A Novel and Compact Ku-band Sensor for Polarimetric SAR Systems Placed in UAVs: Development and Syst

International Journal of Remote Sensing Applications Volume 4 Issue 2, June 2014 doi: 10.14355/ijrsa.2014.0402.02

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A Novel and Compact Ku-band Sensor for Polarimetric SAR Systems Placed in UAVs: Development and System Performances S. Sánchez-Sevilleja*, A. Arenas-Pingarrón*, J. Del Castillo-Mena, J.R.Larrañaga-Sudupe * Aerospace and engineering Department (ISDEFE-INTA). Calle Beatriz de Bobadilla, 3. 28040 Madrid Radar Laboratory, Department of Radiofrequency and Electronic Technologies, National Institute of Aerospace Technology, (INTA), Crta Ajalvir, km 4, 28850, Torrejón de Ardoz, Spain sar7.pers_externo@inta.es; sar1.pers_externo@inta.es; castillomja@inta.es; sudujr@inta.es Received 21th August 2013; Accepted 03rd March 2014; Published 3rd June 2014 © 2014 Science and Engineering Publishing Company

Abstract

Keywords

A new compact Ku band patch array antenna with dual linear polarization and excellet isolation values have been designed, implemented and measured. The antenna is placed in a new no-tripulated UAV platform. This platform is smaller than the last one used by INTA in which others no-compact prototypes were placed in. A lot of advantages have been achieved with this smaller and compact new system.

Antenna; Uav; Sar; Resolution; Nesz; Aasr; Rasr

Two antenna prototypes have been designed and manufactured in Ku-band. Both designs have improved some important characteristics compared with the previous and bigger one developed by INTA in X-band. The new prototypes focus the work on RF components miniaturization, isolation between polarizations (<-29dB), size and weight of the complete sensor (300mm x 100mm), spatial resolution and the azimuth and range ambiguities.

Finally, system performances evaluation has been carried out showing the current differences in resolution and ambiguities levels between both antenna prototypes. The first prototypt (4x8 array antenna in Ku band) is better for narrow coverage up to 4Km and very high azimuth resolution of 0.10m. In addition to that, it can be used to provide very high spatial resolution products up to 20cm x 10cm single look with a narrow range coverage below 4 Km and tight NESZ and AASR values of -20dB and -17dB respectively.

On the other hand, the second prototype 4x32 array antenna in Ku band, provides moderate coverage up to 6Km with high resolution of 0.3m and it can be used to provide high spatial resolution up to 30cm x 30cm with better range coverage below 6Km and better sensitivity and AASR values around -27dB and -35dB respectively.

Introduction QUASAR (Quick look Unmanned Aerial SAR) project started as a part of INTASAR program activities, in order to involve INTA Radar Laboratory developments for UAVs and lightweight platforms [1,2].

In SAR systems a huge array with a small aperture antenna is synthesized taking advantage of the movement of the UAV platform as it travels along a path [3,4]. Regarding this, planar antennas are important for SAR systems placed on Unmanned Aerial Vehicles (UAVs) due to low profile and lowcost manufacturing. In this context, a novel, compact and very light Ku band antenna with dual polarization and a very good isolation value has been developed and measured with excellent results not only in radio electric characteristics but also in range and azimuth ambiguity. The antenna will be placed inside a pod under the wings of the platform (FIG 1). The antenna of the first prototype design is composed by a 4x8 dual polarized square patch array fed through a very efficient and novel structure based on a cross-shape slot to achieve a large bandwidth and, overall, to achieve an excellent isolation between polarizations needed in a polarimetric system such as the one INTA which has developed [5, 6, 7, 8]. Likewise, the antenna second prototype is composed by a 4x32 dual polarized square patch array working in the same way

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International Journal of Remote Sensing Applications Volume 4 Issue 2, June 2014

of the first prototype.

