I ns t i t ut eofManage me nt & Te c hni c alSt udi e s RADAR AND NAVI GATI ON AIDS
500
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RADAR AND NAVIAGATION AIDS
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UNIT I
01-18
FUNDAMENTALS OF RADAR Radar range equation,Radar frequency pulse considerations,Minimum detectable signal,Receiver noise,Integration of radar pulses,Pulse repetition frequency.
UNIT-2
19-37
VARIOUS RADAR SYSTEMS
Pulse radar,FM-CW radar,MTI radar,Pulse Doppler radar,Monopulse tracking radar,Radar receivers.
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1 UNIT-I
RADAR FUNDAMENTALS: RADAR, an acronym for “Radio Detection And Ranging”, is an active device that transmits and receives electromagnetic energy in microwave wavelengths. The majority of current operational imaging radars use wavelengths between 1 mm to 1 m. Two distinctive features characterise microwave wavelengths from a remote sensing point of view: (i) microwaves are capable of penetrating the atmosphere under virtually all conditions, and (ii) microwave reflections or emissions from surface materials bear no direct relationship to reflectance in the visible or thermal portions of the spectrum.
The launch of orbital civil radar systems since the 1970s made possible the acquisition of a unique view of natural resources over the Earth’s surface. Although much progress has been made in the fields of both active and passive microwave remote sensing, they are still important research areas for the environmental sciences, engineering and the military. PRIMARY RADAR OPERATION: The following figure shows the operating principle of a primary radar. The radar antenna illuminates the target with a microwave signal, which is then reflected and picked up by a receiving device. The electrical signal picked up by the receiving antenna is called echo or return. The radar signal is generated by a powerful transmitter and received by a highly sensitive receiver.
BLOCK DIAGRAMM OF PRIMARY RADAR
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All targets produce a diffuse reflection i.e. it is reflected in a wide number of directions. The reflected signal is also called scattering. Backscatter is the term given to reflections in the opposite direction to the incident rays. Radar signals can be displayed on the traditional plan position indicator (PPI) or other more advanced radar display systems. A PPI has a rotating vector with the radar at the origin, which indicates the pointing direction of the antenna and hence the bearing of targets.
APPLICATION OF RADAR
Radar has been employed to detect targets on the ground, on the sea, in the air, in space, and even below ground. The major areas of radar application are briefly described below.
MILITARY:
Radar is an important part of air-defense systems as well as the operation of of-fensive missiles and other weapons. In air defense it performs the functions of surveillance and weapon control. Surveillance includes target detection, target recognition, target tracking, and designation to a weapon system. Weapon-control radars track targets, direct the weapon to an intercept, and assess the effectiveness of the engagement (called battle damage assessment).
A missile system might employ radar methods for guidance and fuzing of the weapon. Highresolution imaging radars, such as synthetic aperture radar, have been used for reconnaissance purpose and for detecting fixed and moving targets on the battlefield. Many of the civilian applications of radar are also used by the military. The military has been the major user of radar and the major means by which new radar technology has been developed.
REMOTE SENSING:
All radars are remote sensors; however, this term is used to imply the sensing of the environment. Four important examples of radar remote sensing are (1) weather observation, which is a regular part of TV weather reporting as well as a major input to national weather prediction; (2) planetary observation, such as the mapping of Venus beneath its visually opaque clouds; (3) short-range belowground probing; and (4) mapping of sea ice to route shipping in an efficient manner.
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Air Traffic Control (ATC) :
Radars have been employed around the world to safely control air traffic in the vicinity of airports (Air Surveillance Radar, or ASR), and en route from one airport to another (Air Route Surveillance Radar, or ARSR) as well as ground-vehicular traffic and taxing aircraft on the ground (Airport Surface Detection Equipment, or ASDE). The ASR also maps regions of rain so that aircraft can be directed around them. There are also radars specifically dedicated to observing weather (including the hazardous downburst) in the vicinity of airports, which are called Terminal Doppler Weather Radar, or TDWR. The Air Traffic Control Radar Beacon System (ATCRBS and Mode – S) widely used for the control of air traffic, although not a radar, originated from military IFF (identification Friend or Foe) and uses radar-like technology.
LAW ENFORCEMENT AND HIGHWAY SAFETY:
The radar speed meter, familiar to many, is used by police for enforcing speed limits. (A variation is used in sports to measure the speed of a pitched baseball0. Radar has been considered for making vehicles safer by warning of pending collision, actuating the air bag, or warning of obstructions or people behind a vehicle or in the side blind zone. It is also employed for the detection of intruders.
AIRCRAFT SAFETY AND NAVIGATION: The airborne – avoidance radar outlines regions of precipitation and dangerous wind shear to allow the pilot to avid hazardous conditions. Low-flying military aircraft rely on terrain avoidance and terrain following radars to avoid colliding with obstructions or high terrain. Military aircraft employ groundmapping radars to image a scene. The radio altimeter is also a radar used to indicate the height of an aircraft above the terrain and as a part of self – contained guidance systems over land.
SHIP SAFETY:
Radar is found on ships and boats for collision avoidance and to observe navigation buoys, especially when the visibility is poor. Similar shore-based radars are used for surveillance of harbors and river traffic/
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SPACE: Space vehicles have used radar for rendezvous and docking, and for landing on the moon. As mentioned, they have been employed for planetary exploration, especially the planet Earth. Large ground-based radars are used for the detection and tracking of satellites and other space objects.
The field of radar astronomy using Earth-based systems helped in understanding the nature of meteors, establishing an accurate measurement of the Astronomical Unit (the basic yardstick for measuring distances in the solar system), and observing the moon and nearby planets before adequate space vehicles were available to explore them at close distances.
