Page 391 - Fundamentals of Radar Signal Processing

FIGURE 5.24 Clutter suppression and windshear detection using an
autoregressive Doppler spectrum estimate: (a) Fourier spectrum of raw data,
(b) frequency response of clutter suppression filter derived from the AR
coefficients, (c) Fourier spectrum of filtered data. (Figure courtesy of Dr. Byron
M. Keel, GTRI.)

5.3.8 CPI-to-CPI Stagger and Blind Zone Maps
Pulse Doppler processing sometimes is combined with pulse cancellers. In this
case, the applicability of the concept of blind speeds is clear. If a pulse

canceller is not used, there is no highpass filter and therefore a target whose
Doppler shift equals an integer multiple of the PRF will not be filtered out as in
MTI processing. However, the target energy will be indistinguishable from
clutter energy since it will alias to the DC portion of the spectrum and combine
with the clutter energy. Targets having Doppler shifts equal to a multiple of the
PRF will still go undetected, so the corresponding target velocities are still

blind speeds.
In CPI-to-CPI PRF stagger, a coherent processing interval of M pulses is
transmitted at a fixed PRF. A second CPI is then transmitted at a different fixed
PRF. Because the blind speeds are different for each PRF used, a target that
falls in a blind speed of one PRF may be visible in the others. This concept is
illustrated in Fig. 5.25, which shows a notional Doppler spectrum for two
different PRFs. The plots are shown on the same frequency scale. First consider

the upper spectrum plot, which corresponds to data collected at PRF . A target
1
whose Doppler equals PRF will be aliased to zero Doppler shift, where it will
1
be undetectable if clutter is present. If the same target scenario is measured with
a lower pulse repetition frequency PRF as shown in the lower half of the
2
figure, the Doppler shift of the target no longer matches the PRF. The target
energy aliases to a nonzero Doppler where it does not compete with the clutter

and is still detectable.