The paper is structured as following: first of all, the antenna designs (first and second prototypes) are showed and divided into the radiation element, the transitions between layers, the feeding networks and the complete antenna. Afterward, the both prototypes measurements are presented. Finally, system performances evaluation has been done with both prototypes and important conclusions are obtained.

designed with H-shape slots (FIG 3) while the two different line layers correspond to the feeding networks : vertical and horizontal (FIG 4).

(a) Manufactured transition

FIG 1. GIMBAL IN UAV IN WHICH THE ANTENNA IS PLACED.

Antenna Design The antenna is based on a micro strip multilayer patch array fed through two slots in Ku band with 2GHz of bandwidth.

The antenna specifications are summarized in TABLE 1.

(b) Transition between layers measurements

TABLE 1. ANTENNA MODULE SPECIFICATIONS.

FIG 3. TRANSITION BETWEEN LAYERS.

Frequency band Bandwidth Gain BW-3dB in elevation plane BW-3dB in azimuth plane Input matching Polarization SLL X-polar value Isolation between polarizations Weight Maximum size

Ku band 15.7GHz-17.7GHz 18-22dBi 18º-22º 8º-12º -10dB Dual linear (horizontal and vertical) -13 dB <-25 dB >30dB 3Kg 300 mm x 100 mm

Each sheet is made up of dielectric RO4350b with a height of 127μm and two cooper layers of 35 μm (FIG 4a, FIG 4b and FIG 5): the upper cooper layer of each sheet contains the ground plane of the feeding network and it includes the transition and radiation slots (FIG 5b). The lower dielectric of each sheet contains one of each feeding network.

The radiation is generated with a square patch fed by two perpendicular and resonant slots placed in the centre of the patch in order to get a better isolation values (FIG 2). (a) upper line layer

FIG 2. RADIATING ELEMENT DESIGN

In addition to that, because of its better isolation characteristic, the transitions between layers are

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(b) lower line layer FIG 4. HORIZONTAL AND VERTICAL FEEDING NETWORK.

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(a) Manufactured patch layer

FIG 8. BACK VIEW OF COMPLETE ANTENNA MANUFACTURED AND MOUNTED.

(b) Ground layer with manufactured slots

FIG 9. FRONT VIEW OF COMPLETE ANTENNA MANUFACTURED AND MOUNTED.

FIG 5. PATCH LAYER AND GROUND LAYER WITH SLOTS.

Each feeding network is excited by a coaxial SMA connector, through a vertical and a novel transition design (FIG 6). The energy is addressed by the feeding lines until the slots, where the field lines are coupled trough the slot to the patch.

Antenna Measurements The completed antenna first prototype (FIG 8 and FIG 9) has been measured in INTA facilities. As it is shown in FIG 10, the isolation between polarizations is better than -28 dB and the input matching for horizontal and vertical polarization is better than -10 dB. Measurement of 4 x 8 array input matching and isolation 0 S11polH meas S22polV meas S12 meas S21 meas

-5 -10

FIG 6. VERTICAL COAXIAL TO MICROSTRIP TRANSITION DESIGN. dB

The completed antenna (first prototype 4 x 8 elements) has been manufactured and mounted joining the different layers including rohacell separators between them and nylon screws to avoid spurious radiation effects (FIG 7, FIG 8 and FIG 9).

-15 -20 -25 -30 -35 -40

15.8

16

16.2

16.4

16.8 16.6 f (GHz)

17

17.2

17.4

17.6

FIG 10. MEASUREMENTS OF INPUT MATCHING AND ISOLATION OF COMPLETE ANTENNA.

FIG 7. STACK-UP OF COMPLETE ANTENNA.

The next graphics represent the antenna radiation measured in INTA anechoic chamber for the central frequency (16.7GHz). Four main cuts are represented which are a kind of combination between polarizations H and V and field vector E direction: FIG 11 shows the E-plane for the horizontal polarization (azimuth). FIG 12 represents the radiation pattern for

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the horizontal polarization and H plane (elevation).