OTHER: Radar has also found application in industry for the noncontact measurement of speed and distance. It has been used for oil and gas exploration. Entomologists and or mythologists have applied radar to study the movements of insects and birds, which can not be easily achieved by other means. Some radar systems are small enough to be held in one’s hand. Other are so large that they could occupy several football fields. They have been used at ranges close enough to almost touch the target and at ranges that reach to the planets. RADAR FREQUENCIES:
Conventional radars generally operate in what is called the microwave region (a term not rigidly defined). Operational radars in the past have been at frequencies ranging from about 100 MHz to 36 GHz, which covers more than eight octaves. These are not necessarily the limits. Operational HF overthe-horizon radars operated at frequencies as low as a few megahertz. At the other end of the spectrum, experimental millimeter wave radars have been at frequencies higher than 240 GHz.
During World War II, letter codes such as S, X, and L were used to designate the distinct frequency bands at which microwave radar was being developed. The original pur-tinct frequency bands at which microwave radar was being developed. The original purpose was to maintain military secrecy; but the letter designations were continued after the war as a convenient shorthand means to readily denote the region of the spectrum at which a radar operated. Their usage is the accepted practice of radar engineers. 3
Table lists the radar-frequency letter-band designations approved as an IEEE Standard. These are related to the specific frequency allocations assigned by the International Telecommunications Union (ITU) for radiolocation, or radar. For example, L band officially extends from 1000 MHz to 2000 MHz, but
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L-band radar is only allowed to operate within the region from 1215 to 1400 MHz since that is the band assigned by the ITU.
There have been other letter-band designations, but Table is the only set of designations approved by the IEEE for radar. It has also been recognized by being listed in the U.S. Department of 4
Defense Index of Specifications and Standards. A different set of letter bands has been used by those working in electronic warfare. It was originally formulated by the U.S. Department of Defense for use only 5
in conducting electronic countermeasure exercise. Sometimes it is incorrectly extended to describe radar frequencies, appropriate. (There may be J-band jammers, but according to the IEEE Letter-Band Standards there are no J-band radars.) Usually the context in which the nomenclature in employed can aid in distinguishing whether the letters refer to radar or to EW.
IEEE Standard radar-frequencies: Band Designation
Nominal Frequency Range
ITU
Specific Frequency Ranges for Radar based on Assignments in Region
HF
3-30 MHz
VHF
30-300MHz
138-144GHz & 216-225 MHz
UHF
300-1000MHz
420-450 MHz & 850-942 MHz
L
1-2 GHz
1215-1400 MHz
S
2-4 GHz
2300-2500 MHz 2300-2500 MHz
C
4-8 GHz
5250-5925 MHz
X
Ku
K
Ka
V
8-12 GHz
12-18 GHz
8500-10,680 MHz 13.4-14.0 GHz 15.7-17.7 GHz
18-27GHz
24.05-24.25 GHz
27-40 GHz
33.4-36 GHz
40-75 GHz
59-64 GHz
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W
75-110 GHz
76-81 GHz & 92-100 GHz
mm
110-300 GHz
126-142 GHz 144-149 GHz 231-235 GHz 238 – 248 GHz
Letter –band nomenclature is not a substitute for the actual numerical frequencies at which radar operates. The specific numerical frequencies of radar should be used when ever appropriate, but the letter designations of Table should be used whenever a short notation is desired.
MINIMUM DETECTABLE SIGNAL (MDS): The ability of a radar receiver to detect a weak echo signal is limited by the ever-present noise that occupies the same part of the frequency spectrum as the signal. The weakest signal that can just be detected by a receiver is the minimum detectable signal. In the radar equation of it was denoted as S min. Use of the minimum detectable signal, however, is not common in radar and is not the preferred method for describing the ability of a radar receiver to detect echo signals from targets. A specific value of minimum receivable power (Pemin) is defined, called the Minimum Detectable Signal (MDS). The minimum detectable signal is defined as the useful echo power at the reception antenna, which gives at the output of the IF amplifier (just before detection), a signal which lies 3 dB above the mean noise level. The MDS is generally expressed in dBm; typical values are between -100 dBm ... -103 dBm. But the actual value of MDS (or Prmin) depends on a number of factors and choices which are ultimately related to the statistics of radar detection. Much effort has been devoted to the design of low-noise input stages (Low Noise Amplifiers) for radar receivers. But however good the design, there is still a certain amount of noise generated in subsequent receiver stages, and in the input transmission line or waveguide. Also additional noise enters the system via the antenna (and radome if fitted).
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MEASUREMENT OF MDS:
DEFINITION OF THE MDS:
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RECEIVER NOISE AND THE SIGNAL-TO-NOISE RATIO
At microwave frequencies, the noise with which the target echo signal competes is usually generated within the receiver itself. If the radar were to operate in a perfectly noise-free environment so that no external sources of noise accompany the target signal, and if the receiver itself were so perfect that it did not generate any excess noise, there would still be noise generated by the thermal agitation of the conduction electrons in the ohmic portion of the receiver input stages. This is called thermal noise or Johnson noise. Its magnitude is directly proportional to the bandwidth and the absolute temperature of the ohmic portions of the input circuit. The available thermal-noise power (watts) generated at the input of a receiver of bandwidth B n (hertz) at a temperature T (degrees Kelvin) is Available thermal-noise power = kTBn Where k=Boltzmann’s constant = 1.38 x 10
-23
J/deg. (The term available means that the
device is operated with a matched input and a matched load). The bandwidth of a super heterodyne receiver (and almost all radar receivers are of this type) is taken to be that of the IF amplifier (or matched filter) In the bandwidth Bn is called the noise bandwidth, defined as 0 |H(f)| df 2
Bn = F0
Where H(f) = frequency – response function of the IF amplifier (filter) and F 0 = frequency of the maximum response (usually occurs at midband).
Noise bandwidth is not the same as the more familiar half-power, or 3-dB, bandwidth. Equation (2.3) states that the noise bandwidth is the bandwidth of the equivalent rectangular filter whose noise-power output is the same as the filter with frequency response function H(f). The half-power bandwidth, a term widely used in electronic engineering, is defined by the separation between the points of the frequency response function H(f) where the response is reduced 0.707 (3 dB in power) from its maximum value.
