RANGE-GATED DOPPLER FILTER

     

The delay-line canceler, which can be considered as a time-domain filter, has been widely used in MTI radar as the means for separating moving target from stationary clutter. It is also possible to employ the more usual  frequency-domain  bandpass  filters of conventional design in MTI  radar to sort the doppler-frequency-shifted targets , The filter  configuration  must be more complex ,however ,than the single ,narrow-bandpass filter.A narrow filter with passband designed to pass the the doppler  frequency components of moving  targets  will  "ring" when excited  by the usual short radar pulse.That is ,its  passband is much narrower than the input. The narrow band filter"smears" the input pulse since the impulse response is approximately the reciprocal of the filter bandwidth. This smearing destroys the target  resolution.If more than one target is present they cannot be resolved. Even if only one target were the noise from the other range cells that do not contain the target will interfere with the desired target signal.                                                                                                                  

The loss of the range information and the collapsing loss may be eliminated by first quantizing the range (time) into small intervals. This process is called range gating The width of the range gates depends upon the range accuracy desired and the complexity which can be tolerated, but they are usually of the order of the pulse width.Range resolution is established by gating. Once the radar return is quantized  into range intervals,the output from each gate may be applied to a narrowband filter since the pulse shape need no longer be preserved for range resolution. A collapsing loss dose not take place since from the other range intervals is excluded.                                                                           

A black diagram of the video of an MTI radar with multiple range gates followed by clutter-rejection filters.the output of the phase detector is sampled sequentially by the range gates. Each range gates opens in sequence just of the long enough to sample the voltage of the video wave from  corresponding to a different range interval in space.The range gate acts as a gate which opens and closes at the proper time.The range gates are activated once each pulse-repetition interval.The output for a series of pulse which vary in amplitude according to the doppler frequency The output range gates is stretched in a circuit called the boxcar generator, or sample-and hold circuit, whose purpose is to aid in the filtering and detection process by emphasizing the fundamental  of the modulation frequency and eliminating harmonics of the pulse repetition frequency.The clutter rejection filter is a bandpass filter whose bandwidth depends upon the expected clutter spectrum                                               

   Following the doppler filter is a full-wave linear detector and an integrator (a low-pass filter). The purpose of the detector is to convert the bipolar video to unipolar video.The  output of the integrator is applied t a threshold-detection circuit. Only those signals which cross the threshold are reported as targets.Following the threshold detector, the output from each of the range channels must be properly  combined for display on the PPI or A scope or for any other appears "cleaner"than the display from a normal MTI radar, not only because of better clutter rejection, but also because the threshold device eliminates many of the unwanted false alarms due to noise. The shape of the rejection band is determined primarily by the shape of the bandpass filter.                                                                             

The bandpass filter can be designed with a variable low-frequency cutoff that can be selected  to the prevailing clutter conditions.The selection of the lower that cutoff might be at the operator  or it can be done deceptively. MTI radar using range gates and filters is usually more complex than an MTI with a signal-delay-line canceler. The additional complexity is justified in those applications where god MTI performance and the flexibility of the range gates and filter MTI are desired. The better MTI performance result from the better match between the clutter filter characteristic and the clutter spectrum.

APPLICATIONS OF CW RADAR

         The chief use of the simple, unmodulated CW radar is fro the measurement of the relative velocity of a moving target, an in the police speed monitor or in the previously mentioned rate -of-climb meter for  vertical -take-off aircraft.In support of automobile traffic,CW radar has been suggested for the control of traffic light, regulation of toll booths, vehicle counting as a replacement for the "fifth-wheel"speedometer in vehicle testing as a sensor in anti lock braking systems, and for collision avoidance. For railway. CW radar can be used as a speedometer to replace the conventional axle driven tachometer.In such an application it  would be unaffected by errors caused by wheelslip  on accelerating or wheelslide when braking. It has been used for the measurement of railroad-freight-car velocity during humping operations in marshalling  yards,and as a detection device to give track maintenance personnel advance warning of approaching trains. CW radar is also employed for monitoring the docking speed of the velocity of missiles, ammunition,and baseballs.                              

