that the receiver output noise remains constant to within + 6.2 db.

-

AN.

6. Frequency Stability. All lkal oscillators shall have a stability or be so recorded that the output frequency uncertainty is less than 2 cgs (standard deviation) and 2 cps man.

7. Output Resietsnce. cf

Lh?

100 ohms to feed the 100 ohm input resistance

C.T.

8. Fzquency Scan. The DOPLCC receiver shall scan the 24-kc expected frequency range in two steps (12 kc per step).

9. Timing Fulses. The DOPLOC receiver ehsll supply "Integ~ateStart" pulses to the CMF st 8 prf of 10 per ssc. These shall be positlye pulse8 aud shall have arqlxlltudes of at least 5 volte, into a 100-ohm load, rise aad fall tbes (10 per cent to 90 per cent of peak amplitude) of not more than 112 wec., and duration (between go per cent points) of not less than 1 gsec nor more than 3 vsec.

D. Circulating M3mory Filter Specification The folloving specification is extracted from Columbia University Report "Summary of the Preliminary Study of the Applicability of the ORD3R System Techniques to the Tracking of Passive Satellites", 11 February 1960, Appendix B. These specifications are quoted here because they do not otherwise appear in a BRL report. Readers not interested in detail of the CMF specification are advised to omit this section.

1. Spectrum to be Analyzed (Frequency Coverage):

*'! ; ;

2. Input Resistance. 100 -+ 2 ohms

,

3. Effective Input Noise Voltage. The noise generated by the CMF

,:.

.

:

shall be such that if a 1.0 mv m s noise voltage per 10-cps bandwidth is applied to the input, the output noise shall increase by no less than 3 db.

The nominal value of effective input noise (self-generated) equal to 1 mv rms gr'l0-cps bandwidth is thus defined. All subsequent voltage levels shall be *seed in db above 1 mv (CMF noise in 10-cps bandwidth is at 0.0 db).

-

:.; !?L,%*:

'

146 $

4.

Nominal W input Signal Levels.

1 0 cps band)

(db above CMP noise i n a

... ....... .......... .........

a. Receiver p l u s CW noise per 10 cpa band b. F+eceivernoim? per 12 kc6 band c. Miniam detectable signal d. kclmum (unlimited) s i g n a l

5.

Nominal Coherent Integration Time.

i n approximately 1 msec s t e p s .

11.0 41.8 21.4 70.0

Variable from 20 msec t o 110 msec

A l l s p e c i f i c a t i o n s s h a l l be met a f t e r inte-

grating during a nominal i n t e g r a t i o n time of

93 msec.

6 . Integration Start. The CMF shall begin t o integrate an input * This voltage not more than 1.0 laa after receiving an i n t e g r a t e start pulse. pulse w i l l have the fallowing ~ t e r i a t i c s :

a. b. c. d. e.

polarity: positive amplitude: 5 v o l t s or g r e a t e r across 100 ohms duration: between 1 and 3 psec risetime: 0.5 Mec (10 per cent t o 90 per c e n t ) period: nominally one pulse every 100 as.

The r e p e t i t i o n r a t e acceptable t o t h e CMF shall be from g pulses per sec t o 40 pulses per sec.

7. Dead Time. The nominal dead time s h a l l be within 1msec of the difference between t h e input pulse period and the s e t integration time but s h a l l never be allowed t o become smaller than 4 msec.

8. Linear Operating Range.

For any s i n g l e sinusoidal, noise free

input s i g n a l within t h e frequency coverage, having an rms amplitude between 16 db and 36 db there s h a l l e x i s t a l i n e a r r e l a t i o n between rms input voltage amplitude and peak video output voltage.

L h e a r i t y i s here defined

t o be within 5 per cent f o r t h e b e s t linear f i t .

9. Frequencycoverage Ripple. For an input sinusoid of constant amplitude between 16 db and 36 db, t h e CMF video output s h a l l be constant t o within

*

+- 1 db

f o r a l l input frequencies within t h e frequency coverage.

Externally supplied 147

10. Selectivity. For a sinusoidal, noise free input signal at any frequency within the frequency coverage and of amplitude no greater than 55 db and no less than 16 db, the output selectivity shall be as shown in Figure 47. All side lobe responses must be at least 40 db below the effective peek response for all inputs up to 55 db, All specifications pertain to that part of the characteristics which lies below 36 db (linear operating range ) , 11. Signal Enhancement. The selectivity characteristic as specified shall be obtained without causing the signal enhancement to fall more than 1.7 db below that obtainable from an ideal coherent integrator, for an input signal not more than 30 db.

12. Spurious Response Levels. a. For a noise free, sinusoidal input voltage of amplitude 55 db rms at any frequency in the frequency coverage, all output spurious responses shall be more than 40 db (with respect to the input) below the output peak response.

Consisting of two ainiisoids

Far a noise free

of

amplitude 55 db rms and at different arbitrary frequencies within the coverage, all spurious responses shall be more than 30 db below the main responses (with respect to the input). This voltage level shall be called the dynamic processing limit.

I , Signal Limiting AGC. The CMF shall supply a signal limiting AGC which doasnot significantly deteriorate the specified selectivity characteristic and spurious response levels (items 9 12). There ahall be an independent AGC for each time multiplexed channel. Each AQC shall be designed so as to meet the minimum specifications shown in Figure 48. In this figure, both the input to the AGC and its output have been normalized with respect to the input terminals of the CW. !The absolute gain and signal .~.-.. levels that are processed by the AGC shall be chosen so as to best compliment CMF design. The salient features of the AGC &e:

-

<L'',!

a. For all inputs between 10 and 40 db the output-input characterls'tic shall be linear to within 5 per cent (best linear fit). ?

%

.."

$

c 5.r!i;,,1

. .. !

-..,

148

OUTPUT I F SATURATION DID NOT OCCUR

OUTPUT WITH NORMAL SATURATION

MAXIMUM SIDELOBE Z

.-20

f

f ,',

0

FREQUENCY

-

fl :f8- f * ' I I

f.1;'

Afs--

DB BELOW EFFECTIVE (UNSATURATE01 PEAK

-2 -3 -6 - lo - I5 - 20

" % -

L

Y .

"

-1

MAXIMUM FREQUENCY SPREAD 1, C P S 0 0.5 13.0 17.0 21.5 28.0 3 2 .O 37.5

0

-I

,

2

f

0

MAXIMUM ASYMMETRY Af C P S

0 1.0 1.5 +_ 1 . 5 2 2.0 2 2.0 22.5 3.0

2

+

A-157

-S -Q O 5 l

C p z Respan== to a :.:axiiil'rirr?i A?iP::tiiee Sinewave (with Time weighting)

7yrypical

8

R MS

INPUT

(dbl

NOT MEASURABLE DUE TO C M F NOISE.

ACCEPTABLE REGION OF AG C ACTION.

Fig.

48

Signal AGC Requirements

db.

b. For all inputs between 40 and 70 db there is an acceptable region no circumatancea shall the of AGC action as shown in Figure 48 but un& output exceed 55 db fm a l l input signals q to 70 db.

For all inputa below 10 db the AM: output ehould be such as to not degndt system performance. c.

d. The AGC gain must be reset at least once during each integration time for each multiplexed channel i.e., it neednotbe continuously in operation during the integration the.

14. Video Outputs.,

!

I % laat circulation time of the integration time

*

shall be called the display period. The thirteen the intervals during the display period which containthe thirteen time: multiplexed channels shall be called the channel display times. The following video outputs with appropriate oscilloscope synchronizing pulses shall be provided by the CW: a. Entire integration time. b. Gated video during display period. c. ~a&d video during each channel display time.

All Qideo outputs shall 'havean amplitude of at least 40 db peak into 100 ohms for a noise free input signal of 20 db.

15. Display Period Start.

synchronizing pulse shall be provided to indicate the start of the display period. This pulse shall occur approximately 1 psec before the beginning of the display period and shall have the electrical characteristics specified for the timing pulses described in Section 6.

A

16. Threshold Levels and Stability. The C W shall provide a video threshold which s h a l l be continuously variable from 15 to 35 db with a

-

'driftof less than + 0.2 db during any t W interval no shorter than 0.1 sec. and no longerthan the time between calibrations. The nominal. threshold level shall be 24.4 db.

This threshold shall operate on all video output signals which result from input signals less than 40 db. For video output signals which result from input signals greater than 40 db, this threshold shall operate on the video after it has been attenuated by 10 db. This operation shall be automatic.

*

Twelve signal channels and one calibration channel 151

-

17. Output Data Pulses. Esca time during the display period that the vldeo output of the CMF is increasing and is equal to the threshold dbecribsd in Section 16 ths CMF shall produce a pulee. !l%a time occur-

* of this pulse

shall have a m a n value within 0.05 of the time thst the threshold is equal to the video output and ehall have a standard deviation of not more than 0.1 wec. These pulses ahall be positive and have en amplitude of,at least 2 volts, rise and fall times (10 per cent to 90 p r cent of peak amplitude) of no more than 0.05 psec, and a duration (bet-n 90 per cent points) of not more than 3 psec nor lese than 0.2 vsec. mnce

Each tlme during the display period that the video output is decreasing and is equal to the threshold describeb. in &ction 16 above, the CMF ehall produce a pulee. The time of occurrence of this pulee shall have a ~ a a n value within 0.05 psec oP the time that the threshold is equal to the video output and shall have a standard deviation of not more than 0.1 psec. The electrical characteristics'of these pulaes shall be the same as those for the data pulses described above.

The pulses marking the positive going threshold crossings shall be provided at a separate output terminal from the pulses markingthe negative going threshold croasinge.

18. Output Impedance Levels. All video outputs, synchronizing pulses, and output data pulses shall be capable of driving 100 ohm im-cerr.

9.External Terminals. a.

Chassis ground

b.

Display period synchronizing pulse Output data pulses (two terminals as provided in Section 17)

c.

AGC voltages External time weighting function generator with disconnect for built-in t b e weighting function d. e.

*

The %be of occurreace of the data output pulses shall be measured with respect to pulses originating fram the calibration signal contained in .':::-the calibration display channel. The time of occurrence specifications shall apply to all input signals of amplitude between 21.4 db and' :-.:;-- 55 db and a threshold setting of 24.4 db.

f. AI1 necessary.monitoringfunctions for alignment and operation as determined by the contractor.

20. Acceptance Tests. All items specified shall be checked for . . conformity with the specifications in the presence of BRL representatives and a reliability run shall be made such that the equipment shall be observed to operate satisfactorily for five consecutive days at eight hours per day. During this five-day period, preventative maintenance and checkout shall be allowed during the period in which the equipment is turned off.

21.. CW Information Available. The contractor shall supply an instruction report sufficiently comprehensive to allow an engineer to perform preventative maintenance and generally service the equipment. The report shall cover theory of operation of the equipment, block diagrams, detailed circuit diagrams, checkout and preventative maintenance procedures, servicing of breakdowns, etc.

22. Installation. Installation and initial. checkout of the equipment at the radar site shall be supervised by a contractor representative.

23. Power Requirements. The equipment shall op&ate from any constant frequency constant amplitude power source whose rms amplitude is greater than 105 volts but less than 120 volts and whose frequency is greater than 50 cps but less than 65 cps.

E. Studies Relative to DOPLCC Receivers 1. BRL Report No. 1093, "The Dynamic Characteristics of Phase Lock

Receivers" by K. A. Pullen. The first receivers used in detecting satellite signals were ccmercial versions of the R-390 A/URR as sold by Collins Radio Company under* model number 5-51. These receivers were used to receive the signals from the first Russian Sputniks. The same receivers along with type R-~~OA/URR were later used for the interFm DOPLCC stations. Since these receivers would not tune above 32 mc modified Tape-Tone converters were used to operate at 108 mc. These circuits were improved through the use of very stable frequency standards and frequency synthesizers as heterodyning frequency sources. Except for some preamplifiers and special

mixing c i r c u i t s t h e interim DOPLOC receiving equipment used e s s e n t i a l l y

off-the-shelf i t e m s .

The phase locked tracktug. f i l t e r s were b u i l t t o

specifications b u t e a r l y models wereavailable when s a t e l l i t e s became a reality.