Although it is not the same as the noise bandwidth, the half-power bandwidth is a reasonable approximation for many practical radar receivers.
5.6
Thus the half-power bandwidth B
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is usually used to approximate the noise bandwidth B n which will be the practice in the remainder of the chapter.
The noise power in practical receiver is grater than that from thermal noise alone. The measure of the noise out of a real receiver (or network) to that from the ideal receiver with only thermal noise is called the noise figure and is defined as
noise out of practical receiver
Nout
Fn =
= Noise out of ideal receiver at std temp T0
kT0BGa
Where Nout = noise out of the receiver, and Ga = available gain. The noise figure is defined in terms of standard temperature T o, which the IEEE defines as 290 K (62’F). This is close to room temperature. (A standard temperature assures uniformity in measurements that might be made at different temperature.)With this definition, the factor kT 0 in the definition of noise figure is 4 x 10
-21
W/Hz, a quantity easier to remember than Boltzmann’s constant. The
available gain Ga is the ration of the signal out, Sout, to the signal in Sin with both the output and input matched to deliver maximum output power. The input noise, N in, in an ideal receiver is equal to kT0Bn. The definition of noise figure given by Eq. (2.4) therefore can be rewritten as Sin / Nin Fn= Sout/Nout This equation shows that the noise figure may be interpret as a measure of the degradation of the signal-to-noise ration as the signal passes through the receiver.
kT0BFnSout Sin = Nout If the minimum detectable signal Smin is that value of Sin which corresponds to the minimum detectable signal-to-noise ration at the output of the IF, (Sout/Nout)min, then
Sout Smin = kT0BFn Nout
min
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Substituting the above into Eq. (2.1), and omitting the subscripts on S and N, results in the following form of the radar equation: P1GAe 4 R max 2
(4) kT0BFn(S/N)min For convenience, Rmax on the left-hand side is usually written as the fourth power rather than take the fourth not of the right – hand side of the equation.
The minimum detectable signal is replaced in the radar equation by the minimum detectable signal-tp-noise (S/N)min. The advantage is that (S/N)min is independent of the receiver bandwidth and noise figure; and, as we shall see in Sec. 2.5, it can be expressed in terms of the probability of detection and the probability of false alarm, two parameter that can be related to the radar user’s needs.
The signal-to-noise ratio in the above is that at the output of the IF amplifier, since maximizing the signal-to-noise ration at the output of the IF is equivalent to maximizing the video output where the threshold decision is made.
7
Before continuing the development of the radar equation, it is necessary to digress and briefly review the concept of the probability density function in order to describe the signal-tonoise ratio in statistical terms. Those familiar with this subject with this subject can omit the next section.
INTEGRATION OF RADAR PULSES: The radar pulse is a 2-microsecond pulse of the 1292 MHz carrier. This carrier was heterodyned to a frequency of approximately 6 MHz where it was sampled at 20 megasamples/sec using the internal clock of the PDA500 data acquisition board. A typical pulse signature sampled by the 8-bit A/D converter is shown in Figure 1. The pulse was distorted somewhat by the long propagation path of 104 km. A weaker delayed pulse can be seen about 3 microseconds after the main pulse, probably due to scattering near the transmitter or receiver.
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Figure 1. Pulse signature sampled by the A/D The pulse shown in Figure 1 was recorded when the radar beam was several beamwidths away from the direction of Green Bank. The GBT receiver was saturated by pulses received when the beam passed over the GBT. Some saturation can be seen on the negative-going part of the waveform in Figure 1. Even at their strongest, the pulses did not appear to blind the receiver. They just caused clipping of the waveform peaks. PULSE REPETITION RATE: The 5-second data set that began about 1.1 seconds before the radar beam swept over the GBT was searched for pulses that exceeded a threshold of 127 A/D counts. Figure 1 shows that this threshold is well above the baseline noise level. The arrival time of each pulse was recorded as the time of the first data sample that exceeded the threshold. This rough measure tends to bias weak pulses to a slightly later arrival time relative to strong pulses, but the maximum error is the 2-microsecond width of the pulse. Figure 2 shows the measured pulse arrival times relative to an expected arrival time assuming a repetition rate of 341.42 pulses per second. This rate was derived by adjusting the arrival times to be constant while the pulses were strongest around 1.1 seconds into the data set. The five parallel delay tracks are due to an intentional transmitted pulse time offset of an integer number of 100 microseconds in a repeating sequence of [0, 4, 0, 3, 1, 2, 1, 3]. Presumably, this offset is to resolve ambiguities due to reflections beyond the range interval of 440 km set by the pulse spacing.
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Figure 2. Pulse arrival delays
PULSE FILTER: Better pulse sensitivity can be obtained by convolving the data with a function that closely matches the pulse signature. An approximation to a pulse filter was derived by a rough fit of a gaussian curve to the power spectrum of the pulse as shown in Figure 3.
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Figure 3. Gaussian fit to a pulse for the purpose of deriving a pulse convolution function. spectrum
The full 102 MB data string was then broken into 64 kB blocks, Fourier transformed, multiplied by the frequency domain filter function, and the transformed back to the time domain. The convolved time series of the pulse shown in Figure 1, squared to get power as a function of time, then looks like the data in Figure 4. The filter function can probably be tuned a bit for a better balance between signal-to-noise ratio and pulse resolution.
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Figure 4. Pulse in Figure 1 filtered by the function shown in Figure 3. PULSE ARRIVAL TIMES All 5 seconds of the data were then filtered and searched for identifiable separate pulses above most of the highest random noise peaks. The measured arrival times with the 100-microsecond offsets and the average expected pulse arrival time removed are shown in Figure 5. The first part of the data run was found to have a constant pulse arrival time by assuming a pulse repetition rate of 341.4142 pulses per second. The drift in arrival times toward the end of the 5 seconds of data was due to either a drift in the radar timing generator or the internal clock of our data acquisition board of about 60 ppm in frequency. This drift was removed empirically with a polynomial time correction function fitted by eye to the arrival times.