The principal advantage of a CW doppler radar over ( non radar) methods of measuring speed is that there need not be any physical contact with the object whose speed is being measured. Most of the above applications can be satisfied with a simple, solid-state CW source with powers in the tens of milliwatts. High-power CW radar for the detection of aircraft and other targets have been developed and have used in such systems as the Hawk missile systems.However, the difficulty of eliminating the leakage of the transmitter signal into the receiver has limited the utility of unmodulated CW radar fro many long-range applications.A notable exception is the space Surveillance  system. The CW radar, when used for short or moderate ranges, is characterized by simpler equipment than a pulse radar. The amount of power that can be used with a CW radar is dependent on the isolation that con be achieved between the transmitter and receiver since the transmitter noise that finds its way into the receiver sensitivity.                                                                                                                                        

Perhaps one of the greatest shortcomings of the simple CW radar is its inability to obtain a measurement of range.This limitation can be overcome by modulating the CW carrier, as in the frequency-modulated radar described in the next section.Some anti-air-warfare guided missile systems activeness homing guidance which a receiver in the missile receives energy from the target,the energy having been transmitted  from an "illuminator"  external to the missile. The illuminator,for example,might be at the launch plat from. CW illumination has been used in many successful systems.It is a tracking  radar as well as an illuminator since it must be able to follow the target as it travels through space. the doppler discrimination of a CW radar allows operation in the presence of clutter and has been well suited for low altitude missile defense systems. A block diagram of a CW tracking illuminator. Note that following the wide-diagram amplifier is a speed gate, which is a narrow-band tracking filter that acquires the target's doppler and tracks its changing doppler frequency shift.

CW AND FREQUENCY-MODULATED RADAR

                                                                                                                                                                   

    

     

      Consider the simple CW Radar.The transmitter generates a continuous(modulated)oscillation of  frequency by the antenna A portion of the radiated energy is intercepted  by the target and is scattered, some of it in the direction of the radar.where it is collected by the receiving antenna. If the target is in  motion with a velocity V r  relative to the radar, the received signal will be shifted in frequency  from the transmitted frequency f 0 by an amount± f d  as given by E q. the plus sign associated with the doppler frequency applies if the distance between target and radar is decreasing  (closing target ), that is when the received signal frequency is greater than the transmitted signal frequency. The minus sign applies if the distance is increasing (receding target). The received echo signal at a frequency  f 0 ± f d  inters the radar via the  antenna and is heterodyned  in the detector (mixer) with a portion of the transmitter signal f 0  to produce a doppler beat not of frequency f d. The sign of f d  is lost in this process. The purpose of the doppler amplifier is to eliminate echoes from stationary targets and amplify the doppler echo signal to a level where it can operate an indicating device. It might have a frequency-response characteristic. The low- frequency cutoff must be high to reject the d-c component caused by stationary targets, but yet it must be low enough to  pass the smallest doppler frequency expected. Sometimes both conditions cannot be met simultaneously and s compromise is necessary. The upper cutoff frequency is selected  to pass the highest doppler frequency expected.The indicator might be a pair of earphones of a frequency meter.If exact knowledge of the doppler frequency is not necessary, earphones are especially attractive provided the doppler frequencies lie within the audio-frequency response of the ear. 