FIG 13 and 14 illustrate the E and H planes for the vertical polarization. The side lobe level is better than 13dB and the cross-polar values lower than -23dB very close to the requirements. There are no phase errors in the feeding networks neither ripple in the main lobe.

As a first conclusion regarding the antenna measurements, it is shown that all requirements are complied with the initial specifications.

Frequency = 16.7 GHz, Antenna 4 x 8 Vertical polarization H-plane 0 Polar meas Polar sim XPolar meas

-5 -10

Radiation Pattern (dB)

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-15 -20 -25 -30 -35 -40 -45 -50

-80

-60

-40

-20

0

20

40

60

80

θ (º)

Frequency = 16.7 GHz, Antenna 4x8 Horizontal Polarization E-plane 0 Polar meas Polar sim XPolar meas

-5

|Radiation pattern| (dB)

-10 -15 -20 -25 -30 -35 -40 -45 -50

-80

-60

-40

-20

0

20

40

60

80

θ (º)

FIG 14. MEASUREMENTS OF THE RADIATION PATTERN VERTICAL POLARIZATION H-PLANE (16.7GHz). ANTENNA 4x8 ELEMENTS.

FIG 15 represents the measurements of gain and directivity of completed antenna for all the frequency band. It is shown that the total losses in vertical polarization (2.15 dB) are worse than in horizontal polarization (1.38 dB) because the feeding networks are different between them as well as due to the alignment process which is complicated and critical.

FIG 11. MEASUREMENTS OF THE RADIATION PATTERN HORIZONTAL POLARIZATION E-PLANE (16.7GHz). ANTENNA 4x8 ELEMENTS Frequency = 16.7 GHz, Antenna 4 x 8 Horizontal polarization H plane 0 Polar meas Polar sim XPolar meas

|Radiation Pattern| (dB)

-10

-20

-30

-40

-50

FIG 15. MEASUREMENTS OF GAIS VS DIRECTIVITY COMPLETE ANTENNA. -80

-60

-40

-20

0

20

40

60

80

θ (º)

FIG 12. MEASUREMENTS OF THE RADIATION PATTERN HORIZONTAL POLARIZATION H-PLANE (16.7GHz). ANTENNA 4x8 ELEMENTS. Frequency = 16.7 GHz, Antenna 4 x 8 Vertical polarization E-plane 0 Polar meas Polar sim XPolar meas

-5

Radiation pattern(dB)

-10

FIG 17 and FIG 18 represent the second manufactured prototype which is bigger than the first one with the same requirements and 4 x32 elements. In this case, the BW-3dB is 2º in azimuth plane and 25.19 dBi of maximum gain in central frequency for H polarization and 24.31 dBi of maximum gain in central frequency for V polarization.

-15 -20 -25 -30 -35 -40 -45 -50

-80

-60

-40

-20

0

20

40

60

80

θ (º)

FIG 13. MEASUREMENTS OF THE RADIATION PATTERN VERTICAL POLARIZATION E-PLANE (16.7GHz). ANTENNA 4x8 ELEMENTS.

90

FIG 16. ANTENNA 4x32 ELEMENTS.

International Journal of Remote Sensing Applications Volume 4 Issue 2, June 2014

Frequency = 16.7 GHz,Antenna 4 xA32. Horizontal polarization E- plane 0

|Radiation Pattern| (dB)

-

Transmitted power, that will influence system NESZ, will be limited by platform available power level and thermal aspects to a total of 50dBm.

-

Transmitter maximum duty cycle has to be also taken into consideration as will limit average received power of radar echoes limiting NESZ. Tx duty cycle will also limit timing of system as shown in swath wide limits provided. A reference value for Tx pulse duty cycle of 10% will be used.

-

Tx Chirp bandwidth will be set to achieve the required range resolution to 600MHz and 1GHz. This will set the required Rx Bandwidth that will limit system Noise power. Chirp bandwidth will set the minimum required sampling frequency of ADCs, but nowadays values in the range of 2GSamples/s are achievable.