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Figure 5. Pulse arrival times for the full 'sweep_minus1s.dat' file data set with the 100-microsecond offsets and the average expected pulse arrival time removed. At least three features in Figure 5 stand out. First, pulses at constant arrival times are present during the full length of the data set. This is no doubt due to the fact that we are seeing pulses from the radar even when the radar beam is pointed well away from Green Bank. Since the receiver saturated with the beam pointed in our direction, we cannot determine the relative strengths of the sidelobes, but they are at least 30 dB below the main beam. The second notable feature of Figure 5 is that the earliest pulses are not the most prevalent. The shortest distance, great circle, diffraction path from the radar to the GBT must produce the earliest arriving pulses. Evidently a longer path has a lower propagation loss since the pulses from the radar antenna sidelobes are most apparent at an extra delay of about 40 microseconds. The plots below will show that this lower propagation loss is produced by a reflection from the high mountain ridge about 8 km west of the GBT. A great circle plot of the terrain profile in the direction of the radar shows that the GBT is about 400 meters below the elevation of the nearest diffraction obstacle about 12 km away. The mountain ridge west of the GBT includes Bald Knob, the second highest peak in West Virginia. The third notable feature of Figure 5 is the cluster of pulses around 1.1 seconds into the data sample. This is when the radar beam passes over Green Bank, and we see reflections from local terrain features in addition to the directly arriving pulse.
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PULSE REPETITION FREQUENCY (PRF): The Pulse Repetition Frequency (PRF) of the radar system is the number of pulses that are transmitted per second. Radar systems radiate each pulse at the carrier frequency during transmit time (or Pulse Width PW), wait for returning echoes during listening or rest time, and then radiate the next pulse, as shown in the figure. The time between the beginning of one pulse and the start of the next pulse is called pulserepetition time (prt) and is equal to the reciprocal of prf as follows: PRT = 1/ PRF The radar system pulse repetition frequency determines its ability to unambiguously measure target range and range rate in a single coherent processing interval as well as determining the inherent clutter rejection capabilities of the radar system. In order to obtain an unambiguous measurement of target range, the interval between radar pulses must be greater than the time required for a single pulse to propagate to a target at a given range and back. The maximum unambiguous range is then given by, Runamb = Co /1/ 2.PRF Runamb = Co . PRT / 1/ 2.PRF
where c0 is the velocity of electromagnetic propagation.
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UNIT QUESTIONS
ASSESMENT QUESTIONS:
1. Explain about basic radar system in detail. 2. Define the radar range equation. 3. write about radar frequencies 4. Explain about pulse repetition frequency. 5. Define signal to noise ratio at the receiver.
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19 UNIT 2
VARIOUS RADAR SYSTEMS PULSE-RADAR SYSTEMS:
INTRODUCTION:
Basic Pulse-Radar System The two basic types of radar systems, pulse radars and continuous-wave radars, use pulse and continuous-wave energy transmission.
There are two
types of pulse radars and the two types of continuous-wave radars.
The pulse-radar systems include radar and the pulse-Doppler radar. The signal of a basic pulse-radar system is gen- erated by the transmitter and is radiated into space by the antenna. Intermediate frequencies from 30 to 60 MHz are commonly used because signal handling is easier to accomplish at the lower frequencies than at the transmitter frequencies. The duplexer enables the use of a single antenna to transmit and receive the radar signal. The
return echo signal is then mixed with a local oscillator (LO) signal
to
produce
an
intermediate frequency (IF) signal at a lower frequency than the transmitter.
The
IF
filter
conditions
the
echo
signal
through amplifying and filtering extraneous
signals. The IF signal is then sent to the second detector where the IF is converted to a lower frequency video signal. At that time, the video signal is processed for display by a video amplifier. The display is usually a cathode-ray tube (CRT) the basic pulse that is monitored by an operator. The timer/synchro- nizer controls the repetition frequency of the transmitter. It can also provide a zero range start signal for the display device. Basic pulse-radar systems are rather complex in their composition, but they all contain the same basic functional areas, with additional equipment included for specific purposes. For instance, a search radar requires additional circuitry to indicate antenna azi- muth position coincident with a particular target echo. radar for moving-target indication (MTI) and to filter out stationary targets, landmasses, and clutter from weather and the sea state.
A tracking radar, such as a fire-control radar, re- quires additional circuitry to measure target range, az- imuth, and elevation. Since circuitry is also required to keep the antenna pointed at the target, fire-control radars
have
ranging
and
angle
tracking
systems
in-
Additional circuitry might also be added to a search — cluded. Figure shows a basic pulse radar.
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CW DOPPLER RADAR:
The radar systems discussed in the previous sections have their operations based upon transmission of pulses of high energy and reception of reflected pulses from the target. The CW radars, on the other hand, transmit electromagnetic waves continuously towards the target and there is a continuous reflection of these waves from the targets.
It was possible to use a single antenna for transmission and reception in pulsed radars and this was achieved with the duplexer switch. In CW such, separate antennae are required for transmission and reception.
The CW radar makes use of Doppler effect for speed measurements of targets. Figure shows the block diagram of a CW Doppler radar.
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BLOCK DIAGRAM OF A CW DOPPLER RADAR
CW oscillator f1
f0 Transmitting antenna
f1
IF oscillator f1+-fd
Detector fd
Transmitter mixerft + fi
fc +-fd
IF amplifier
Receiver mixer
ft+fd Receiving antenna
AF amplifier fd To frequency counter
The transmission section is a low power microwave oscillator such as reflex klystron that generates sinusoidal signals in the microwave rage. This signal is transmitted by the transmitting antenna. A small fraction of the transmitter signal is fed to the transmitter mixer, to which is also fed the IF signal generated by IF oscillator. Sum of the transmitter signal frequency (f 1) and the IF signal (fc) is selected at the output of the transmitter mixer.