With t h e development of t h e concept of scanning beam antennas f o r t h e DOPLCC system, it became obvious t h a t more sophisticated receivers would %e required.

The f i r e t study made resulted i n BRL Report No. 1093, January 1960, e n t i t l e d "The Dynamic Characteristics of Phase-Lock Receivers",

by Keats' A. Fullen, Jr. Series.

88

This was Report No. 8 i n the ARPA S a t e l l i t e Fence

The r e p o r t t r e a t s of the general theory of phase locked c i r c u i t s

applied t o small amplitude signal detection.

Except f o r the following

conclusions r e l a t i v e t o the receiver, the reader i s referred t o the o r i g i n a l report.

a.

"Constant s e n s i t i v i t y t o phase changes requires the use of a

phase detector i n the signal c i r c u i t and a cross correlation type detector i n the gain control c i r c u i t , so t h a t t h e g a i n action i s controlled by t h e desired s i g n a l alone and not the s i g n a l and noise together." b.

"The l i m i t a t i o n of phase e r r o r requires e i t h e r the use of a caammon

system f o r a p a i r of signals or the use of considerable l o c a l feedback

i:f.

on a l l of the r.f. and 1 . f . amplifiers.

The use of a c i r c u i t without l o c a l

feedback around t h e 1 . f . amplifiers does not introduce phase s t a b i l i z a t i o n ; it only introduces a compensating phase i n t o the feedback path.

Since t h e

phase of t h e s i g n a l i n the feedback path i s used a s a measure of the received phase, no removal of i n t e r n a l phase e r r o r s r e s u l t s unless l o c a l feedback over t h e in&P~ridunlatages c

.

ifi

used."

"Adjustment of the phase-lock loop f o r minimum position, velocity,

and acceleration e r r o r requires the use of a r a t h e r d i f f e r e n t type of . .. ,

c i r c u i t r y than has been used i n general i n t h i s type of equipment.

The

presence of frequency compensating c i r c u i t s between the phase detector and t h e : ~ a c t a n c emodulator introduces neither position nor velocity e r r o r correction, but contributes only t o the correction of the acceleration . ..,.. .-

error.

Correction f o r position and velocity e r r o r must be accomplished

iiietiie feedback loop .'.P .-..

.,

:"., .

?.i i .rp. '..

".$&,.

."

1.54

d.

"The introduction of f u l l correction f o r v e l o c i t y e r r o r requires

the use of a phase network i n the feedback path of r a t h e r unusual characteristics.

Further investigation of t h i s portion of the c i r c u i t appears

t o be justified.''

2.

DOPLCC Receiver Design f o r CMF.

BRL Technical Note No. 1345,

August 1960 e n t i t l e d "The DOPLOC Receiver f o r Use w i t h t h e Circulating kmory ~ i l t e "r by Kenneth H. Patterson.

Report No. 18 i n the ARPA Satel'

l i t e Fence Series, discusses a radio receiver which was proposed f o r use with t h e Circulating W o r y F i l t e r .

Figure 49 i s a schematic block diagram

of this receiver. Basically it consists of an r . f . amplifier operating a t 150.79 mc and three i . f . amplifiers w i t h automatic gain control and t h e s t a b l i i z e d mixer frequency sources.

The l s t , 2nd and 3rd i . f .amplifiers

operate a t 19.217296, 2.770708 and 10.994002 mc respectively.

The

receiver provides the specified 11 mc signal output t o t h e c i r c u l a t i n g The required frequehcy s t a b i l i t y i s provided by bringing

memory f i l t e r .

the required mixer frequencies from frequency standards and synthesizers.

The receiver a l s o supplies t o t h e f i l t e r the "start integrate" pulses.

For

a more d e t a i l e d discussion reference i s made t o t h e above mentioned report.

3.

Tests of Parametric Pre-amplifiers.

BRL Technical Note No. 1354,

October 1960 e n t i t l e d "Parametric Pre-Amplifier Results" by K. H. Patterson.

Report No. 19

, ARPA

S a t e l l i t e Fence Series describes the r e s u l t s of a

s e r i e s of t e s t a w i t h some parametric amplifiers supplied by Microwave Associatee, Inc.

These amplifiers were rather simple i n s o far as physical

construction was concerned. i n j e c t i o n and a pickup loop.

,

They consisted of a c o a x i a l cavity rdth an The 50-ohm loops appeared t o be i d e n t i c a l .

Each was provided with a standard type N coaxial cable f i t t i n g . A coarse and Pine tuning adjustment were provided. The latter was a c t u a l l y a modified micrometer mechanism. Provision was a l s o made f o r mounting a varactor diode and 'an adjustment t o vary a capacitor connected i n series w i t h the diode.

F i r s t msults w i t h the parametric amplifiers w e r e very disappointing.

It was then discovered t h a t t h e recommended pump frequency was the wrong value f o r the amplifiers. L a t e r laboratory t e s t s showed the amplifiers

to be practical-and to give a reduction of about one db in the noise figure of the receiver system. However, later attempts to use these amplifiers atthe field stations gave unsatisfactory results. The high "Q" circuit seemed .tobe sensitive to temperature drift. This sensitivity combined with the narrow bandwidth seriously changed the overall performance. Since the improvement over vacuum tubes was only about one db and since the external noise was frequently higher than the receiver input noise, no further attempt was made to overcome the problems associated with the parametric amplifiers.

F. Study of Orbital Data Handling and Presentation

BRL Technical Note No. 1265, June 1959, entitled "Orbital Data Handling and Presentation" by R. E. A. Putnam, Report No. 3 ARPA Satellite Fence Series, discusses the problem associated with handling the data for the DOPLCC system. This is a preliminary study, the purpose of which is to present the relative merits and limitations of various alternates proposed; to indicate the particular methods worthy of serious consideration and further study; and to outline the time factors and development costs likely to be involved in procuring prototype equipment.

Discussion is limited to a general, rather than to a specific treatment of the subject, because numerous pertinent details of instrumentation and orbital computational procedures we- not stabilized at the time the report was written. The subtitles treated are "preliminary data editing, orbital data catalog, orbital comparisons, visual data displays". The following is a summary of concluaians reached. a. Preliminary data editing by visual inspectlon of the Doppler "St' curve is believed to be essential, at least during the early stages of the project. b. Orbital garmeter data should be catalogued at the data analysis center in printed tabular form for operator reference, also in digital form far satellite identification and plotting board input. The choice of digital storage medium will be governed by facilities locally available, equipment,ccnnplexity,ccmrmaercial availability and cost factors.

157

c. A plotting board display appears to be justifiable for providing a visual check on satellite identification in doubtful cases. d. Orbital charts produced on plotting board and distributed in advance to ground receiver stations may sene a useful purpose in alerting operators to the Fmminent approach of known satellites and the* dinction of approach. e. It is recommended that prototype items of equipment hardware be aesaplb&ed to demonstrate their suitability in handling actual orB'lital data before gttempting to prepare final apeoifications for a field installation.

G. Synchronization of Tracking Antennas BRL, Technical Note No. 1278, September 1959, entitled "Synchronization of Tracking Antennas" by R. E. A. Putnam, Report No. 6, ARPA Satellite Fence Series, considers the problems associated with synchronizing scanning antennas.

Synchronizing a two-station complex to within

+- 1 degree, relative to

each other, requires that each individual station be synchronized to within + 0.5 degree, relative to a ccormron reference standard. In the table which follows are presented estimates of the relative component and total error magnitudes which might be expected from each of the three types of scanning antenna designs capable of meeting performmice requirements.

-

Estimated Error Usgnitudes for Various Antenna Types Error Source

1.

2.

3.

4.

5.

6.

.

.I

Station Timing Antenna Orientation Antenna Pattern Phasing Structural Deformation Scanning Beam Drlve Totals

Rotatable

Element Switching

+- 0.03~

-+ 0.02 -+ 0.05 -+ 0.10

-+ 0.05 -+ 0.10 -+ 0.35'

Controlled Phase Shift

-+ 0.03" -+ 0.02 -+ 0.05 - 0.35 -+ 0.05 -+ 0.00 -+ 0.50' +

,

It is to be noted that error ratings of the three antenna systems are identical for the first three error sources, a d are believed to be reasonably representative of values that can be maintained under ordinary operating conditions. Incidentally, they add up to 20 per cent of the total error permissible. Inasmuch as error magnitudes from almost any source can be progressively reduced by concentrating on equipment and circuit refinement, it is reasonable to suppose that all three antenna type8 under discussion can be developed to the point where they meet the necessary performance requirements for this proposed project, if cost is not a factor. However comparative figures tabulated above strongly indicate that the switched element type of antenna has the distinct advantage of a lower error susceptibility, hence the probability that it will carry the lower price tag and a simplified maintenance program. Other conclusions reached are: a. Sky-scanning antennas, synchronized to within + 1 degree, appear to be technically feasible for station spacings up to, and somewhat in excess of 1000 miles.

-

b. Each station should include a precision time generator to permit continuing operation for a reasonable period following interruption of the external source of synchronizing signals. c. Economic and other considerations indicate the desirability of synchronizing each station directly with W timing signals. d. Sky-scanning rates Wluence station synchronization procedures an8 data transmission problems. It is desirable that a scanning rate be selected such that the resulting number of scan cycles per minute becomes a whole number. e. It is not.essentia1 that the angular scan rate be completely unlform throughout the sweep range, but it is important that any nonlinearities be identical for both stations of a pair.

f. Sky-scannFng afiangements with a reciprocating a m p may be eubject to non-linear caaponents of motion in one direction that a'e not duplicated in the reverse direction, thus cmnplicating synchr~nizati~n procedures. For this reason, bema that sweep contFnuously in One direction are to be preferred. g. A fixed antenna structure, where in a scanning beam i6 genersted by prbgresaive switching of radiating elements, appears to be the most praC%ical antema type.

5.

Same moderate decrease in beam widtb, or some increase in beam scanning rate, appear to be feasible, if desired. However, such changes tend to impose increasingly severe requirements on the synchronization prop?m, and at some point a limit will be reached beyond which further changes will be impractical. Whether synchronization requirements can be met with beemwidths of only one degree is problematical.

.

E. Erecision Frequency Measurement of Noisy Doppler Signals BRL Report No. 1110, June 1960, entitled "PrecisLon Frequency Measurement of Noisy Doppler Signals", by William A. Dean, Report No. 15 in the BRL DOPLOC Satellite Fence Series, gives the rerults of a brief study of the uncertainty expected in measuring the frequency of a radio Doppler signal from the data gathering equipment of the DOPLCC satellite tracking station.

Two methods of measurement are alternately uaed depending q o n the cycle count and the desired precision. One method simply counts Doppler cycles from the tracking filter for a given time period, for example, one second.

The second method is indirect and integrates a chosen number of Doppler cycles over which period a high frequency, say 10 mc standard frequency is counted. Since the resolution of the counter is + 1 cycle, yielding a cycles per second when both ends of the interval counting precision of are concerned, the precision of the 10 mc count is considerable improved over that of a Doppler count of from zero to a few thousand cycles/second.

+G

-

me random scatter varies with propagation phenomena and the angle of elevation of the satellite with respect to the observation station, but may be as much as 3 cycles per second. This scatter is of such character V2.l

that it does not a f f e c t t h e operation of the tracking f i l t e r .

Typical

conditions are p l o t t e d i n t h e r e p o r t where a t an input frequency of 10 kc an input s i g n a l t o noise r a t i o of -18db, and a tracking f i l t e r bandwidth of 1 0 cps, peak excursions of 0.16 cps were observed and the ws frequency

error w a a 0.652 cps

.

I. Contract DA-04-200-m~-674, Stanford Research I n s t i t u t e , System Studies Under contract DA-04-200-ORD-674,

Stanford Research I n s t i t u t e conducted

a DOPLCC! system study which lead t o two r e p o r t s .

The f i r s t of these i s

e n t i t l e d F i n a l Report, P a r t A, S t a t i o n Geometry Studies f o r t h e DOPLCC System by W. E. Scharfman,

H. Rothman and T. Morita.

i n t h e BRL DOPLCC s a t e l l i t e fence s e r i e s .