-

Rx noise figure will be set to 3dB taking into account a Noise figure of 2dB of a typical GaAs LNA and losses due to antenna, connectors, polarization switch Tx/Rx circulator and Rx limiter, which can sum up to 2dB approx.

-

One important aspect is the platform height that will influence coverage, spreading losses and then SNR. A maximum platform height of 6100 m will be used.

Polar meas Polar sim XPolar meas

-10

-20

-30

-40

-50

-80

-60

-40

-20

0

20

40

60

80

θ (º)

FIG 17. MEASUREMENTS OF THE RADIATION PATTERN HORIZONTAL POLARIZATION E-PLANE (16.7GHz). ANTENNA 4x32 ELEMENTS

System Performances Analysis Regarding system performances, there will be several system limits or constraints that have to be defined to be taken in the performance simulations. These constraints are defined by available technological limitations and system engineering aspects.

Frequency = 16.7 GHz, Antenna 4 x 32 Vertical Polarization H-plane 0 Polar meas Polar sim XPolar meas

|Radiation Pattern| (dB)

-10

-20

-30

System Sensitivity

-40

-50

-80

-60

-40

-20

0 θ (º)

20

40

60

80

FIG 18. MEASUREMENTS OF THE RADIATION PATTERN VERTICAL POLARIZATION H-PLANE (16.7GHz). ANTENNA 4x32 ELEMENTS.

Noise equivalent sigma nought is a figure that presents the minimum backscattering coefficient (σ0) of a target to be detected as its received power equals the systems noise power. Based on the radar equation:

PRx =

The main aspects to be taken into account are related to QUASAR RF front-end and are shown in Table 2 [9].

⋅

TABLE 2: QUASAR SYSTEM PARAMETERS.

System parameters Operating frequency Max Tx power Max Tx bandwidth Max Tx duty cycle Rx noise figure PRF range Platform Height Platform speed Terrain Height Antenna Length Antenna Height

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Value 16.7GHz 37.2dBm 1000MHz 20% 3dB [1500:6500] 610m 70m/s 700m 4x8 (96mm) and 4x32 ( 400 mm) 4x8 ( 60 mm)

Operating frequency will be set to Ku band, with a centre frequency of 16.7GHz.

PTx G AntTx (θ , ϕ )G AntRx (θ , ϕ )λ 2 G Rxσ

(4π )3 R 4 Ltot

cτ 3 dB ⋅ R ⋅ θ Az 2 sin (θ inc )

⋅ (1)

Being PTx the peak transmitted power, G AntTx / RX antenna gains in Tx and Rx respectively, λ the wavelength, σ the target backscattering coefficient,

θ inc

the incidence angle and

3 dB θ Az

the antenna

azimuth beamwidth.

The NESZ can be derived by equalling Eq. (1) to noise power and solving for σ [10]:

(4π )2 R 4 Ltot KT0 BWRX F NESZ ( R ) = ⋅ PTX G antTx (θ ,ψ )λ2 G RX 2 sin (θ inc ) B D ⋅

cτ

⋅

PRF

(2)

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Where BWRX is receiver Bandwidth, F is noise figure,

Where

fd+mPRF is

the

Doppler

frequency of

and BD the azimuth Doppler bandwidth.

mth azimuth ambiguous area.

System Spatial Resolution

System Performances Results

Resolution represents the minimum space distance to discriminate two close targets. It is measured as a resolution cell and can be separated into Range and Azimuth Resolutions.

This section shows the performance results which are obtained by simulation using QUASAR system parameters and antenna characterization data.