The receiver antenna picks up the waves reflected form the target and for moving targets, the received signal frequency equals f 1+-f. This signal is given to the receiver mixer where it mixes with the
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output of the transmitter mixer. At the output of the receiver mixer is obtained the difference frequency signal at (fc+-fd). This signal is amplified by the IF amplifier and given to the detector stage. The detector circuit recovers the Doppler frequency from the IF signal and passes it to the AF amplifier where it is amplified. The amplified signal is given to a frequency counter. Since the Doppler frequency shift f d is proportional to the velocity of the target, the output of the counter gives an indication of the target speed.
The frequency counter is so designed that at its output the target speed is displayed directly in kilometer/hour rather than showing the Doppler frequency. However, the display does not give indication as to whether the target is approaching or receding, because sign of the Doppler frequency shift is lost. The CW Doppler radar is not capable of giving the range of the target.
An important advantage of CW Doppler radar is that it uses low transmitting power, low power consumption, simple circuitry and small size. This makes it mobile. It can be used to give accurate measurement of relative velocity of the target and the reading obtained is unaffected by the presence of stationary objects. It is capable of measuring a large range of target velocities from a very low value to a high value quickly. The most common application of CW Doppler radars is in checking the speeds of vehicles and for this purpose it is widely used by Police. It is also used in aircraft navigation for speed measurement and as rate-of-climb meter.
In spite of these applications, it has certain drawbacks. Firstly, it is limited in the maximum power it transmits and this places a limit on its maximum range. Secondly, it is not capable of indicating the range of the target and can show its velocity only. Lastly, if a large number of targets are present, then it gets confused rather easily.
FREQUENCY MODULATED CW RADAR: CW Doppler radar cannot give the range of a target, because the transmitted signal is unmodulated. As a result, the receiver cannot sense which particular cycle of oscillations id being received at at moment. If the transmitted carrier is frequency modulated, then it should be possible to eliminate this main drawback . Using frequency modulation will, however, increase the bandwidth and thus it is seen that for conveying more information, more bandwidth is required.
Figure shows the block diagram of a frequency modulated CW radar used in aircraft for measurement of their altitudes. For this reason, it is commonly named as airborne altimeters. Here a sawtooth wave is used for frequency modulating signal but the sawtooth waveform gives the simplest circuit arrangement. Thus the frequency of the transmitted signal increases linearly with the increasing amplitude of the modulating signal. It this case the target is the earth which is stationary with respect to the aircraft.
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BLOCK DIAGRAM OF A FM - CW DOPPLER RADAR
Receiving
Transmitting
antenna
antenna
Mixer
CW Transmitter modulator
Sawtooth generator
Amplifier
Limiter
Frequency modulator
Frequency counter
Indicator
Since increase in the amplitude of modulating signal is uniform with time, therefore, the rate of increase in frequency in the transmitted signal brought by the modulating signal is also uniform with time. For a given height of aircraft, a known time will be required for the waves to travel from earth to the aircraft. Thus during this time a definite change in signal frequency will take place. Viewing it the other way, if we are ale to measure the frequency change in the signal, it will give an indication of the height of the aircraft. This suppose we switch on a frequency counter just when the modulating signal has zero frequency i.e., the transmitted frequency equals (f c). This signal frequency is picked up after its reflection from earth and may be used to switch – off the counter. The final reading of the counter will give an indication of the change in the frequency and hence about the height of the aircraft. This is indicated by the indicator connected in the receiver.
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Now let us consider the case when the relative velocity of the aircraft and the earth is not zero. This will produce another frequency shift due to the Doppler effect and this frequency shift will be superimposed on the frequency difference discussed in the preceding paragraph. This frequency shift can now be used to measure the relative velocity of the aircraft in the same way is in Doppler radar. However, the time difference between the transmission and reception of a particular cycle of the signal will be constant and hence the average frequency difference will also be constant. Therefore, correct height measurement can still be made on the basis of average frequency difference.
The major application of FM-CW radar is as altimeter in aircraft and because of short range involved, it is used in preference to pulsed radars. Another advantage is that it has it has quire a low power requirements as compared to pulsed radars. Another advantage is that is has quire a low power requirement as compared to pulsed radars. The size of this equipment is small and quite suitable for aircraft installations. Because reflection has to take place from earth which has a large size as compared to aircraft a small size can be used. The transmitting powers used are quire small (a few watts) and for this reason devices such as IMPATT diode. Or reflex klystron may be used in the transmitter.
TRACKING RADAR:
Once a target has been located by a search-radar, it may then be tracked. For this purpose, radars with pencil beam radiation are used. A radar that is used purely for tracking may employ conical scan or monopoles system. A radar that provides angular information of the target accurately is said to be tracking angle. If it provides range information continuously and accurately, it is said to be tracking in range.
In ground and shipbrone use, there is sufficient space available and separate search and tracking radars may be employed. In aeroplanes, the availability of space is limited and generally the same radar is used for search as well as tracking.
MOVING TARGET INDICATOR (MTI)
In PPI display, there is a lot of clutter due to echoes corresponding to stationary targets. When it is desired to remove this clutter or grass from the display. MTI or moving target indicator may be employed. The MTI makes use of Doppler Effect for its operation.
DOPPLER EFFECT:
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The apparent frequency of electromagnetic or sound wave is dependent upon the relative motion of the source and the observer. To understand this phenomenon, consider an observer standing on a platform approaching a fixed source of radiation with a relative velocity +v r. If both the observer and the source of energy are stationary and the frequency of radiation is f n then he would note ft crests of wave per second padding beyond him. If he is moving forward at a velocity V r, he will come across more than f 1 crests per second. The number of crests observed under this condition is given by
f1 + fd = ft 1 + Vr Vc Where Vc is the velocity of the wave and fd is the Doppler frequency difference of shift. In radar, the signal undergoes the Doppler frequency shift when impinging upon a moving target. As this target reflects the waves, we may consider it as a moving source, transmitting energy towards a stationary observer. Thus we have another Doppler shift. Hence Doppler frequency in a radar is given as
2Vr.f1 Fd
= 2fd’ = Vc
= 2vr
f1
=
Vc
It should be noted that this Doppler frequency shift will take place only if the target moves radially and not in tangential motion.
