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.
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