This r e p o r t i s number 9

It p e r t a i n s e n t i r e l y t o t h e interim

type system which u t i l i z e s fixed antenna beams oriented i n such a manner a s t o obtain data f o r segments of an "St' curve.

An analysis was made of the

r e l a t i o n between the layout of the receiving and transmitting s t a t i o n s of the BRL CW r e f l e c t i o n Doppler system ( f o r the detection of non-radiating s a t e l l i t e s ) and such f a c t o r s a s antenna gain, p a t t e r n shapes, time of t r a n s i t of a beam by a s a t e l l i t e and coverage f o r fixed-beam antennas.

The r e s u l t s of t h e study show the regions i n space where s u f f i c i e n t data would be obtained t o determine t h e s a t e l l i t e ' s o r b i t a l parameters, and t h e complexity of t h e required antenna system for various s t a t i o n geometries and s a t e l l i t e altitudes.

This report is c l a s s i f i e d confidential and i s therefore

not: abstracted i n d e t a i l here.

The second SRI r e p o r t i s e n t i t l e d "Final Report, P a r t B, DOPLOC System Studies" by W. E. Sharftnan, H. Rothman, H. Guthart and T. Morita and is Report No. 10 i n the BRL DOPLW S a t e l l i t e Fence Series.

P a r t B of t h i s f i n a l r e p o r t shows t h a t because of the v a r i a t i o n i n r e f r a c t i v e index of t h e ionosphere, an e r r o r i s introduced i n t h e determinat i o n of the radial v e l o c i t y of a moving t a r g e t .

This e r r o r r e s u l t s because

t h e d i r e c t i o n of t h e r e f r a c t e d ray a t t h e t a r g e t d i f f e r s from t h e line-ofsight direction.

The Doppler e r r o r introduced a r i s e s from conditions a t the

a c t u a l instantaneous position of the s a t e l l i t e r a t h e r than from t h e integrated e f f o r t of conditions along t h e e n t i r e propagation path.

This e r r o r

is greatest when the satellite is at the level where the ionization is nost dense, at the msximm of the F2 layer (300KM). Here the error amounts to about 0.25 per cent of the Doppler frequency for a carrier frequency of 100 mc.

The Faraday effect causes a rotation of the plane of polarization Of a radio wave passing through an ionized medium in the presence of a megnetic field. As a typical example the number of rotations that a 100 mc vavc might incur in a double passage through an ionosphere 260 KM thick, at a height of 150 KM, varies from about 20 at zero degrees elevation to about 5 at 90 degrees elevation.

This report shows that the Luxembourg effect, or ionospheric crossmodulation effect, can be neglected for frequencies above 4 mc. Hence cross-modulation at 100 mc shourld be extremely small. It is also shown that the atmospheric noise generated in the troposphere is at least 40 db less than cosmic noise at 100 mc and can therefore be neglected.

Forward-scatter propagation involves scattering either by ionospheric or tropospheric inhomogeneities in the conmnon volume of the transmitting and receiving antenna beams. For frequencies above 100 mc and separation less than 600 miles, the tropospheric transmission component appesrs to dominate. Typical features of the tropospheric scatter are that it decreases rapidly with increasing distance, and decreases very slowly with increasing frequency. For a station separation of 450 miles, the average signal level with optimum antenna orientation will be about 90 db below that expected at the same distance in free space with the same power and same antennas. Free-space attenuation at this frequency and separation is 130 db. This received signal level is considerably greater than the received signal expected from forward scattering of the satellite. The actual tropospheric signal received is further below the free space signal since the side lobes and not the main beams will be forming the common valume

.

,s --t

,<

-

SRI s p e c i f i c a l l y investigated t h e question, a s t o whether the noise f i g u r e could be reduced by reducing the temperature of the antenna or of any reasonable volume surrounding t h e antenna.

A t 100 mc the background

noise temperature i s about 800 degrees K, while some areas of t h e sky (notably m e g a l a c t i c c e n t e r j nave temperatures up t o thousands of degrees Kelvin.

It i s shown t h a t i f a receiver has a 3-db noise f i g u r e and an

apparent antenna temperature of 800 degrees K, t h e operating noise figure w i l l be

5.5 db.

It i s therefore concluded t h a t no reduction i n noise

f i g u r e would be achieved by cooling e i t h e r the antenna or the volume immediately surrounding it.

The e f f e c t s of meteors and meteor t r a i l s have been discussed b r i e f l y elsewhere i n t h i s r e p o r t .

SRI presents an extensive analysis of t h e e f f e c t s

t o be expected from both meteors and meteor t r a i l s .

This treatrhent i s too

extensive t o a b s t r a c t here except f o r t h e following remarks.

The specular

echo count f o r meteor t r a i l s with i n i t i a l l i n e densities of 1014 electrons per meter i s d i r e c t l y proportional t o t h e square r o o t of t h e transmitter output power and t h e receiver s e n s i t i v i t y .

The interim DOPLOC system was

s e n s i t i v e t o t r a i l s with l i n e d e n s i t i e s of t h e order of 1012 electrons per meter. If t h e system were t o become s e n s i t i v e t o l i n e d e n s i t i e s of l e s s .-10 - . thm lo electrons per meter, t h e count might become d i r e c t i y proportionai t o these system f a c t o r s , and possibly even proportional t o t h e i r square or cube.

Various methods such a s increasing the frequency, the incorporation

of f i l t e r s , e t c . a r e available t o discriminate against meteor t r a i l s i g n a l s .

The remainder of Part B of t h e SRI report t r e a t s t h e e f f e c t s of antenna phase p a t t e r n s and suggests antenna systems f o r t h e DOPLOC system.

For this treatment t h e reader i s r e f e r r e d t o the basic r e p o r t .

J.

Comb F i l t e r Development Since the l e a d time f o r t h e development of t h e c i r c u l a t i n g memory

f i l t e r was known t o be a t l e a s t 12 months and the c o s t approximately $250,000, it was decided t o look f o r a second method of obtaining narrow band f i l t e r a c t i o n on t h e data from the scanning antenna DOPLOC system.

The search and lock-on time f o r t h e phase locked tracking f i l t e r s was too 163

long t o permit these f i l t e r s t o operate with scanning antennas.

Therefore

a progr~mwas i n i t i a t e d t o develop ~ ~ u l t a n e o u e lwith y t h e circuhting m+oq filter a comb type filter.

S t a r t i n g with a 10-cycle f i l t e r pass band and a possible 12-kc band of Doppler frequencies t o be covered, it is a t once apparent that 1200 individual f i l t e r s would be required t o cover the e n t i r e band. It was therefore decided t o build only 8 section of the f i l t e r f o r test purposes.

The e s u l t was one hundred and eighty individual ten cycle f i l t e r s spaced 20 eps a p a r t t o cover a t o t a l of 3600-cps bandwidth. This input was t o be switched three t b e s t o cover t h e 12-kc band. The r a t e of change of Doppler frequency v a r i e s with a l t i t u d e of the s a t e l l i t e from approximately 25 t o 100 cps. This maximum r a t e of 100 cps determines the minimum usable bandwidth of 10 cps. This i n turn s e t s the maximum time available t o recognize the Doppler signal a s 100 milliseconds. This maximum t i m e determines the maximum scan r a t e f o r a given antenna beamwidth. Since twelve d a t a points a r e desired t o insure s u f f i c i e n t data f o r a convergent solut i o n from a single pass, there i s a l s o a minimum scan r a t e associated with t h e low a l t i t u d e passes f o r R scan r a t e t h a t would give 12 data points on a pas6 and an integration time of 100 m . s . , 1 degree i s indicated.

a beamwidth of approximately

Describing the f i l t e r action b r i e f l y , t h e output of the receiver i s f e d through a noise actuated automatic gain control c i r c u i t , thence t o t h e f i l t e r comb, each f i l t e r of which has an amplifier, AGC, and threshold control c i r c u i t which feeds a multi-pen recorder. A record indicates a s i g n a l of frequency corresponding t o the f i l t e r frequency present a t the input t o the comb. This comb filter has been completed and t e s t e d on both r e a l time and simulated tracking data and found t o perform satisf s c t w i l y . It i s described i n f u l l d e t a i l i n BRL Memorandum Report No. 1349 e n t i t l e d "A Comb F i l t e r f o r Use i n Tracking S a t e l l i t e s " , by R. Vitek.

Figures 50 t o 57 inclusive a r e photographe of a c t u a l recordings made a t the output of the comb f i l t e r . Figures 50 t o 54 ahtm the performance c h a r a c t e r i s t i c s of the f i l t e r with simulated date..

Figurea 55 t o 57 ara

OPTIMUM

NOISE

LEVEL

FOR

MAXIMUM SENSITIVITY VARIABLE

NOISE

vs SIGNAL

1

...,

.

c

8

. :. ..

LEVEL 2

NOISE L E V E L : I 0 0 MV RMS ' I .

SWEEP R A T E : 8 0 CYCLESISEC.~ SWEEP LEVELS : AS SHOWN

FREQUENCY

SCALE

-

,,

:'!. 'i !

C.P.5

Figure 50 Optimum Noise Level for Maximum Sensitivity (Variable Noise vs Signal Level)

- .......

..........

.- . . . . . . . . .

SENSITIVITY CHART

VARIABLE

NOISE

DETERMINE

,

.... ....

'

SIGNAL

LEVEL

-.._

.........:

'

.

'

'

" ' . . ....... ,

-2,*)l

I.

NOISE LEVEL : 7 0 UV RMS SWEEP RATE ! 100 CYCLESISEC.' SIGNAL LEVEL : AS SHOWN

....-..

-

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*

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TO

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...... .. '....

.

AND

OPTIMUM

FREQUENCY SCALE

-

C.P.S.

Figure 51 Sensitivity Chart 166

.

DYNAMIC RANGE RANGE OF

MAXIMUM TO

GIVING SINGLE

. .. i

MINIMUM L E V E L S

P E N OPERATION

. I.

:

.. :

I

CHART

NOISE L E V E L : I 5 0 MV RMS SWEEP RATE : 80 CYCLESISEC.~

:

!..I

?

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:j , I . .,,I ;I. ,'-;: :::I ., SIGNAL LEVELS : AS SHOWN . . ......: , : .. ;I:!. : ... .i : . . .," . 1 . k ii '.:. : .. :. '.'- '. >:, :;., , n , , . - 3 0 d b ::' ,I, .. .I I :, , , :1. ,t .. . ,.. ( . I * 1' .iL :'.: .I1. I;, ., ,I .. . .... .. . . . . . . II ., .. *,i . . ; .I 4 ;. .!: i , . 6 : ; . . . . ' i -'.:.'!. . . ,,, jl.

,I

I

1,.

I

,.?

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,,I

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8.

..

,

,t. , #j 1 :

l.

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i

0

, , {

,

C

;,

; !i',.;!~l

.

1 ,, .. . : j :

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I, .: . ': ,... !I. .. I.: _. I; .. >. . ,: : ,. . ,, .,, . : . ... . . I. :. .

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6

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3

.

.

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. ! : , . : s:. ' I .. , . , , I ! .... ,..'.. .:';.I . : .. . ., ., 6,.

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a

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9

P

.

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j

f

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t

8

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(

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FREQUENCY

SCALE

-

'1.

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'

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/ I

C.P.S.

Figure 52 Dynamic ~ange'chart 167

#8

P

; 1s

,

AGC' ACTION LARGE

AMPLITUDE SWEEP

/

,

LEVEL :

;I:

,

S W E E P RATE : 77.5 C Y C L E S / S E C . ~ , S I G N A L L E V E L S : AS SHOWN

:

.. .'. .'.

.;, .

. ,:.

L E V E L .: 150 M V R M R FIXED SIGNAL : - 6 d b

.

'

!

j

SIGNAL

SIGNAL

. ' NOISE

21.

.I

F I X E D FREQUENCY VS

.

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CHART

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Figure 53 AGC Action Chart

~~

.

.

TWO *;:. , ..., :I .:,. .i a;..

,,. :.

9

CROSSING

SIGNALS .! 1::

WITH

VARIABLE

FREQUENCY

SWEEP RATES

SCALE

-

'I

::

I:. ..