The Doppler frequency shift may be used to determine the relative velocity of the target. Thus moving targets can be distinguished from stationary targets on the basis of Doppler frequency shift.
MTI PRINCIPLE:
In principle, a moving target indicator system compares a set of received echo pulses with those received during the previous sweep. The echoes belonging to the stationary targets cancel out while those corresponding to moving targets do not cancel and shoe only a phase change. Thus, the clutter is
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completely removed from the display and it reduces the time taken by the operator to observe the target. It allows easy detection of moving targets whose echoes are hundreds of time smaller than those of near by stationary targets. This would not have been possible without the use of MTI. Block diagram of a MTI radar is shown in Figure.
The radar transmitter frequency in the MTI system, shown in Figure, is given by the sum of two oscillators produced at the output of mixer 2. The first of these oscillators is the STALO which stands for stable local oscillator and oscillates at a frequency f 0. The other oscillator is the COHO or coherent oscillator, oscillating at a frequency fc. This frequency is the same as the intermediate frequency of the receiver and for this reason it is termed as the coherent frequency. The sum frequency (f 0 + fc) is given as input signal to the output tube which is a multicavity klystron amplifier in this case. This amplifier amplifies the signal and provides a high power pulse when modulator switches on this tube. The transmitter output pulse is passed on to the antenna through the duplexer.
The transmitted pulse is received back by the radar antenna after its reflection from the target. In case of a moving target, the received pulse undergoes a Doppler frequency shift. The received pulses are passed on to the mixer 1 of the receiver. The Mixer heterodynes the received signal of frequency (f 0+fc) with the output of the STALO at f0 and gives the output at the difference frequency f c. The stages Mixer 1 and Mixer 2 are similar in all respects except that the output frequencies are different. It is the difference frequency in Mixer 1 and sum frequency in Mixer 2.
The difference frequency signal present at the output of mixer amplified by the IF amplifier and given to the phase sensitive detector, This director compares this IF signal with the reference signal obtained from COHO stage and gives an output depending upon the phase difference between the two signals. Since all received signal pulses will have a phase difference compared with the transmitted pulse, phase will have a phase difference compared with the transmitted pulse, phase detector gives output for stationary as well as moving targets. While the phase shifts for the stationary targets remain constant, for moving targets remain constant, for moving targets phase shifts for the stationary targets remain constant, for moving targets phase shifts are changing.
This happens because of Doppler effect in moving targets. A change of half cycle in the Doppler frequency shift would cause an output of opposite polarity in the phase detector output. Thus the output of the phase detector will have an output that has different magnitudes and polarities for successive pulses in case of a moving target, whereas in fixed targets the magnitude and polarity of the output will remain the same for all the transmitted pulses as shown in figure 13.6.
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Block Diagram of MTI radar with klystron output tube
Klystorn amplifier
Duplexer
f0+fc
f0+fc
Modulator f0
fc
Mixerfc 2
Mixer 1 fc
Stalo f0 IF amplifier Coho Fc
fc
Video
Phase detector T = 1/PRF
Amplifier 2
Subtractor
Amplifier 1
Delay line
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The video output of the phase detector so obtained is applied to a delay line which has a delay time that exactly corresponds to the time period of the transmitted pulses. The delayed output is amplified by amplifier 1 and given to the subtractor. At the same time the undelayed video output of the detector is also amplified and given to the subtractor. For a stationary target signal pulse, such as (1) in Figure 13.6(a) the delayed pulse at the subtractor. For a stationary target signal pulse, such as (1) in Figure), the delayed pulse at the subtractor input will be (2) of the Figure and when subtracted these will cancel out each other, as shown in Figure.
However, for and when subtracted these will cancel out each other. However, for moving target pulses, the delayed pulses and the following pulse will have different magnitude/phase and will not cancel each other when subtracted in the subtracted circuit. As a result, the subtractor output for moving targets is shown by the display.
PULSE-DOPPLER RADAR SYSTEM:
A pulse-Doppler radar has certain advantages over a basic pulse radar or a continuouswave radar. It can detect both stationary and moving targets and can also determine range. In addition, it can distinguish between two targets with the same radial velocity, but at different ranges. The radial velocity is the apparent speed that the target is closing on or going away from the radar.
A
pulse-Doppler
radar
has
certain
disadvantages too:
blind
target
ranges
and
velocities, range and delivery ambiguities, and reduction in maximum range capabilities. These disadvantages
can
be
compensated for by using additional circuitry. A pulse radar’s echo
signal also contains velocity information in the Doppler frequency information, but the signal is not normally used by a basic pulse radar. By using the Doppler signal available on an echo signal, a basic pulse radar can detect a weak signal from a moving target in the presence of strong signals from large targets, such as landmasses and heavy seas.
Pulse-Doppler radars use the Doppler shift signal. These radars can detect an aircraft flying close to a hill or mountain where the strong landmass echo would block detection with a basic pulse radar. Circuitry in the pulse-Doppler radar normally would reject the stationary target, allowing easy detection of the weak signal from the moving target. The ranging system of
a
pulse-Doppler
modulated,
radar
continuous-wave
is more (FM-CW)
complex
than
that
of
a
pulse
or
a
frequency-
radar.
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PULSE-DOPPLER RADAR SYSTEM
A pulse-Doppler radar senses both range and velocity by time-sharing its waveform between these functions. To detect a Doppler frequency from the target echo, most pulseDoppler radars use a much higher pulse- repetition frequency (PRF) than basic pulse radars. Higher
PRF
decreases
the
pulse-repetition
time (PRT)
between
pulses,
resulting
in
the
possibility of a target echo returning at the time of the next trans- mission. This, in turn, results in a blind spot in the range.