Range resolution is limited by chirp transmitted bandwidth ( BWTX ) following the equation [10]: (3)

Azimuth resolution is limited by azimuth illumination time of the target, or in other words by the available Doppler bandwidth of the imaged target [10]:

R AZ = 0.886 ⋅

λ ⋅ Vg 2 ⋅ Vsat ⋅ θ

3 dB AZ

system sensitivity-NESZ -14

-16

(4)

-18

Where V g is ground is projected velocity and Vsat is satellite forward velocity.

NESZ(dB)

c 2 ⋅ BWTX ⋅ sin (θ inc )

RGR = 0.886 ⋅

The results have been obtained at worst case of platform attitude of 6000m. Three look angles configuration were stabilised in order to calculate performance of different QUASAR image products.

Pulse length is limited to 40μs in order to allow receive window to be open-ended before Near range echoes reach the system at 30º look angle configuration.

System Ambiguities

Range ambiguities are due to echoes from previous or subsequent pulses that reach radar receiver at the same time the desired echo. It is defined by Range ambiguity to signal ratio equation [10]:

R 2 sin(θ inc ) ⋅ GantTX ( R) GantRX ( R) σ 0 ( R)

⋅

m m m G antTx ( Ramb ) G antRx ( Ramb ) σ 0 ( Ramb )

Nr

∑

m m Ramb sin(θ incAmb ) 2

m = − Nr m≠0

AASR( R) =

m = − NA

− 0.5 BD

∫

0.5 BD

− 0.5 BD

92

GantTx ( fd ) GantTx ( fd )GantRx ( fd ) df

25

30

35

40 45 look angle

50

55

60

65

FIG 19 SYSTEM PREDICTED NESZ USING 4X8 ELEMENTS ANTENNA

FIG 19 depicts system sensitivity with 4x8 elements array antenna versus look angle configuration. At this configuration Tx and Rx bandwidth has been set to a maximum of 1GHz in order to get the better range resolution.

Beams coverage 4

3.9

(6)

3.8

Range coverage (km)

Azimuth ambiguities are due to the fact SAR azimuth Doppler bandwidth is sampled at a frequency equal the PRF. Azimuth returns from a target are not band limited and there will be aliasing of returns coming from secondary lobes[10].

GantTx ( fd + mPRF )GantRx ( fd + mPRF ) df

20

(5)

distance to ambiguous returns.

∑∫

θlook=45º θlook=60º

-28 15

m

0.5 BD

θlook=30º

-26

Being R the distance to desired target and the Ramb

NA

-22

-24

There will be undesired radar returns that will reach receiver at the same time the desired ones will do which are known as ambiguities.

RASR( R) =

-20

3.7

3.6

3.5

3.4 θlook=30º 3.3

3.2 15

θlook=45º θlook=60º 20

25

30

35

40 lookangle

45

50

55

60

65

FIG 20 SYSTEM PREDICTED COVERAGE USING 4X8 ELEMENTS ANTENNA

International Journal of Remote Sensing Applications Volume 4 Issue 2, June 2014

FIG 20 presents the system coverage for a NESZ better than 15dB while system spatial resolution is shown in FIG 21.

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Azimuth Ambiguity to Signal Ratio - AASR -17.1545

θlook=30º θlook=45º

-17.1545

θlook=60º

RASR and AASR are depicted in FIG 22 and 23 respectively. RASR is negligible as expected in airborne based SAR comparing with AASR values. In any case, AASR values are limited values as equation 5 do not take into account the different spreading losses suffered by ambiguous returns and desired echoes and a further effect due to the different azimuth compression gain during image processing steps as will be demonstrated in next section.

Rg θlook=30º AZ θlook=30º AZ θlook=45º

resolution(m)

RG θlook=60º θlook=60º

0.4

0.3

-17.1545 -17.1545 -17.1545 -17.1545 -17.1545 15

20

25

30

35

40 45 look angle

50

55

60

65

TABLE 3: QUASAR PREDICTED PERFORMANCES

0.2

0.1

0 15

-17.1545

It can be seen how 4x8 configuration can be used to provide very high spatial resolution products up to 20cm x 10cm single look with a narrow range coverage below 4 Km and tight NESZ and AASR values of-20dB and -17dB respectively.