C.P.S.

Figure 54 Two Crossing Signals with Variable Sweep Rates

........

i

.........

.-.. . . . . . . . . . . . . . . .

';

...................

60 DELTA REV. 172 NORTH -CENTER - S O U T H ANTENNAS NORTH- SOUTH PASS

-

DOPPLER

FREQUENCY

-

C.P.S

Figure 55 60 Delta Rev. 172

7

DOPPLER FREQUENCY

-

C.P.S.

Figure 56 Delta Rev. 165

- ..

. . .. .- .60 DELTA R E 124 ~ NORTH - C E N T E R - S O U T H ANTENNAS NORTH SOUTH PASS

..

.d

-

.

. .

.

DOPPLER. -FREQUENCY

- C.P.S.. ..

Figure 57 60 Delta Rev. 124

copies of s a t e l l i t e pass data from magnetic tapes recorded on the interim DOPLOC system a t Forrest City, Arkansas. a l s o available of the performance of the ALO ment on these passes.

-

Chart records a r e

tracking f i l t e r equip-

The arrows on t h e records indicate t h e frequency

l i m i t s of the tracking f i l t e r records.

In general t h e comb filter

tracked the s i g n a l f o r a longer period than did t h e tracking f i l t e r .

This was expected since the b e s t s e n s i t i v i t y of t h e ALO was 21 db.

It

could not therefore pick up t h e s i g n a l as soon a s t h e comb f i l t e r .

After a lock was obtained the tracking f i l t e r was independent of t h e ALO and i t s s e n s i t i v i t y was i d e n t i c a l t o t h a t of t h e comb f i l t e r .

The t e s t

records of simulated and a c t u a l s a t e l l i t e signals a r e indicative of a high l e v e l of performance. requirements.

The comb f i l t e r prototype met a l l of t h e design

The u n i t i s f u l l y capable of being expanded t o the f u l l

complement of 1200 f i l t e r s and of providing an e f f e c t i v e tracking capab i l i t y f o r t h e proposed DOPLOC scanning system.

It could provide an

excellent means of tracking any short duration signals having a high r a t e of frequency change.

The frequericy accuracy i s limited t o

the 10-cps bandwidth f i l t e r elements.

-+ 2

cps with

VII. SATELLITE ORBIT DFLTERMINATIONFROM DOPLOC DATA

R. B. Patton, Jr. A. Introduction Computing methods have been developed which provide the DOPLOC system with the capability of detecting and identifying non-radiating satel4itee.

While these methods have been derived specifically for Doppler-type tracking syst,ems, the computing procedures are sufficiently flexible that, with minor modification, they may be applied to observations from any satellite or ICBM tracking system. Moreover, they provide relatively accurate orbital deterininations when the tracking observations are limited to a few minutes of data recorded at a single receiver in the course of a single pass of a satellite.

This capability is fundamental for a detection system. In addition, the computing program has been extended to provide a highly accurate solution when the input data are increased to include observations from more than a single pass of the satellite or from more than one receiver.

A prime requirement for a satellite detection system is the capability of establishing an orbit within minutes after the first:pass over the tracking system. The computing program presentedhere provides the DOPLOC systep with the capacity for orbit determination within one to five minutes after satellite passage. Further,.theae methods function effectively with as few as 6 to 12 observations recorded at a single receiver over a 2 to 3 minute interval. It is doubtful whether any other satellite detection systemcan provide equivalent accuracy for first pass detection. Moreover, many have no capability whatever for orbital determination from single-pass observations.

Computing programs for multiple-pass solutions have been under consideration for sometime. Although the development of these methods has not been completed, a relatively simple procedure is emerging which provides sufficient accuracy for positive identification. In addition, these results may be used to predict position with enough precision to be used as an aid in future acquisition. Although this work is not complete, the method

appears to be both practical and useful. Moreover, the initial results from solutions with actual field data are very promising and ha&, therefore, been included in the section, "Computational Results".

In practice, orbital determination from DOPLOC observations is accomplished in three phases: 1. Initial approximations for position and velocity are established from frequency and slope measurements near the inflection point of the Doppler frequency time curve for each set of observations.

These results are used as input to obtain single-pass solutions for individual sets of observations. Such computations yield moderately accurate Keplerian orbital parameters to be used for initial identification.

2.

3. The single-pass results are used to initiate a multiple-pass solution which simultaneously combines observations from several'passes to compute a perturbed orbit with sufficient accuracy to provide both positive identification and practical prediction.

The first two steps in the computing procedure have been completely developed and thoroughly tested with numerous sets of actual field data.

Development of the multiple-pass solution is nearing completion. Limited, but successful, testing has been accomplished with field observations.

B. Description of Observed Data Doppler observations consist of recordings of Doppler frequency as a function of time. Here the Doppler frequency is defined as the frequency obtained by heterodyning a locally generated signal against the signal received from the satellite followed by a correction for the frequency bias introduced as a result of the difference between the frequency of the local oscillator and that of the signal source. The Doppler frequency is defined to be negative when the satellite is approaching the receiving site and positive when it is receding. If the Doppler frequency, as defined, is plotted as a function of time, one obtains curves of the form shown in Figure 58, usually referred to as "St' curves. The asymmetry of the curves is typical for a tracking system with a ground-based transmitter and a

receiver separated by an appreciable distance. Only for a Satellite, whose orbital plane either bisects or is orthogonal to the base line, will the Doppler data produce a symmetrical "s" curve with a reflection system. With a satellite-borne transmitter, the "St' curve is very nearly s~metrical,being modified slightly by the earth's rotation and the refractive effect of the ionosphere. If continuous observations are made and sampled at frequent intervals, such as once per second, Figure 58 (a) illustrates an analog plot of the data available for computer input.

However, with a ground-based transmitter it is necessary to limit the number of observations in order to minimize equipment cost and complexity.

For example, the three beam antenna complex of the interim DOPLOC system provides three sections of the "St'curve as shown in Figure 58 (b). Another possibility is the use of a scanning antenna beam to provide discreet observations at regular intervals as shown in Figure 58 (c). Such data could be obtained by an' antenna with a thin, fan-shaped beam which scanned the sky repetitively.

Any of these sets of data may be used readily as input for the computing procedures. Whenever possible, this input consists of the total cycle count rather than the Doppler frequency, i.e., the area under the curves or arcs of curves presented in Figure 58 (a) and (b). Hence, this method of solution is based, in a sense, upon every available observation rather than upon a selected few of the total observations (which would be the case if the computing input were limited to a representative number of frequency measurements). Experience has shown this to yield a very significant gain with regard to the accuracy and convergent properties of the solution.

C. The Single-Pass Solution The method of solution consists of a curve-fitting procedure in which a compatible set of approximations for the orbital parameters, are improved by successive differential corrections. The latter are obtained from a least-squares treatment of an over-determined system of equations of condition. The imposed limitation of single-pass detection permits several assumptions which considerably simplify the computing procedure. Among these

176

A

'e2 L

0

. . . .

. . .

. . .

. . ..

TIME

(c)

Figure 58 (a) (b) (c)

-Doppler

frequency-time curves.

,

is the assumption that the Earth may be treated dynamically as a sphere while geometrically regarding it as an ellipsoid, in addition, it iu assumed that no serious loss in accuracy will result if drag is neglected as a dynamic force. With these assumptions, it is apparent that the setellite may be regarded as moving in a Keplerian orbit. An additional simplification in the reduction of the tracking data is warranted if the frequency of the system exceeds 100 megacycles; for it then becomes feaslble to neglect both the atmospheric and ionospheric refraction of the transmitted signal.

In formulating the problem mathematically, it is helpful to regard the instrumentation as an interferometer. In this sense, the total number of Doppler cycles observed within any time interval will provide a measure of the change in the sum of the slant ranges from the satellite to the transmitting and receiving sites. If.h is the wavelength of the radiated signal and N the total number of Doppler cycles resulting at the ith i5 receiver from themotion of the satellite between times t and t J 5 J +l it follows that v hNIJ is a measure of the total change in path length i5 from the transmitter to the satellite to the ith receiver between times t and t5+l. Let gi5 be defined as the actual change in path length. It 5 fcll~wsfrcz Pig=e 56 that:

.

where T is the transmitting site, R the location of the ith receiver, i S the position of the satellite at time t5+l, and S the position 5+l J of the satellite at time t In the event that the observations consist 3' only of discrete measurements of frequency, the same definitions will will be limited to one second. apply, but the time interval from t to t 'I 'I+l The solution consists of improving a set of position and velocity components which have been approximated for a specific time. The latter will be defined as to, and as a matter of convenience, it is generally taken as a time near the inflection point of the "St' curve. Although the location of.the coordinate system in which the position and velocity vectors are

defined is relatively immaterial from the standpoint of convergence, a eyetem fixed with reepect to the Earth's surface doee provide two dietinct advantagee. Firet, the function g ie eomewhat simplified iJ since the coordinates of the instrumentation sitee remain fixed with respect to time. Of greater significance, however, is the fact that the method may be altered to accept other types of measurements for input by merely changing the definition of the finction g and correspondingly, ij the expressions for its derivatives, with no additional modification to the balance of the procedure which in fact, constitutes the major portion of the computing process.

The set of initial approximations for the position and velocity iO). The components are defined for time to as (x0, yo, z0, Go, reference frame is the xyz-coordinate'systemwhich is defined in the following section. If second and higher order terms are omitted from the Taylor expansion about the point (x0, yo, z , , , io), the equations of condition may be mitten in matrix form,

iO,

where

for all values of i and j. Hence, J is a matrix of order (i. jx6), AV a

179

matrix of order (i*jxl), and

m

a matrix of order (6x1).

Since there are

six unknowns, a minimum of six equations are required for a solution. In practice, sufficient data are available to provide an over-determined system, thus permitting the least squares solution,

m

=

(J* J)-~J*AV,

(4)

where J* is the transpose of the Jacobian J. Finally, improved values for the initial conditions are obtained from

x

+

m,.

where

As a matter of convenience, no subscripts were introduced to indicate iteration; but at this point, the improved values of X are used for the initial point and the process is iterated until convergence is achieved.

Since the function g cannot be expressed readily in terms of X ij directly, the evaluation is obtained implicitly. An ephermeris is computed for the assumed values of the position and velocity vectors at time to.

Then using equation (1) may be evaluated for all values of i and j. gij In the process of this evaluation, it is expedient to use two rectangular coordinate'systems in addition to the Earth-bound system whose origin is at the transmitting site. Referring to Figwe 60 these coordinate systems are defined as follows.

Figure 59 -

Problem geometry.

181

COORDINATE SYSTEMS

Figure 60

-

Coordinate systems.

1. The xyz-coordinate system is a right-hand rectangular system

with the origin on the Earth's surface at the transmitter. The x axis i~ positive south, the y axis is positive east and the z axis is normal to the Earth's surface at the transmitting site.

2. The x'ylz'-coordinate system is a right-hand rectangular

system with the origin at the Earth's center. The x t axis lies in the plane of the equator and is positive I n the direction of the vernal equinox, while the positive a' axis passes through the north pole. The y 1 axis is chosen 60 as to cmplete a right-hand system.

3. The x"y"z"-coordinate system is likewise a right-hand rectangular system with the origin at the Earth's center. The xl'y" plane lies in the orbital plane of the satellite, with the positive x" axis in the direction of perigee. The positive z" axis forms with the positive z' axis an angle equal to the inclination of the orbital plane to the plane of the equator.

The y" axis is chosen so as to complete a right-hand system.

The first step in this phase of the computation consiets of computing values for the initial position and velocity components in the x'y'et-coordinate system. This may be achieved by one rotation and one translation which are indepenhent qf time, and a second rotation which varies with time. Iat the notation ~ ~ ( c r indicate ) the matrix performing a rotation through an angle a about the ith axis of the frame of reference such that the angle ie positive when the rotation is in the right-hand direction and i is equal to 1, 2 or 3, according to whether the rotation is about the x, y or z axis respectively. The desired transformation follows,

where

63 ( - 00 ).

t h e time d e r i v a t i v e of R ( - Q j ) when Q

= O0'

t h e r i g h t ascension of the transmitting s i t e a t time t

0

-=

fl

r t h e geodetic l a t i t u d e of t h e transmitting s i t e ,

p#

z

J

t h e r a d i u s vector from t h e E a r t h ' s center t o a p o i n t on t h e E a r t h ' s sea l e v e l surface a t t h e l a t i t u d e

A

=

J'

6,

t h e difference between t h e geodetic and geocentric l a t i t u d e s a t the transmitting s i t e .