If the echo from the first pulse returns after the second pulse is transmitted, then a range
ambiguity occurs. The range blind spot and ambiguities can be compensated for by
changing the PRF over a wide range.
For example, the fire-control computer could adjust the radar PRF based on the expected range of a designated target. If the designation were for a target at 50 nmi (100 kyd), the PRF could be changed so that the second pulse would not occur until enough time had elapsed for the target echo to return for that range, plus an additional range interval for the acquisition and tracking
gates.
Varying
the
PRF
over
a
wide range
by
computer
control
can
resolve
range ambi- guities and blind ranges.
DOPPLER SHIFT THEORY:
A Doppler shift allows distinguishing between the target and the trans- mitter leakage. The amount of Doppler shift is deter- mined by the radial velocity of the target since the radial velocity is the apparent speed that the target is closing on or going away from the radar.
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A target can move in any direction and in a wide range
of
speed;
therefore,
the
radial
velocity can change considerably. If the target is moving at a 900 angle to the radar, then no Doppler shift is produced.
However, if the target moves straight at or away from the radar, radial velocity will equal the actual target speed. The amount of Doppler shift is also dependent on the resulting from
wavelength
the transmitter fre- quency. A target radial velocity that produces a spe- cific
Doppler shift at 5,000 MHz would produce twice as much at 10,000 MHz.
MONOPULSE
SCANNING:
With monopulse (simultaneous lobing) scanning, range, bearing, and elevation angle information
of
a target is obtained from, as the name implies, a single pulse. This type of
tracking radar normally produces a narrow circular beam of pulsed RF energy at a high pulserepetition rate (PRR). Each pulse is divided into four signals that are equal in amplitude and phase. The four signals are radiated at the same time from each of four feed horns that are grouped in a cluster. The radiated energy is focused into a beam by a microwave lens.
In turn, energy reflected from the target is refocused by the lens into the feed horns. The amount of the total energy received by each horn varies, depending on the position of the target relative to the beam axis. The four targets are at different posi- tions with respect to the beam axis. A phase inversion takes place at the microwave lens similar to the image inversion in an optical system. The amplitude of returned signals received by each horn is continuously compared with those re- ceived in the other horns, and error signals are generated that indicate the relative position of the target with respect to the axis of the beam. Angle servo cir- cuits receive these error signals and correct the posi- tion of the radar antenna and the director to keep the beam axis on target.
An important advantage of a monopulse-tracking radar over a radar using conical scan is that the instan- taneous
angular
measurements
are
not
subject
to
er- rors caused by
target scintillation. Scintillation is the rapid fluctuation of the echo signal amplitude as the target maneuvers or moves, resulting in radar beams bouncing off different areas of the target and causing random reflectivity, which may lead to tracking er- rors. A monopulse-tracking radar is not subject to this error
because
each
pulse
provides
an
angular
meas- urement without
regard to the rest of the pulse train; therefore, scintillation does not affect the measure- ment.
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An additional advantage of monopulse tracking is that required,
such
as
a scanner and the monopulse variations
of
no
mechanical
received
energy
action with
is
target
positions.
RADAR RECEIVER: The received RF-signals must transformed in a videosignal to get the wanted informations from the echoes. This transformation is made by a super heterodyne receiver. The main components of the typical superheterodyne receiver are shown on the following picture:
Figure 1: Block diagram The superheterodyne receiver changes the rf frequency into an easier to process lower IFfrequency. This IF- frequency will be amplified and demodulated to get a videosignal. The Figure shows a block diagram of a typical superheterodyne receiver. The RF-carrier comes in from the antenna and is applied to a filter. The output of the filter are only the frequencies of the desired frequency-band. These frequencies are applied to the mixer stage.
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The mixer also receives an input from the local oscillator. These two signals are beat together to obtain the IF through the process of heterodyning. There is a fixed difference in frequency between the local oscillator and the rf-signal at all times by tuning the local oscillator. This difference in frequency is the IF. This fixed difference an ganged tuning ensures a constant IF over the frequency range of the receiver. The IF-carrier is applied to the IF-amplifier. The amplified IF is then sent to the detector. The output of the detector is the video component of the input signal. IMAGE-FREQUENCY FILTER: A low-noise RF amplifier stage ahead of the converter stage provides enough selectivity to reduce the image-frequency response by rejecting these unwanted signals and adds to the sensitivity of the receiver. Many older radar receivers do not use a low-noise pre-amplifier (RF stage) as the receiver front end; they simply send the echo signal directly to a crystal mixer stage. This has any disadvantages. It is possible for these receivers to receive two different stations at the same point of the dial. MIXER STAGE: The mixer stage is used to increase the received frequency to an intermediate frequency. The mixer also receives an input from the local oscillator. These two signals are beat together to obtain the IF through the process of heterodyning.
fif = frx - flocal oscillator fif = flocal oscillator - frx There aren't any components which can distinguish a negative frequency of a positive frequency.
therefore we can measure the magnitude of the frequency only: fif = | flocal oscillator - frx. | The result is a second reception frequency as a „mirror image” around the intermediate frequency.
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IF- AMPLIFIER The IF amplifier has the capability to vary both the bandpass and the gain of a receiver. After conversion to the intermediate frequency, the signal is amplified in several IF- amplifier stages. Most of the gain of the receiver is developed in the IF amplifier stages. The overall bandwidth of the receiver is often determined by the bandwidth of the IF stages. Gain must be variable to provide a constant voltage output for input signals of different amplitudes. DETECTOR:
Figure 2: Scan from a screen of an oszilloscope The detector in a microwave receiver serves to convert the IF pulses into video pulses.