RG θlook=45º

0.5

-17.1545

FIG 23 SYSTEM PREDICTED AASR USING 4X8 ELEMENTS ANTENNA

Spatial resolution 0.7

0.6

Azimuth ambiguity ratio(dB)

-17.1545

20

25

30

35

40 45 look angle

50

55

60

65

FIG 21 SYSTEM PREDICTED RESOLUTION USING 4X8 ELEMENTS ANTENNA

TABLE 3 summarizes the results obtained including also the ones achieved by using 4x32 elements array antenna. With the last configuration the only modification on system parameters is the use of a limited Tx and RX bandwidth to 600MHz as azimuth resolution is decreased due to the use of a narrower beamwidth, thus sampling a smaller Doppler bandwidth.

Range Ambiguity to Signal Ratio - AASR -82 -84

Antenna Look Angle [degrees] PRF NESZ [dB] AASR [dB] RASR [dB] Spatial Rg Resol.

Az [mxm] Coverage [Km]

4x8

4x32

30

45

60

30

45

60

1200

1200

1200

1000

1000

1000

-23.3

-20.5

-17.3

-30.2 -27.6

-24.1

-17.1

-17.1

-17.1

-35

-35

-35

-97.7

-92

-86.1

-102

-96.7

-90.3

0.35 0.09

0.23 0.09

0.18 0.09

0.6 0.32

0.38 0.32

0.3 0.32

3.2

3.8

4

3.7

4.5

5.9

4x32 configuration can be used to provide high spatial resolution up to 30cm x 30cm with better range coverage below 6Km and better sensitivity and AASR values around -27dB and -35dB respectively.

Range ambiguity ratio(dB)

-86

Azimuth Ambiguity Analysis

-88 -90 -92 -94 -96 θlook=30º

-98

θlook=45º

-100 -102 15

θlook=60º 20

25

30

35

40 45 look angle

50

55

60

65

FIG 22 SYSTEM PREDICTED RASR USING 4X8 ELEMENTS ANTENNA

Due to the fact the azimuth response in a SAR system is sampled by the pulse repetition frequency (PRF), the azimuth bandwidth is limited inside the band between -PRF/2 and +PRF/2. Because received Doppler frequencies extend beyond the bandwidth given by the PRF, it exists in aliasing which causes replicas of point and extended targets when processing azimuth signals. Simulations to estimate the relative amplitude of such replicas that appear on the image are shown in this section.

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Azimuth signals are modulated in amplitude depending on the antenna pattern, since each Doppler frequency is related to one pointing angle by equation

f doppler =

2⋅v

λ

sen (θ )

(7)

Where v is the platform speed (50m/s), λ the wavelength (18mm) and θ the pointing angle to the target. Thus, depending on platform speed and wavelength, the azimuth spectrum is amplitude modulated.

Azimuth Ambiguity Results In this section the azimuth ambiguity results are shown. In FIG 24 and FIG 25 the azimuth antenna pattern (red, left axis) for horizontal polarization patterns with 32 and 8 elements respectively, with aperture of 120˚ (from -60˚ to 60˚), together with the aliased wrapped Doppler frequencies (green, right axis) are plotted. PRF is 1500Hz, so bandwidth is limited by -750Hz and 750Hz. azimuth pattern: A32PH planeE-f=16,7GHz.mat 0

1000

-10

Amplitude [dB]

500 -20

250 0

-30 -250 -40

-500

Doppler frequency [Hz]

750

the main ramp, the better for lower power level replicas. However, because the azimuth compression is performed by correlating with a matched filter, the ambiguous targets have lower level than expected, because beyond the main response, the azimuth response (frequency modulation) is different, since in the main response the pointing angle in equation 7 varies almost linearly, and beyond it doesn’t, so the filter is not matched. This can be observed in FIG 26, where the main response (centre of the image, normalized to 0dB) and the expected replicas after azimuth compression simulation are represented with pattern and frequency modulation of FIG 24, for the case without considering antenna pattern (black plot) and antennas with 4x32 (H and V polarizations, red and magenta respectively) and 4x8 elements (H and V polarizations, blue and cyan respectively).