.me next

s t e p i n t h e computation involves the evaluation of the follow. .

ing o r b i t a l parameters: a

semi-major a x i s ,

e

eccentricity,

o

E

mean anomaly a t epoch, inclination,

i

R

E

r i g h t ascension of t h e ascending node,

o

2

argument of perigee.

The evaluation of these o r b i t a l parameters i s obtained from t h e following

n

equations :

*

Derived by D r . B. Garfinkel, B a l l i s t i c Research Laboratories, Aberdeen Proxring Ground.

184

where g is the mean gravitational constant and R is the radius of the Earth, which is assumed to be spherical in the development of the equations,

where

o

= Eo

- e sin Eo - nto ,

hl = y l i l 0 0

% =

Ztkr 0 0

-

X~$,r

00'

1

-

-x

= =

-53 , w h e r e O T i zn, a(cos Eo - e ) , cos

where

1

P

[k; + S(x;k2

~ g n

2

=

0 =

where 9

-

=

(-1)9 cos-1 N1

9

- Y&

Having obtained values f o r a, e, a, i, evaluated f o r each time of observation.

n,

and w, g i j may be

The f i r s t s t e p in the computing

procedure consists of solving f o r the eccentric anomaly E equation,

i n Kepler's

.

I

The position of the s a t e l l i t e as a function of time i s determined i n the x"y"z"-coordinate

r~

= a(1

-

system.

-e

cos E

y" = r s i n f j J .

I

.

z" i s zero according t o the d e f i n i t i o n of the xny"z"-coordinate system.

.

I

A transformation t o the x'y'z'

coordinates can be achieved by three

r o t a t i o n s as follows:

,'!:I

Finally the position i n the xyz-coordinate system may be obtained by two additional r o t a t i o n s and a t r a n s l a t i o n . sin A

p

[

~

]

-

[

~

~

-o

s (381 ~

Referrt o (11, gij may be then exyresaed i n tern of the s a t e l l i t e ' s positions a t time t and t

J

9+1

.

where (xi, yi, zi) i s the surveyed position of the i t h receiver.

the residuals Av

ij

Finally,

may be determined from

Having evaluated the vector AV, there remains the problem of Cie?e.mlni~g+he : T R c o ~ ~ R.Tc ~

necessary differentlation may be carried out numerically, but the computing time w i l l be reduced an& the accuracy

increased i f the derivatives are evaluated from analytical expressions.

Recalling t h a t f o r all values of I and j,

let

where J1

=(

Bl0'

"

Xj+l' Y j + l '

"

gij,

Zj+l'

... ,y ,zj

=,

The orders of the above matrices are (I*jx6) for J1, and (6 x 6) for J2, J and J,+. A distinct advantage of this method of evaluating J results 3

from the fact that the function g appears only in J1. Hence, if the iJ solution is applied to other types of measurements, only J1 needs revision lor the appropriate evaluation of J. This is trivial compared to the effort The expressions required to derive the derivatives contained in J2 and J 3' for many of the elements of these Jacobian matrices are rather long and involved, and therefore will not be presented here, but the complete results are reported elsewhere. 5

D. Initial Approximations for the Single-Pass Solution Convergence of the single-pass computation rests primarily upon the adequacy of the initial approximations for position and velocity. It has been established that, for a system consisting of a single receiver and an earth-bow& transmitter at opposite ends of a 400 mile base line, convergence is assured when the error in each coordinate of the initial estimate is not in excess of 50 to 75 miles and the velocity components are correct to within 112 to 1 mile per second. However, if single pass measurements are available from two or more receivers, the system geometry is greatly strengthened. Cocvergence can then be expected when the initial approxilnations are within 150 to 200 miles of the correct value in each coordinate and 1 to 2 miles per sxond in each velocity component. Larger errors may occasionally be tolerated, but the figures presented are intended to specify limits within which convergence may be reasonably assured.

Therefore, it hes 'ken rlecessary to develop a supporting computation to provide relatively accurate initial approximations to position and velocity for the primary computation. Several successful methods have been developed for this phase of the problem; but discussion will be confined to a few applications of a differential equation which approximately relates the motion of the satellite to the tracking observations. The following definitions will be useful in the derivation of this equation..

4, $1

Ex;,

(9, .jrl 0 ) j

3

PA

_P

tllft psitien of ths txaumaitting site.

=

the pasitixm d' the atellib3 a t t h e tj.

2

the astan~l, fzm the +zammitting site t o the satellite at the tj.

; the

distance

Pnap

the i t h receiver t o

* satellite

a t th? t

J*

H

E

the Pltituda

v

=

the valocitg of the Satellite.

t b s a t e l l i t e &we the Earth's

To smliip the problem, certain a@flsaptionahave been made:

1. the satellite mom Fn a c h u l a r Keplerian orbit, 2. $ i s ~ l a t i p e l yaaall throughout the period of obsemtion, 3. the Earth i s not rotating.

A number of useful relationships BBJ be derived as a result of these

aas~tions.

where H la constant.

The reiarenca fmw for thLe derivation ia the x"y"a"-aaxaiwte -tea

which has been defined

ma.It fallowe frat the

definition of pj that

Miferentisting twice with respect t o t h e yields

be almpllfied t o

which

4

.

Z -R-i P

It followa that

A expression may be derived for p Recalling the d e f h i t i o n i J' for gij, M C M C ~ Uthat ~ ~

-

A

.\r B,, ,

where

- PJ. 2 P~

!

A

- Pi,2 "LJ

,

k t , us define a r i g h t - W d r e c ~ a coordinate c system, as

n h m L~IFigure 61, with the origin a t the transmitter and the z-axis positive i n the d i r e c t i o n of the v e r t i c a l . The y-axis i s formed by

the interoaction of the tangent p l m e &tthe theranamitter with the plane detem.ber! by the transmitter, the i t h receiver, and the Fkuth's center.

If the me yeceiver w i l l then be a t the known point (0, yi, zi). variable p o b t (x , y , z ) indicates the poeition of the satellite, the

J

J

J

almt ranges f r a the t r a n d t t e r and the i t h receiver are respectively given by

Prom which it follows that

I n the three-beam mode of operation, the s a t e l l i t e w i l l be approximately i n the YE-plane a t to, which i s aefFned as t h e time halfway between the i n i t i a t i o n and termination of tracklng i n the center beam. Let the s a t e l l i t e ' s position and velocity a t this time be defined as (xo, Yo, zo) and (ko, $o, i 0 ) , respectively. Obviously, xo may be approximated by zero and we may s a f e l y assume that the v e r t i c a l component of velocity i s small and can well be approximated by zero. The last p a i r of equations then reduce t o

TO

EARTH'S CENTER

GEOMETRY FOR OETERMlNlNG THE INITIAL APPROXIMATIONS

Figure 61 193

iiobe

the Doppler frequency and rate of change of frequency fnr the i t h receiver a t to. I t follows t h a t

Let fio and

From equation (481 wc conclude

Fxpressing equations (55) and a i d mlociei caqcntat:: of the

(56)i n terms of the position coordinates =~t;-plli(P at

+

yo

(yo

J(y,

- yiJr

time, to, yields

- yi) + (z o

Let ue assume a speclfic o r b i t a l Inclination.

ii

- z112

With our previous

assmption of circular motion, may readily be cmputed as a function of yo and zo. Then equations ( 5 7 ) and (58) w i l l lFlrewise provide fio and

iioa~

functions of position i n the yz-plane.

Thus, f o r a given

inclination, families of curves may be computed and plotted i n the yz-plane f o r both f,, and i,,,. Figure 62 presents such a plot, f o r an inclination A"

J."

of 80째, with the transmitter and receiver separated by 434 miles and with both located 35' off the equator.

To a t t a i n symmetry and s ~ l l f the y

construction of such charts, zi was assumed t o be zero, which is a reasonable approximation f o r this approach t o the problem. If similar charts are prepared for a number of inclinations, satisfactory i n i t i a l appraximations may be rather quickly and e a s i l y obtained by the following operations:

1. Assume an inclination. This, of course, is equivalent to selecting a chart. Accuracy is not essential at this atage since the estimate may be in error by 15' or more without preventing convergence.

2. Enter the chart with the observed values of fio and

Pie to determine

an appropriate position within the yz-plane.

3. Approximate the velocity components. These should be consistent with the assumption of circular motion, the height determined in step 2, and the assumed inclination.

4.

Determine the position and velocity components in the coordinate system for the primary solutionby an appropriate coordinate transformation.

In addition to the graphical method, a digital solution has been devised for eqs. (57)and (58). Aa in the previous development, we have two m e a s w ments available and desire to determine three unknowns. In this approach, one unlrnom is determined by establishing an upper bound and assuming a value which is a fixed distance from this bound. The distance has been selected to place the variable between its upper and lower bounds in a position which is favorable for convergence of the primary computation. In this method, we chose to start by approximating eo. It may be obeerved in Fig. 62 that, for larger values of fio, the maximum value of zo occurs above either the transmitter or receiver while, for smaller values of fio, the maximum value of zo occurs over the mid-point of the base line. The first step in the is computation is to determine a maximum value for zo. To this end, to yield an expression which varies only eliminated from eqs. (57) and (9) in yo and zo. A appears in this expression, but it is also a function of these variables. The resulting equation may be solved by numerical methods for so with yo = 0 and then, solved a second time for zo with yo = (1/2)yj.

The larger of these results is to be used as a value for ( z ~ which ) ~ is defined to be the maximum possible value of zo. Assuming the altitudes of all satellites to be in excess of 75 miles, we may conclude, from the general characteristics of the family of curves for ? in Fig. 62, that the 10

io

TRANSMITTER

RECEIVER

Y - A X I S (EAST

IN MILES)

DOPLOC FREQUENCY AND RATE OF CHANGE OF FREQUENCY AS A FUNCTION OF POSITION IN THE Y Z PLANE (FOR 80' INCLINATION

-

Figure 62 196

satellite's altitude will differ from ( z ~by ) no ~ m r e than 100 miles.

Since an error of 50 miles may be tolerated in the approximetion for each coordinate, EzO)M 5 q is a suitable value for zo. With the altitude thus determined, we may solve eqs. (57) and (58) for yo and to. In the process, A and hence the velocity, will be determined.

With i0 assumed as zero, ko may be readily evaluated to complete the initial approximations which consist of the position (0,yo, zo) and the velocity (io,io, 0). It is worth noting that there are a pair of solutions for yo and $o. Further, the method does not determine the sign

-

of Po. If, in addition, we accept the possibility of negative altitudes for the mathematical model at least, we arrive at eight possible sets of initial conditions which are approximately symmetrical with respect to the base line and its vertical bisector. It is an interesting fact that all eight, when used as input for the primary computation, lead to convergent solutions which exhibit the same type of symmetry as the approximations themselves., Of course, it is trivial to eliminate the four false solutions which place the orbit underground. Further, two additional solutions may be eliminated by noting that the order in which the satellite passes through the three antenna beams determines the sign of Go. In the two remaining possibilities, io is observed to have opposite signs. Since the y-axie of the DOPLCC system has been oriented *om west to east, the final ambiguity may be resolved by assuming an eastward component of velocity for the satellite certainly a valid assumption to date. In any event, all ambiguity may be removed from the solution by the addition of one other receiver. Moreover, this would significantly improve the geometry of the system and thereby strengthen the solution.

-

These two methods, for obtaining initial approximations, h a w been developed for a system which provides observations of the type displayed in Fig.S%(b). If (very')minor modifications are made in the procedures, both methods may readily be applied to data of the type presented in Fig. 5 8 ( d ) .

Indeed, with any tracking system that provides observations of satellite

w?lr?clty components, eq. (48) furnishes an adequate base for establishing an approximate orbit to serve as an initial solution which m y be refined hy more sophisticated methods.

