Page 419 - Fundamentals of Radar Signal Processing

apparent Doppler shift F, the actual Doppler shift must be of the form F +
t
t
k·PRF for some integer k. Use of the DFT for spectral estimation results in
quantization of the Doppler spectrum into Doppler bins (equivalently, velocity
bins), analogous to range bins in the range dimension. The same techniques used
for range disambiguation can therefore be used to resolve velocity ambiguities
as well.
The coincidence algorithm can be readily extended to simultaneously

disambiguate detections in both range and Doppler. Other extensions to deal
with radar systems that do not use the same range resolution in each PRF
(typically so as to maintain constant duty cycle) or the same velocity resolution,
as well as references to other disambiguation algorithms are given in Alabaster
(2012). A newer approach using the emerging technique of sparse
reconstruction has been proposed in Shaban and Richards (2013). It also works
in range and Doppler simultaneously and appears tolerant of reasonable

numbers of missed detections due to blind zones or other causes, false alarms,
and other data inconsistencies.

5.6 Clutter Mapping and the Moving Target Detector

5.6.1 Clutter Mapping
All of the MTI and pulse Doppler processing discussed so far has been focused
on reducing the clutter power that interferes with the signature of a moving
target so as to improve the SIR and ultimately the probability of detection.

These techniques are not effective for targets with little or no Doppler shift, and
that therefore are not separable from the clutter based on Doppler shift. Clutter
mapping is a technique for detection of moving targets with zero or very low
Doppler shift. It is intended for maintaining detection of targets on crossing
paths, that is, passing orthogonal to the radar line of sight so that the radial
velocity is zero; such targets are discarded by MTI and pulse Doppler

processing. Clutter mapping can be effective if the target RCS is relatively large
and the competing clutter is relatively weak, a situation depicted in Fig. 5.40
that can arise for instance in a ground-based air surveillance radar. In that
situation the antenna is tilted upward so that mainlobe ground clutter is not
present to compete with the target echo (though weather clutter may be); the
clutter is primarily from the sidelobes.