Figure 3: a simple detector The simplest form of detector is the diode detector. It detects the pulse envelope: The condenser has got the function of a lowpass and blocks the IF- frequency.In addition to the shown Amplitude Modulation there are possible other types of modulation too. VIDEO AMPLIFIER: The video amplifier receives pulses from the detector and amplifies these pulses for application to the indicating device. A video amplifier is fundamentally an RC coupled amplifier that uses high-gain
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transistors. However, a video amplifier must be capable of a relatively wide Frequency response. The output stage of the receiver is normally an emitter follower. The low-impedance output of the emitter follower matches the impedance of the cable. The video pulses are coupled through the cable to the indicator for video display on the crt. LOCAL OSCILLATOR: The local oscillator excite a frequency for mixing with the incoming signal to get the intermediate frequency.Most radar receivers use megahertz intermediate frequency (IF) with a value between 30 and 75 megahertz. The IF is produced by mixing a local oscillator signal with the incoming signal. The local oscillator is, therefore, essential to efficient operation and must be both tunable and very stable. The power output requirement for most local oscillators is small (20 to 50 milliwatts) because most receivers use crystal mixers that require very little power. The local oscillator output frequency must be tunable over a range of several megahertz in the 4,000megahertz region. The local oscillator must compensate for any changes in the transmitted frequency and maintain a constant 30 or 75 megahertz difference between the oscillator and the transmitter frequency. A local oscillator that can be tuned by varying the applied voltage is most desirable. The exiting frequency is either higher or lower than the incoming frequency. An RF amplifier stage ahead of the converter stage provides enough selectivity to reduce the image-frequency response by rejecting these unwanted signals and adds to the sensitivity of the receiver. AUTOMATIC GAIN CONTROL (AGC)
Figure 2: automatic gain control block diagram Gain control is necessary to adjust the receiver sensitivity for the best reception of signals of widely varying amplitudes. A complex form of automatic gain control (agc) or instantaneous automatic gain control (IAGC) is used during normal operation. The simplest type of AGC adjusts the IF amplifier bias (and gain) according to the average level of the received signal. With AGC, gain is controlled by the
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largest received signals. When several radar signals are being received simultaneously, the weakest signal may be of greatest interest. IAGC is used more frequently because it adjusts receiver gain for each signal. The AGC circuit is essentially a wide-band, dc amplifier. It instantaneously controls the gain of the IF amplifier as the radar return signal changes in amplitude. The effect of IAGC is to allow full amplification of weak signals and to decrease the amplification of strong signals. The range of IAGC is limited, however, by the number of IF stages in which gain is controlled. When only one IF stage is controlled, the range of iagc is limited to approximately 20 dB. When more than one IF stage is controlled, iagc range can be increased to approximately 40 dB. LOGARITHMIC AMPLIFIER: The logarithmic amplifier is a non-saturating amplifier that does not ordinarily use any special gain-control circuits. The output voltage of the logarithmic amplifier is a linear function of the input voltage for low-amplitude signals. It is a logarithmic function for high-amplitude signals. In other words, the range of linear amplification does not end at a definite saturation point, as is the case in normal IF amplifiers. Therefore, a large signal does not saturate the logarithmic amplifier; rather, it merely reduces the amplification of a simultaneously applied small signal.
Figure 3: logaritmic amplifier block diagram A typical circuit for obtaining a logarithmic response is shown in the figure. If detectors 2 to 5 were not present, the output voltage would be limited by the saturation point of the final IF stage, as it is in a normal IF section. However, when the final stage of the logarithmic amplifier is saturated, larger signals cause an increase in the output of the next to last stage. This increase is detected by detector 2 and summed with the output of detector 1. This sum produces an increase in the output even though the final stage is saturated. Detector 3 causes the output
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to continue to increase after the second stage saturates. The overall gain becomes less and less as each stage saturates, but some degree of amplification is still available. The proper choice of IF stage gains and saturation points produces an approximately logarithmic response curve. CLUTTER: Radar returns are produced from nearly all surfaces when illuminated by a radar. Therefore, in competition with the return from an aircraft, there are many sources of unwanted signals. Unwanted signals in a search radar are generally described as noise and clutter. Clutter is the term used and includes ground returns, sea returns, weather, buildings, birds and insects. The definition of clutter depends on the function of the radar. Weather is not clutter in a weather detecting radar. Since aircraft usually move much faster than weather or surface targets, velocity-sensitive radar can eliminate unwanted clutter from the radar indicator. Radar systems that detect and process only moving targets are called Moving-Target Indicators (MTI). The basic types of clutter can be summarized as follows:
Surface Clutter – Ground or sea returns are typical surface clutter. Returns from geographical land masses are generally stationary, however, the effect of wind on trees etc means that the target can introduce a Doppler Shift to the radar return. This Doppler shift is an important method of removing unwanted signals in the signal processing
part of a radar system. Clutter returned from the sea generally also has movement associated with thewaves.
Volume
Clutter
–
Weather
or
chaff
are
typical
volume
clutter.
In the air, the most significant problem is weather clutter. This can be produced from rain or snow and
can
have
a
significant
Doppler
content.
Point Clutter – Birds, windmills and individual tall buildings are typical point clutter and are not extended in nature. Moving point clutter is sometimes described as angels. Birds and insects produce clutter, which can be very difficult to remove because the characteristics are very much like aircraft.
Clutter can be fluctuating or non-fluctuating. Ground clutter is generally non- fluctuating in nature because the physical features are normally static. On the other hand, weather clutter is mobile under the influence of wind and is generally considered fluctuating in nature.
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Clutter can be defined as homogeneous if the density of all the returns is uniform. Most types of surface and volume clutter are analysed on this basis, however, in practice this simplification does not hold good in all cases. Non-homogeneous clutter is non uniform clutter where the amplitude of the clutter varies significantly from cell to cell. Typically non-homogeneous clutter is generated by tall buildings in built up areas.
ASSESMENT QUESTIONS:
1. Explain about pulse radar system with neat diagram. 2. Explain about CW-radar. 3. Write a note on FMCW radar. 4. Explain the principles of MTI radar. 5. Write about pulse Doppler radar. 6. Write about monopulse tracking. 7. Explain about radar receivers.
--------------------------------------------------------------------THE END--------------------------------------------------------
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