E.

Multiple-Pass Solution

Single-pass orbit determination is unquestionably a primary requirement for any satellite detection system. Of no less importance is the capability of the system to generate suff'i~ient accuracy in the computed orbit to permit pooitive identification and predictions which are adequate for later acquisition. Imp~ovedaccuracy is most readily obtained by increasing the number of observations and lengthening the time interval over which data are caliected. Both of these objectives may be realized by simultaneously using data fram several passes in an orbital determination. Further improvement will result if the system may be expanded to include additional receivers.

The single-pass solution has already been programed to accommodate several receivers; but it is inadequate for a solution with input data from more than one pass. Therefore, a separate solution for dealing specifically with t M s phase of the problem has been under development for sometime.

The derivation of the computing procedure is essentially complete; and in fact, a few encouraging results have already been obtai.ned. However, the method still requires the introduction of a few minor refinements as well as extensive testing to verify its compatibility with a wide range of input data.

The multiple-pass solution was designed to allow a major portion of the computing procedure for the single-pass method to be used in the actual computation. It is assumed that the satellite moves in an unperturbed, or Keplerian, orbit for those short intervals of time that it is under observation by the DOPLOC System. Otherwise, the satellite is considered to move in a perturbed orbit. The latter is determined by thirteen parsmeters from which six time-varying Keplerian orbital parameters may be readily derived. For a given period of observation, the Keplerian parameters are determined for to, a fixed time within the tracking interval. These

parameters a r e then assumed t o determine the o r b i t f o r t h e few minutes

that t h e satellite remains w i t h i n the volume c k e g e of the DOPLOC System.

Thus, if data are available f o r Q revolutions, Q s e t e of Keplerian o r b i t a l parsrpeters, each constent within a given i n t e r v a l of observation, w i l l be expmesed i n terms of the t h i r t e e n unknowns. The l a t t e r J I I ~ Ythen be adjusted by a s e r i e s of d i f f e r e n t i a l corrections t o obtain a least-squares f i t t o all observations f o r two o r more revolutions. No attempt has been made t o determine p a r t i c u l a r perturbing elements euch a s drag. Rather, it has been assumed t h a t an empirical determination of the t h i r t e e n unknowns w i l l adequately r e f l e c t the t o t a l e f f e c t of a l l perturbations upon the o r b i t .

To conserve computing time, it i s considered desirable t o use input that consists of t h e integrated Doppler frequencies over time i n t e r v a l s of several seconds as recamended f o r t h e single-pass solution.

Let the subscripts i a n d j be defined a s before.

A t h i r d subscript,

q, w i l l be introduced t o sp&clfy t h e s a t e l l i t e revolution number. The perturbed o r b i t a l a r e expressed i n terms of ah where h ranges from one through t h i r t e e n . F o r t h e q t h revolution, we define t h e unperturbed parameters a s follows:

where 90

toq

5 -

the time f o r which the unperturbed parameters a r e computed f o r the qth revolution,

AYq :Tq ,

:

y9

,

:the minimum value of q

-

y90

'

the angular distance along the o r b i t from the equator t o t h e satellite a t time t

oq

.

199

From eq. (62), we conclude that

The set of unperturbed paraetere for qth revolution is completed with the camputation of a from the equations 9

7 may be evaluated with sufficient accuracy from the single-pass solution 4 far positian at the the, t 1 0 oq, e a m d a constitute a set

,

,

q' q 4 9 'a of unperturbed parameters for each revolution q for which tracking observations are available. U t h e r , they are functionally related to %. Hence, the observed data may be axpressed as a function of the set of parametera aq'

5,.

As in the single-pass method, the solution is obtained by using leastsquares methods to compute a series of differential corrections to initial approximations for the set of parameters, 4. For each value of q, eqs.

(32) through (40)may be used as before to compute the residuals, aVijq.

The least-squares solution is obtained from the matrix equation

where AV and Crr are column matrices whose element8 are respectively the residuals and the differential corrections for the set of unknowns ~ 6 .

J is a Jacobian defined as follows:

-

J where

Jq

=

,&) ,

=pi2, ...... ..,..

gi

!q.,.

.. . .5 .3 .

(68)

is similar to Chat for g of the single-pass The definition for g iJs ij solution. Let

where

It will be noted that Jlq and J have definitions which are similar to 2q J1 and J2 of the single-pass solution. Hence, the analytical expressions for the elements of these matrices are identical to those for J1 and J2 The elements of Ja have not which have been reported upon previously? been published but are easily derived, and therefore, will not-be presented.

To summarize, initial appraximations for q, are derived from the aingle-paas solutions for each value of q. We assume the perturbed parameters a eq, oq, co , 0 and i can be adequately expressed in terms of 9' 9 9 % by eq. (59) t h r o w 763). Further, it is assumed that they remain conetant during each short period of observation, but otherwise vary in accordence with this same set of equations. Hence, we have sets of Keplerian parameters for each period of observation. They are used to compute the elements of the AV matrix for the least-squares solution to determine appropriate differential corrections for the estimated values of ah. The process is iterated until convergence is achieved. The resulting set of provide perturbed orbital parameters ae a function of time and revolution number. The results may also be used to predict the time and position of future passes of the satellite as an aid in later acquisition.

F. -

Computational Results Numerous convergent s o l u t i o n s have ljgen obtained with b o t h simulated

and a c t u a l f i e l d d a t a oerving a s computer input.

Since t h e l a t t e r a r e of

major i n t e r e s t , -the discussion w i l l be r e s t r i c t e d t o r e s u l t s obtained from r s a l data.

The p r e s e n t results were obtained from observations which were

recarded during a period when t h e WSIa receiver was inoperable and, theref o r e , a r e derived from d a t a recorded by a s i n g l e r e c e i v e r .

Included with

the DOPLOC results m e o r b i t a l parameters which were determined and pubAs

l i s h e d $;y t h e National Space Surveillance Control Center (Space Track).

an a i d 4.n camparing t h e two s e t s of determinations, t h e Space Track parame t e r s have been converted t o t h e epoch times of *he DOPLOC reductions.

The i n i t i a l s ~ i c c e s s f d lreduction f o r t h e F o r t S i l l , F o r r e s t C i t y system

9937 of Sputnik 111. The DOPLE observations, ao well a s t h e r e s u l t s , a r e presented i n Fig. 63. hkasurements were recorded f o r 28 seconds i n the south antenna beam, 7 seconds i n t h e c e n t e r beam, end 12 seconds i n the north beam with two gaps i n t h e d a t a of 75 was achieved for Revolution

seconds each.

Thus, observations were recorded f o r a t o t a l of 47 seconds

within a time i n t e r v a l of 3 minutes and 17 seconds.

Using t h e graphical

method described i n t h e previous s e c t i o n t o obtain i n i t i a l approximations, convergence was achieved i n t h r e e i t e r a t i o n s , on t h e f i r s t pass through t h e computing machine.

I n comparing t h e DOPLCC and Space Track s o l u t i o n s , it

w i l l be noted t h a t t h e r e i s reasonably good agreement i n t h e values f o r a , e , I , and 0, p a r t i c u l a r l y f o r t h e l a t t e r two.

This i s c h a r a c t e r i s t i c of

t h e single-pass s o l u t i o n when t h e e c c e n t r i c i t y i s small and t h e computat i o n a l input i s l i m i t e d t o Doppler frequency. c i r c u l a r , a and

u,

Since t h e o r b i t i s almost

a r e l e s s s i g n i f i c a n t than t h e other parameters and l i k e -

wise, a r e more d i f f i c u l t f o r e i t h e r system t o determine a c c u r a t e l y . ever, a s a r e s u l t of t h e small e c c e n t r i c i t y , t h e sum of

u,

How

-

and o i s a r a t h e r

good approximation t o t h e angblar distance along t h e o r b i t from t h e nodal p o i n t t o t h e p o s i t i o n of t h e s a t e l l i t e a t epoch time and a s such, provides

a b a s i s of comparison between t h e two systems. (u,

+ a)

= 32.66'

I n t h e DOPLCC s o l u t i o n ,

while t h e Space Track determination y i e l d s a value of

35.73

0

,

I I

-

--

--tRBO

- . -

-

0..

.

B

DOPLOC

4149mi 0.0153 ~ E - . T R M ; ~ - - 41 ~ ~1 I m i 0.0130 DIFFERENCE .

~

65.37' 65.W0

381111 0.0023

~~

n

~

~

~

- .

-r

178.24O 104.62' 178.22O-- . t37.9S0

0.31째

0.02.

I

I I

I

;..(w+-+7L

I

1

I

~

--

~

.-

77Wb

.I

I

1

~

I

.

I 1 I j

w

3

7200 K

Y

6700

ny

I

A

I

a

I I

a

g

6200

28

I

~

~ I -

I

I

!

;I

..

1

~. .

.

. - ~ - ~

I

II

I I

I

II

I

I

I

I I

I

I I I

I

I

~

I

I I I

1

' 12J75-7-

75

I

I I

:' CSKE!? &??TEHY& l SLOPE 89 CPS/S

-~~

Z

IV

~

..

f

1 -3.07.

I

I

0 0

132.66.

- 35-7SC-

f

I

1

30.26.

. ..

~

1

288.04' 1 257778'--I----

-33.33.

.

~

I

- (I

. . ~ ~ ~ ~ ~ . ~~

ltym

I

- 0 0 .

IS)

I Ni$7AN+Nru Jurrr

ARPA-BRL DOPLOC DOPPLER RECORD OF 58 DELTA REV CITY. ARKANSAS - - p - . - -9937 - . - - p - . - - . . .FORREST -.

~

-?.

--

I

I

5700

I

MEASURED 0643:OO Z. PREDICTED 0 6 4 2 Z AiTiTUDE is6 Y i i E S . 372 Y i i E S EAST Fi. S i i i SOUTH -CENTER NORTH ANTENNAS. S N PASS BFO 1 0 8 , 0 0 7 , 0 0 0 CPS TRANSMITTER 1 0 8 . 0 0 0 , 0 0 0 CPS

-

-

-

i (.+ @.+---

3200.- . ... . ..

-

.

-. .

/" SOUTH ANTENNA - . .. . .. :54 0 6 4 2 : ~:I4 ~~

I

.

sLom 23 CPSE

1

24

:Y

:44

~~

:24

34

.

~~. .

:44 Figure 63

Z.TIME ...-. ..:s4.

31 MAR. osii:m 60 :,,

-

.. ..

-.

- .

....

-

- ..24

:34

. .

~

144

.

~

-

:S40644:W:14

-

.

~

.

:e4

. .

-

:H

.

. :44

a differeuce of 3.07' between the two sets of results. To summarize, when lFmitqd to single-pass, single-receiver observations, the DOPLOC system provides an excellent determination of the orientation of the orbital plane, a good determination of the shape of the orbit, and a fair-to-poor determination of the orientation of tt-e ellipse within the orbital plane.

Occasionally, exceilent results have been obtained for both a and u; but in general, the interim DOPLOC system with its present limitations fails to provide consistently good single-pass evaluations of these two quantities.

Therefore, in presenting the remaining single-pass DOPLOC reductions, a and o, have been eliminated from further consideration. Results have been indicated in Table I for six revolutions of Dihcoverer XI, including number 172 which was the last known revolution of this satellite. As a matter of interest, the position detenined by the interim DOPLOC system for this pass indicated an altitude of 82 miles as the satellite crossed ihe base line 55 miles west of Forrest City. To provide a basis for evaluation of the DOPLOC results, orbital parameters, obteined bj converting Space Wac& determinations to the appropriate epoch times, haye been included in the table. Table I also contains a listing of the amount of data available for each reduction in addition to the total time interval within which the observations were collected. The observations recorded for the Fort Sill, Forrest City complex are plotted in Fig. 64 for the six revolutions of Discoverer XI which are surmnarized in Table I. In Fig. 65, results are tabulated for a reduction based on only seven frequency observations from a single pass over the interh DOPLM: system. These have been extracted from the complete set of observations previously presented for Sputnik 111. They were selected to serve as a crude example of the type of reduction required for the proposed DOPLOC scanning-beam system. The example shows that the method is quite feasible for use with periodic, discrete measurements of frequency. Of course, the proposed system would normally yield several more observations than were available in the example.