Compressed response 0

-20 -30 -40 -50 -60 -70 -80 -90

-50

-40

-30

-20

-10

0

10

20

30

40

1000

250 0

-30

-250 -500

-40

Doppler frequency [Hz]

500

-20

-750 -50 -50

-40

-30

-20

-10

0

10

20

30

40

50

w/o pattern A32PH planeE A32PV planeH A8PH planeE A8PV planeH

-20 -30 -40 -50

750 -10

10

Compressed main response

Amplitude [dB]

azimuth pattern: A8PH planeE-f=16,7GHz.mat

5

0 -10

FIG 24 DOPPLER FREQUENCY PATTERN MODULATION (4 x 32 ELEMENTS) 0

0

FIG 26 COMPRESSED RESPONSE OVER FULL APERTURE

-1000

50

-5

Azimuth [Km]

Angles [degree] from target point of view

Amplitude [dB]

-10

-750

-50

-1000

Angles [degree] from target point of view

FIG 25 DOPPLER FREQUENCY PATTERN MODULATION (4 x 8 ELEMENTS)

Each ramp of the green saw tooth profile represents an azimuth chirp to be compressed in the azimuth compression step during image processing, and hence all except the one around the main pattern lobe will be replicas of the target, so as lower the pattern outside

94

w/o pattern A32PH planeE A32PV planeH A8PH planeE A8PV planeH

-10

Amplitude [dB]

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-60 -70 -0.9165

-0.916

-0.9155

-0.915

-0.9145

-0.914

-0.9135

-0.913

Azimuth [Km]

FIG 27 COMPRESSED MAIN RESPONSE

Although in FIG 24 azimuth pattern in -60º (furthest left ramp) is higher than other ambiguous ramps, its amplitude level after compression is lower, due to the fact the filter is unmatched. FIG 28 to FIG 31 contain the compressed main response, nearest left (negative angles), furthest left, nearest right (positive angles) and furthest right ambiguities respectively that are a zoomed view of responses in FIG 27. In FIG 28, the different responses for the antenna with 4x32 and 4x8 elements are due to the different bandwidth. The ambiguous level is always -50dB lower than the main

International Journal of Remote Sensing Applications Volume 4 Issue 2, June 2014

response (0 dB), which means that ambiguous target amplitude response in SAR image is 50dB lower than its corresponding real target.

Compressed nearest left ambiguity -30

w/o pattern A32PH planeE A32PV planeH A8PH planeE A8PV planeH

-40

Amplitude [dB]

-50

Conclusions

-70 -80

Two prototypes of a novel, broadband and a compact Ku-band SAR antenna have been designed, manufactured and measured in INTA. The antennas, which will be placed in a UAV platform, present a very good isolation between polarizations (<-28 dB), a good input matching better than -10 dB in 2GHz of bandwidth and a very good radiation pattern performances such as main lobe, side lobe level, XPolar, gain and directivity, consequently, it is complied with the initial requirements.

-90

-110 -120 -2.9

-2.85

-2.8

-2.75

-2.65

-2.7

Azimuth [Km]

FIG 28 COMPRESSED NEAREST LEFT AMIBIGUTY Compressed furthest left ambiguity -40

w/o pattern A32PH planeE A32PV planeH A8PH planeE A8PV planeH

-50

Amplitude [dB]

-60 -70 -80 -90 -100 -110 -120 -130 -12

-11

-10

-9

-8

-7

Azimuth [Km]

FIG 29 COMPRESSED FURTHEST LEFT AMIBIGUTY Compressed nearest right ambiguity -30

w/o pattern A32PH planeE A32PV planeH A8PH planeE A8PV planeH

-40 -50

Amplitude [dB]

levels are considered only the azimuth dimension to synthesize azimuth signals, so range curvature and its curvature correction existing in real images have not been taken into account. Since for ambiguous targets this correction is not properly carried out (alike for matched filters), real measurements over a SAR image should give even lower ambiguous levels.