 Isolation between transmitter and receiver :A signal antenna serves the purpose of transmission and reception in the simple CW radar described above. In principle, a signal antenna may be employed since the  necessary isolation between the transmitted and the received signals is achieved via  separation   in frequency as a result of the doppler effect. In practice, it is not possible to eliminate completely the  transmitter leakage. There are to practical effects which limit the amount of transmitter leakage power  which can be tolerated at the receiver. These are the maximum amount of power the receiver input circuitry can withstand before it is physically damaged are its sensitivity reduced and the amount of transmitter noise due to hum, micro phonics, stray pick -up, and instability which enters the receiver from the  transmitter. The amount of isolation required depends on the transmitter power and the accompanying transmitter noise as will as ruggedness and the sensitivity of the receiver. For example if the safe value of power which might be applied to s receiver  were 10 m W and if the transmitter power were 1 K W, the isolation between and receiver must be at least 50 dB The amount of isolation needed in a long - range CW radar  is more often determined by the noise that accompanies the transmitter leakage signal rather than by any damage caused by high power.For example, suppose the isolation between the transmitter and receiver were such that 10 m W of leakage signal appeared at the minimum detectable signal were 10-13 watt (100 dB below  1 m W), transmitter noise must be at least 110 dB (preferably 120 or 130 dB ) below the transmitted carrier       The transmitter noise of concern in radar includes those noise components that lie within the same frequency range as the doppler frequencies.The greater the desired radar range, the more stringent will be the need for reducing the noise modulation accompanying the transmitter signal. If complete elimination of the direct leakage signal at the receiver could to achieved, it might not entirely solve the isolation problem since echoes from nearby fixed targets can also contain the noise components of the transmitted signal.                                                                                                                                      

It will be recalled that the receiver of  a pulsed radar is isolated and protected from the damaging effects of the transmitted pulse by the duplexer, which short-circuits the receiver input during the transmission  period. Turning off the receiver during transmission with a duplexer is not possible in a C W radar since the transmitter is operated continuously.  Isolation between transmitter and receiver might be obtained with a signal antenna by using a hybrid junction, circulator, turnstile junction, or with separate polarization.                                                                                                                           

Ferrite isolation devices such as the circulator do not suffer the 6-dB loss inherent in the hybrid junction.practical devices have isolation of the order of 20 to 50 dB. Turnstile junction achieve isolation of  high as 40 to 60 dB                                                                                                                             

The largest factor are obtained with two antennas -one for transmission, the other for reception -physically separated from one another. Isolation of the order of 80 d B  or more are possible with high -gain antenna. The more directive the antenna beam and the greater the spacing between antenna.  

          

THE RADAR RECEIVER

   THE  RADAR  RECEIVER        

        The function of the radar receiver is to detect desired echo signals in the presence of noise, interference,or clutter.It must separate wanted from unwanted signals, and amplify the wanted signals to a level where target information can be displayed to an operator or used in an automatic data processor. The design of the radar receiver will depend not only on the type of waveform to be detected,but on the nature of the noise, interference,and clutter echoes with which the desired echo signals must compete.In this chapter,the receiver design is considered mainly as a problem of extracting desired signals from noise.Noise can enter the receiver via the antenna terminals along with the desired signals, or it might be generated within the receiver it self.At the microwave frequencies usually used for radar, the external noise which enters via the antenna is generally quite low so that the receiver sensitivity is usually set by the internal noise generated within the receiver.Good receiver design is based on maximizing the output signal-to-noise  ratio.As described in Sec.10.2,to maximize the output signal-to-noise ratio, the  receiver must be designed as a matched filter, or its equivalent.The matched filter specifies the frequency response function of the I F part of the radar receiver. Obviously, the receiver should be designed to generate as little internal noise as possible, especially in the input stages where the desired signals are weakest.Although special attention must be paid to minimize the noise of the input stages,the lowest noise receivers are not always desired in many radar applications if other important receiver properties must be sacrificed.