There has not been sufficient time thus far to completely evaluate the computing methods for the multiple-pass solution. However, one set of reeults has been obtained for observations from revolutions 124, 140 and

TIME

- SECONDS

Figure fA

1

+

.

DOPPLER FREQUENCY EXTRACTED FROM ARPA- B R L DOPLOC RECORD FOR 1958 DELTA REV. NO. 9937 TRANSMITTER AT FORT SILL RECEIVER AT FORREST CITY

A.

+

WPLOC SPACE TRACK DIFFERENCE

a

0

I

0

4105 ml.

4 I I l ml.

-6 mi.

0.0221 0.0158 0.009 1

65.36.

65.06O 0.30.

l70.QS0 178.2Z0 -0.13O

+

+

7

100 SEC.

TINE

t

- SECONDS

Figure 65

Comparison of Single-pass DCPLOC snd Space Ilack Results for Discoverer XI Revolution .Number DOPL(X: Space Track Difference

Difference DOPLOC Space R g c k Dlfference Daaoc Space aaclr Dlfference DOPLOC Space Track MfYerence

DOPm Space Track

'6--.

Dlfference

a

(miles)

e

1 (dew=-)

n (aegrees)

Total Amount of Data (seconds)

Interpal of (Ibeervation ( e c m )

156 of Discover XI. These are presented in Table I1 where, as before,

8

comparison is made with Space Track results for the same re~0l~ti0nS. *era11 agreement is excellent, and in general, the differences between the two determinations are of the same magnitude as the error to be expected in either' set of parameters. Disagreement is excessiye only in a, the mean anomaly at epoch. For this particular parameter, the major portion of the error must be in the Space Track results, for the DOPLOC reduction is designed to force the satellite's calculated position to lie in the narrow east-west vertical beam of the tracking system at the actual tima of ~bservation. The resulting error in the DOPLOC determination of satellite passage is certainly less than one second. Hence, the angular error in position is relatively insignificant.

As an indication of the feasibility of using the multiple-pass solution for prediction, the DOPLOC results in Table I1 were used to compute the times of satellite passage through the mid-point of the vertical beam for revolutions 165 and 172. These results were compared to the observed thes. The predictions were found t.0 be two minutes early for 165 and seven minutes early for 172. Since the latter was the final revolution for this satellite, these predictions covered a period in which drag was increasing rapidly and extrapolation was rather hazardous. These results are considered reasonable for this portion of the orbit and would certainly have been adequate as an aid in acquisition.

G. Conclusion These methods of solution have been shown to be both practical and useful by numerous successful applications with real as well as simulated data. Although results have been presented for passive data only, the computing procedure has been altered slightly and applied to active data with considerable success. Computing times are reasonable since convergent solutions have required from 20 to 40 minutes on hhe ORDVAC with the coding in floating decimal, whereas more modern machines would perform the same computation in a fraction of a minute. These methods allow the determination-'of a relatively accurate set of orbital paremeters with as little %..

Camparison of Space 'R9ck and Multiple-pass DOPLOC Results for Discoverer X I Revolution Number DOPLOC Space Track

Difference Space rack 0

\O

Difference

DOPLOC

-

Space hack Difference

a (miles)

e

i (degrees)

R (degrees)

a

( d e p e s)

0)

(degrees)

/

as 1.5 to 3 minutes of intermittent observations from a single receiver when the signal source is a ground-based transmitter. While multi-receiver systems provide numerous distinct advantages, it has been shown that it is possible to obtain, routinely, quite satisfactory results with observations from a single receiver.

However, it should be noted that a system with two

receivers will reduce error propagation in the computations to approximately one tenth of that to be expected from a single receiver system. &tension

of the computational methods to include multiple-pass

solutions results in a well-rounded and complete computing program for satellite detection and: identification. Not only are the orbital elements much Setter determined, but the increased accuracy is sufficient to permit adequate prediction for future acquisition.

It should be emphasized that solutions have been obtained for actual field data, and further that such solutions were completely independent of other measuring systems. Results from the latter were used only for comparison and did not enter any phase of the computations leading to these solutions.

In conclusion, the methods are quite general and need not be confined to either ~opplermeasurements or Keplerian orbits. It is merely necessary to modify J1, J , g.. and g.. to allow these methods to be applied to 1J IJq observations from any satellite or ICBM tracking system.

VIII.

POTENTIAL OF DOPLOC SYSTEM IN ANTI-SATELLITE DEFENSE

If it can be assumed that within a reasonable period of time the population of earth bound artificial satellites will reach the predicted number of 10,000, it csn be assumed with equal certainty that any adequate system of satellite detection, tracking and orbit cataloguing equipment will utilize the Doppler principle and at least some Qf the DOPLOC techniques. DOPLCC was certainly the first and probably is still the only system to have demonstrated its ability to consistently compute orbits within minutes after first detection by a single station from single pass data. The performance capability of the DOPLCC system is equal to that of any other known system. Continuous wave bi-static Doppler Radar and narrow band reception techniques combine to provide a uniquely economical utilization of the physically limited power and receiver sensitivity. The DOPLOC system has its greatest potential as a surveillance system for cataloguing and identifying satellites. Its application as an anti-satellite or ICBM defense system is limited in its present form. Either as a surveillance system by itself, or as a member of a family of sensors, it offers a low cost dependable method of detecting and cataloguing satellites over an extremely large volume of space. With modifications to the scan configuration the system may have application to the ICBM and the antisatellite defense problem. An early warning feature would need to be incorporated since DOPLOC is presently effective primarily after one pass over the sensor.

The author wishes t o ac1;nowledge t h a t most of t h e work done under t h e DOPLCC p r o j e c t (ARPA Order8:-58) was d i r e c t e d and supervised by Mr. L. G. de Bey, r e c e n t l y deceased.

A s indicate& by t h e t i t l e , t h i s r e p o r t i s

intended t o be a t e c h n i c a i summary covering t h e e n t i r e project.

The material

p~eeen$adwas therefore compiled, from various r e p o r t s and eourcee and repres e n t s the work of most of t h e technical s t a f f of the Electronic,kasurements Branch of t h e B a l l i s t i c MEasureroent Laboratory over t h e period July 1958 30 June 1960.

-

Section V I I , " S a t e l l i t e Orbit Determination from DOPLOC Data"

was authored e n t i r e l y by M r . R. B. Patton, Jr.

Others who made major contri-

butions t o the program were V. W. Richard, C. L. Adams, K. A. Richer, K.

P. P l l e n , J r . , R .

K. Patterson.

E. A . Pntnum,

Vitekj FI. Lnotenn, C : ShaPP-r and

Material extracted from the work of contractors i s referenced

i n the t e x t .

A.

H. HODGE

REFERENCES 1,. Muon, L. A. Variation of Cosmic Radiation with Frequency. Nature, Vol. 158, No. 4021: 758 759, November 1946.

-

2. Cattony, H. V. and Hohler, J. R. Cosmic Radiation in W e V.H.F. Band.

Proc. IRE: 1053 1060, September 1952.

-

3. Gabor, D. Theory of Communications, Part I. Journal of Institute of Electrical Engineers: 429, 1946.

4. Woodward, P. M. Probability and Information Theory with Applications to Radar. Permagon Press, 1955.

5. Patton, R. B., Jr. A Method of Solution for the Determination of Satellite Orbital Parameters from DOPLOC &asurements.

Ground: BRL MR 1237, September 1959.

Aberdeen Proving

1. Patton, R . B., Jr. Orbit Determination from Single Pass Doppler Observations. IRE Trat~ssctionson Military Electronics, Vol. MIL-4, Numbers 2 and 3 : 335 344, A p r i l July, 1960.

-

2.

-

Pnt$onn, R. B., Jr. A Method of Solution f o r t h e Ihtermination of ; S a t e l l i t e Orbital Paraneters from CCPLCC Measurements. Aberdeen Proving ,Gr.omd: BRL MR 1237, BRL DOPLCC S a t e l l i t e Fence Series, Report No. 7 i n t h e S e r i e s p September 1959.

3. Guier, W. H. and Weiffenbach, G.

C . Theoretical Analysis of Doppler Hadio Signals from Earth S a t e l l i t e s . Applied Physics Laboratory, S i l v e r Spring, : Bumblebee Report No. 276, April 1958.

.

4. Richards, Paul E.

Orbit b t e r m i n a t i o n of a Non-transmitting S a t e l l i t e Using Doppler Tracking D a k . Space Sciences Laboratory, General E l e c t r i c Company, Philadelphia, Pa.: BRL DOPLOC S a t e l l i t e Fence Series, Report No. 12 i n the S e r i e s , October 1960.

5.

Besag, P. L. Polystation Doppler System. Western Development Laboratories.,.

Philco Corporation, Palo Alto, C a l i f . : BRL DOPLCC S a t e l l i t e Fence Series, Report No. 11 i n the Series, May 1960.

6.

Davis, R . C. Techniques f o r S t a t i s t i c a l Analysis of Continuous-Wave Doppler Data. U. S. Naval Ordnance Test S t a t i o n , China Lake, C a l i f . : NAVORD Report 1312, April 1951.

7.

Maulton, F. R . An Introduction t o C e l e s t i a l Mechanics.

Macmillan and Company, Ltd., 1902.

8. Richard, V. W .

DOPLCC Tracking F i l t e r .

New York:

Aberdeen Proving Ground: BRL

MR 1133.

9.

10.

Bolgiano, L. P., Jr. Doppler Signals and Antenna Orientation f o r a DOPLOC System. Aberdeen Proving Ground: BRL R 1055, BBL DOPLCC S a t e l l i t e Fence Series, Report No. 1 i n t h e S e r i e s .

Maxon, L. A. Variation of Cosmic Radiation With Frequency.

Vol. 158, No. 4021: 758 759 , November 1946.

-

Nature

11. Cattong, H. V. a n d J o h l e r , J. R. Cosmic Radiation i n the V. H. F. Proc. IRE: 1053 1060, September 1952.

-

12.

Band.

Balgiano, L. P., J r , and Gattschalk, W, M. Quantum Mechanical AnaLyeis of Radio Frequency Radiation. E l e c t r i c a l Engineering Dept., University of Delaware, Newark, Del.: BRL DOPLCC S a t e l l i t e Fence S e r i e s , Report No. 13 i n the S e r i e s , 15 June 1960.

c

BIBLIOGRAPHY ( m n t . 'd)

13. Lootene, H.

D O P E Observations of Reflection Croes Sections o f S a t e l l i t e s . Aberdeen Proving Ground: BRL R 1330, BRL DOPLOC Satellite Fence Series, Report No. 22 i n the Series.

1 4 . Scharfman, W . E., Rothman, B., Guthart, A. and Morita, T. DOPLOC System Studies, Stanford Resemch I n s t i t u t e , Menlo Park, C a l i : ' Final Report P a r t B, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 10 i n the Series.

15. Adams,

C . L. DOPLOC Instrumentation System f o r S a t e l l i t e Tracking.

Aberdeen Proving Ground: BRL R 1123, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 2 1 i n t h e Series.

16. Summary of t h e Preliminary Study of the Applicability of the Ordir System Techniques t o the Tracking of Passive S a t e l l i t e s . School of Engineering, Columbia University: Final Report No. ~1157,BRL DOPLOC S a t e l l i t e Fence Series, Report No. 1 4 i n the S e r i e s .

17.

de Bey, L. G . , Richard, V. W., Hoage, A. H., Patton, R. B. and Adams, C . L. F i r s t Semi-Annual Technical Summary Report for t h e Period 1 July 1958 31 December 1958. Aberdeen Proving Ground: BRL MR 1185, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 2 i n the Series, January 1959.

-

18. Putnam, R. E. A.

Orbital Data Handling and Presentation. Aberdeen Proving Ground: BRL TN 1265, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 3 i n the Series.

19. de Bey, L.

G. An Approach t o t h e Doppler Dark S a t e l l i t e Detection Problem. Aberdeen Proving Ground: BRL TN 1266, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 4 i n t h e Series, July 1959.