-60

-100

By means of performance simulations using QUASAR system parameters related to radar front-end and using the measured radiation patterns of these antennas, a comparison between the two different antenna options has been performed. It can be concluded how different products are in terms of coverage, resolution and ambiguities level. The 4x8 array antenna is better for narrow coverage up to 4Km and very high azimuth resolution of 0.10m while 4x32 array antenna provides moderate coverage up to 6Km with high resolution of 0.3m.

-60 -70 -80 -90 -100 -110 -120 0.9

0.95

1

1.05

1.1

1.15

Azimuth [Km]

FIG 30 COMPRESSED NEAREST RIGHT AMIBIGUTY Compressed furthest right ambiguity -40

w/o pattern A32PH planeE A32PV planeH A8PH planeE A8PV planeH

-50 -60

Amplitude [dB]

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The simulations performed to obtain the ambiguous target levels in a SAR image by using these antennas show a maximum level of 50dB lower than the level of the corresponding real target. Although azimuth pattern of the antenna with 32 elements has a secondary lobe about -16dB in angle 60˚, due to it is not processed with a matched filter, has a similar peak response than nearer ambiguous targets, so from ambiguities point of view further side lobes have lower impact.

REFERENCES

-70 -80

Boerner, W.-M. et al.: “Polarimetry in radar remote sensing:

-90 -100 -110

Basic and applied concepts’” in Henderson, F.M., and

-120

Lewis, A.J. (Eds.): “Principles and applications of

-130 7

8

9

10

11

12

13

Azimuth [Km]

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S. Sánchez-Sevilleja was born in Madrid, Spain in 1982. She received the Telecommunication engineer degree and the PhD Master in systems and communications from Universidad Politécnica de Madrid (UPM), in 2007 and 2011, respectively. Currently she is realizing her PhD Thesis on active and reconfigurable arrays

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She worked in Radiation Group of Department of Signal, Systems and Radio-communications of UPM in an IFF radar antenna and EADS Casa-Espacio as Antenna and RF engineer in space-related projects. From 2008 she works in Spanish Institute of Aerospace Technology (INTA) in antenna and RF projects for radar applications. She has been author and co-author of more than 12 national and international papers in journals and conferences. Her research works include active arrays antennas, planar antennas and beam forming.

A. Arenas-Pingarrón was born in Madrid, Spain in 1982. He received the Telecommunication engineer degree in 2006 and the PhD Master in systems and communications in 2013, both from Universidad Politécnica de Madrid. He has worked in Spanish Institute of Aerospace Technology (INTA) in the field of Synthetic Aperture Radar (SAR). His main interests are SAR systems (processing and simulation), Time-Frequency distributions and Sonar.

J. Del Castillo-Mena was born in Madrid in 1983.

He received the Telecommunication engineer degree from ETSIT UPM in 2006. He works as radar system engineer for INTA SAR Radar laboratory since 2006, where he focus on radar MW subsystem development and synthetic aperture radar characterisation and calibration. Since 2008 he is also working on PAZ mission being part of technical assistance to Space segment provider and CALVAL centre team. He is also studying a MSc in communication systems and technology at Madrid Polytechnic University.

J.R. Larrañaga-Sudupe was born in Azkoitia, Spain in 1966.

He received the Telecommunication engineer degree from Universidad Politécnica de Madrid (UPM), in 1992. He is working on development of Synthetic Aperture Radar systems since 1993. Head of Radar Laboratory and SAR program at INTA since 2005. Currently, he is involved in the development of advanced SAR systems for unmanned platforms.