DIGITAL SIGNAL PROCESSING

                                                   DIGITAL  SIGNAL  PROCESSING     

  The introduction of practical and economical digital processing to M T I radar allowed a significant increase in the option open to the signal processing designer. The convenience of digital processing means that multiple delay-line cancelers with tailored frequency-response characteristics can be readily achieved.A digital M T I processor does not, in principle, do any better than a well-designed analog canceler; but it is more dependable, it requires less adjustments and attention,and can do some tasks easier.Most of the advantages of a digital M T I processor are due to its use of digital delay lines,rather than analog delay lines which are characterized by variation due to temperature, critical gains, and poor on-line availability.A simple block diagram of a digital M T I processor,from the output of the I F amplifier the signal is split into two channels.One is denoted I, for in-phase-channel.To the other is denoted Q for quadrature channel,since a 90 degree phase change is introduced into the coho reference signal at the phase detector. This causes the output of the two detectors to be 90 degree out of phase.The purpose of the quadrature channel is to eliminate the effects of blind phases, as will be described later. It is desirable to eliminate blind phases in any M T I processor, but it is seldom done with analog delay-line cancelers because of the complex of the added analog delay lines of the second channel.

Add caption

TRACKING WITH RADAR

TRACKING WITH RADAR                                                                                                                              A tracking-radar systems measures the coordinates of a target and provides data which may be used to determine the target path and to predict its future position. All or only part of the available radar data-rang, elevation angle, azimuth angle, and doppler frequency shift may be used in predicting future position: that is, a radar might track in  range, in angle,in doppler,or with any combination.Almost any radar can be considered a tracking radar provided its output information is processed properly. But in general, it is the method by which angle tracking is accomplished that distinguishes what is normally considered a tracking radar from any other radar. It is also necessary to distinguish between a continuous tracking radar and a track-while-scan(T W S) radar. The former supplies continuous tracking data on a particular target, while the track-while-scan supplies sampled data one or more target.In general,the continuous tracking radar and the T W S radar employ different types of equipment.The antenna beam in the continuous  tracking radar is positioned in angle by a servomechanism actuated by an error signal.The various methods for generating the error signal may be classified as sequential lobing conical scan,and simultaneous lobing or monopulse. The range     and doppler frequency shift can also be continuously tracked,if desired,by a servo-control look actuated by an error signal generated in the radar receiver.The information available from a tracking radar may be presented on a cathode-ray-tube (C R T )display for action by an operator,or may be supplied to an automatic computer which determines the target path and calculates its probable future course.

PULSE DOPPLER RADAR

    PULSE DOPPLER RADAR                                                                                                                A pulse radar that extracts the doppler frequency shift for the purpose of detecting moving targets in the presence of clutter is either an M T I radar or a pulse doppler radar. The distinction between then is based on the fact that in a sampled measurement system like a pulse radar, ambiguities can arise in both the doppler frequency and the range measurements. Range ambiguities are avoided with a low sampling rate and doppler frequency ambiguities are avoided with a high sampling rate. However,in most radar applications the sampling rate, or pulse repetition frequency,cannot be selected to avoid both types of measurement ambiguities. Therefore a compromise must be made and the nature of the compromise generally determines weather the radar is called an M T I or a pulse doppler. M T I usually refers to a radar in which the pulse repetition frequency is chosen low enough to avoid ambiguities in range but with the consequence that the frequency measurement is ambiguous and results in blind speed. The pulse doppler radar, on the other hand, has a high pulse repetition frequency that avoids blind speeds, but it experiences ambiguities in rang, It performs doppler filtering on a single spectral line of the pules spectrum. One other method should be mentioned of achieving coherent  M T I. If the number of  cycles of the doppler frequency shift contained within the duration of a single pules is sufficient, the returned echoes from moving targets may be separated from clutter by suitable R F or IF filters.this is  possible if the doppler frequency shift is at least comparable with or greater than the spectral width of the transmitted signal.It is not usually applicable to aircraft targets. but it can sometimes be applied to radars designed to detect extraterrestrial targets such as satellites or astronomical bodies. In these cases, the transmitted pulse width is relatively wide and its spectrum is narrow.The high speed of the extraterrestrial targets results in doppler shifts that are usually significantly greater than spectral width of the transmitted signal.