20.

de Bey, L. G., Richard, V. W. and Patton, R. B. Second Semi-Annual Technical Summary Report f o r the Period 1 January 1959 30 June 1959.

Aberdeen Proving Ground: BRL MR 1220, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 5 i n the Series, July 1959.

21.

Putnam, R. E. A. Synchronization of Tracking Antennas. Aberdeen Proving Ground: BRL TN 1278, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 6 i n the S e r i e s .

22.

Fullen, K. A. The Eynamic Characteristics of Phase-Lock Receivers.

Aberdeen Proving Ground: BRL MR 1093, BRL DOPLOC S a t e l l i t e Fence Series, Report No. 8 i n the Series, March 1960.

23.

Scksrfman, W. E., Rothmsn, H. and Morita, T. S t a t i o n Geometry Studies f o r t h e DOPLOC System. Stanford Research I n s t i t u t e , h n l o Park, C a l . : F i n a l Report P a r t A, BRL DOPLOC S a t e l l i t e Fence Series, Report No. g i n t h e Series, SRI Control No. FO-0-93, July 1960.

-

BIBLIOGRAPHY (~ont. 'd) Dean, W. A . Precision Frequency Measurement of Noisy Doppler Signals.

Aberdeen Prcving Ground: BHL R 1110, BRL DOPLOC Satellite Fence Series, Report N3. 15 in the Series, June 1960.

Third Technical Summary Report for the Period July 1959 thtough:June 30, 1960. Aberdeen Proving Ground: BRL MR 1287, BRL DOPLCC Satellite Fence Series, Report Xo. 16 in the Series.

The Theory of Phase Synchronization of Oscillators Kreindler, E. with Application to the DOPLW Tracking Filter. Columbia University: TN T-1/157, BRL DOPLOC Satellite Fence Series, Report No. 17 in the Series, August 1, 1959.

Patterson, K. H. The DOPLOC Receiver for Use With the Circulating Memory Filter. Aberdeen Proving Ground: BRL TN 1345, BRL DOPLOC Satellite Yence Series, Report No. 18 in the Series, August 1960.

Patterson, K. H. Parametric Pre-Amplifier Results. Aberdeen Proving Ground: BRL TN 1345, BRL DOPLOC Satellite Fence Series, Report No. 19 in the Series, October 1960.

Vitek, R. A Comb Filter For Use in Tracking Satellites. Aberdeen Proving Ground: BRL MR 1349, BRL DOPLOC Satellite Fence Series, Report No. 24 in the Series.

Lootens, H. T. Satellite-Induced Ionization Observed With the DOPLOC System. Aberdeen Proving Ground: BRL MR 1362, BRL DOPLOC Satellite Fence Series, Report No. 23 in the Series. :de Bey, L. G., Berning, W. W., Reuyl, D. and Cobb, H. M. Scientific Objectives and Observing Methods for a Minimum Artificial Earth Satellite. Aberdeen Proving Ground: BRL R 956, October 1955.

32. Patterson, K. H. The Design for a Special Receiver for Satellite Ionosphere Experiments. Aberdeen Proving Ground: BRL TN 1158, April 1958.

33. Patterson, K. H. A Pre-Amplifier Design for Satellite Receivers.

Aberdeen Proving Ground: BRL MR 1154, July 1958.

34. Katz, L. and Honey, R. W. Phase-Lock Tracking Filter. Interstate Engineering Corp., Anaheim, Cal.: Final Engineering Report for BRL, 22 M Y 1957.

35. Bentley, B. T., Prenatt, R. E. and de Bey, L. G. Summary of Doppler Satellite Tracking Operations for Soviet Satellite 1957 Alpha 2 and 1957 Beta. Aberdeen Proving Ground: BRL MR 1174, October 1958.

.

BIBLIOGRAPRY (~ont'd) Bentley, B. T., Prenatt, R. E. and de Bey, L. G. Summary of Doppler Satellite Tracking Operations During the First Week After 1958 Alpha Launch. Aberdeen Proving Ground: BRL TN 1224, October 1958.

Instrumentation for Tracking Satellite Ionospheric Payloads. Ballistic Measurements Laboratory, Aberdeen Proving Ground: BRL TN 1267, July 1959.

Cruickshank, W. J. Instrumentation Used for Ionosphere Electron Density Measurements. Aberdeen Proving Ground: BRL TN 1317,. k y 1960.

Zancanata, H. W. ~alliiticInstrumentation and Summary of Instrumentation Results for the IGY Rocket Project at Fort Churchill. Aberdeen Proving Ground: BRL R 1091, January 1960.

Marks, S. T. Summary Report on BRL-IGY Activities. Aberdeen Proving Ground: BRL R 1104, April 1960.

Department of the Army Technical Manual Radio Receiver, R-~~~A/uRR. TM 85&, Jan~ary1956.

-

Phase-Lock Tracking Filter Model 1V. Operating Manual, Interstate Electronics Corporation, Anaheim, Cal.: October 1958.

Radio Receiver, Model 2501. Operating Manual, Nema-Clarke Company (subsidiary of Vitro corporation), Silver Spring, Ma. Fre-Amplifier, Model Pr-203. Operating Manual, Nems-Clark Company.

Frequency Synthesizer, Rhode and Schwarz Type XUA, BN Operating Manual.

- 444463,

Schwab, D. and Taneman, H. D. 108-M: Precision Frequency Generator.

U. S. Army Signal Research and Development Laboratory, Fort Monmouth, N. J. : USASRDL Report No. 2166, November 1960.

1

Department of the Army Washington 25, D. C.

The entagon Washington 25, D. C.

Commanding Officer 1 Diamond Ordnance Fuze Laboratories ATTN: Technical Information Office Branch 0l2 Washington 25, D. C.

Commanding General White Sands Annex BRL White Sands Missile Range New Mexico

2

10

10

Commander Armed Services Technical Information Agency A'ITN: TIPCR Arlington Hall Station Arlington 12, Virginia Cammander Air Force Systems Command ATPN: SCTS Andrews Air Force Base Washington 25, D. C.

1

Ccmunander Electronic Systems Division L. G. Hanscom Field Bedford Massachusetts

1

Coimnander Air Proving Ground Center A m : PGTRI E g i i n Aii'Force Base, Flori&&

-

Commanding General Army Ballistic Mlesile Agency AWN: Dr. C. A. Lundquist Dr. F. A. Speer Redstone Arsenal, Alabama Director Advanced Research Projects Agency Department of Defense Washington 25, D. C. Director National Aeronautics & Space Administration 1520 H Street, N.W. Washington 25, D. C.

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a58 vhich led to tbe pmposal of a system f o r

orbiting s a t e l l i t e s . An interim research of 81 U h & m t b transmitter and two receiving statione with flnd enrss first established to detelmine the f e a s i b i l i t y of using tar prineiplr with d r e n l y narrow bandvidth phase-locked tracking interim system established the f e a s i b i l i t y of filmas a scnsm satem. miag tbe Ibpplu nrtbDd apd -tad in r e a l data far which computational mcthode wen dnewhich aoQna mbital parameters n-m s W pass data aver a spopulation problem of detection and ein&le station. m ret t b c -tad ldentlficatim ard to aa m a d m m range with p r a e t i a l emitted power, a f m beam saten proposed in a trmsdtter and a receiver a n t e m -ted by Im,ailcs soula be -ly scarmed about the earth chord Jtba aa at ions to a u CJof saee vahme i0a) miles lous aPb2mvi.lEa-cU3omnFles.

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W B C p l P L -

lQUrrpm+-istsdatechknl~oftbcresearchanddnrrlDpmmt rmdv AEBl (Wn 8-56 which led to the propxal of a system fcn propam dewand orbit* s a t e l l i t e s . An interim rese-h h u t y -isting d an lll-ullg M t t u and tuo receivFng stations with miirsr ~ l l s h e to d &the I-mfiity of using tbc PoppLr prtmziph with -4 Darros h a i v l d t h rhaue-loebcd tmckjng rum as a aemax -tea.

The i n t e r i m s a t e n est&m&d the fuuribillty cb ustas n~pplcr and -tea in rrsl aata f o r which computatirmal mthods mere dmlopa which aoQna orbital pmmetue fx-m single p s data opu a single station. m ret tbc eqected space poplstim @Icm cb detection and rezse vltb suactical emittad m, a idcmtiiieatim ard to obtain a a t a a u a d t t a d a receiver antem4 sewmed 1- bs8 qatP pcp3sd in scanned about the earth chrPd sepanted by aaoot lorn dlcs mmld be Jtas aa statlms to nrpp a bsli EJof space wahm 1000 miles lcms

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InCIASSIPIED

Accession no.

Ballistic .-. ucvlrch .1Tab-tarie. ~.

Pnul BYXUEAL KERlRT R r i o d 20 JM 1958 30 Jlme 1961 ADJPLOC sAZELLI1S ISVSCTIM COI(PIEI[ ARPA satellite Rncc Series RCpcDt 110. 4 in the S v i e ~

Radar

- Dctcetirm - sa&tes

backiw

mlu tracuse SFDen*

'

-

Zhis report consistll of a tec4olcal of th research and d m l w pmgram sponsored under ARPA CTder 8-58 which led to the prof a system far detecting md treeking non-radiating orbiting s a t e l l i t e s . An interin research f a c i l i t y consisting of an illmimating tmmdtter and two r e a i v l n g stations with fixed antenna arrays -as f i r s t established to determine the f e a s i b i l i t y of using the Doppler principle cmpled with extremely n a r r w bandwidth phase-1tnrebin( f i l t e r s as a sensor aystem. The interim system established the f e a s i b U t y cb using the Doppler method and resulted in r e a l d a t a Xcn which ccmputatian~lmetho& were developed which produced o r b i t a l parameters *am si&e psss data over a single station. To meet the expected space population problem of detection and identificaticm and t o obtain a marrange with -tical mlttcd p e r , a scanned fan beam syntem -as proposed in which a tmamdtter and a receiPcr anseparated by about 10a) miles vovld be s~nebr~nously scanned about the carth ehmd joinins t h e two stations to -P a bsli cy-r or s p w r ~ l t 10a) p ~ riles lols and ha* a -us of 3COO miles.

A,,=ession W. U s t i c Research w t m i c e r' PnullEmDKc&aanaRIRriod20Jlmel95830 Jlmc 1961 BRL-ARPA DOFl.CC -08 C w ARPA S a t e l l i t e Pence Series Report Ila. 4 in th & N s satellites Qtection A. x. Hodgc Radar Satellites m ~ c p m BO. t uJ6 JulJ 1961 m 1 e r tmcnlu system M ~ o m.j 5 0 ~ - 0 6 - ( ~ 1as , codt m. m o . n . 1 4 3 Den*

--l - -

-

UEIASsIPIED BePn-t

5m rrport carslsts of a technical mmary d the research and &d-t pmgran m o r e d lmdv ARPA (Wer 8-58 which led t~ the proposal cb a m t e m for detecting and t m c k i q nm-radiatiag orbiting ~atellltes. ~n interim re-ch f a c i l i t y ccmsisting of an illumlnatiag traPsmitter end two r e e e i r b @ stations with r i n d ~~ntem arraJs a vas first established to dcte-â&#x20AC;˘ tbe f i e a s i b u t y of using the principle CmPlCd atb -4 -0y bandwidth phase-lacked trmking filmas a sensor system. 'Ih interim Byatem established the f e a s i b i l i t y of using th Doppler metbod and rrsulted in real data rm which c c q u t a t i m m l rcthods YM dewlOpCd which o r b i t a l paramctus tr(l single pass data orrr a sin& station. m meet the u p c t e d nzacc popvlatlrm @la of detection and ~ t i f l e a t i m aob to Dbtain a mslmm with m-aetical d t t e d a s c a n n d fan hem -ten uae PnmC553 in which a ta'8bmdtte.r and a r r e e i n r antema -tcd W a b d 1000 miles mmld be -4 scanned about tba chord Joining tba two Stationa to -P a u OX space vo1lQlO riles 1 and h a r i n s a r a d i u s af JOOOnFles.

mlu

-,

-