Page 132 - Fundamentals of Radar Signal Processing

observed using the same target and waveform, but with the aspect angle

changing 0.7 mrad per pulse. The total angle change over 20 pulses is then 13.3
mrad, nearly nine times the decorrelation interval of 1.5 mrad. The target echo
amplitude and phase will then both fluctuate significantly, raising the sidelobes
and smearing the target energy over a wider frequency range, effectively
whitening the spectrum significantly.

FIGURE 2.19 Effect of intra-CPI target fluctuations on Doppler spectrum.

2.3 Clutter

In radar the term clutter refers to a component of the received signal due to
echoes from volume or surface scatterers. Such scatterers include the earth’s
surface, both terrain and sea; weather echoes (for example, rain clouds); and

man-made distributed clutter, such as so-called chaff clouds of airborne
scatterers, typically made out of lightweight strips of reflecting material. Clutter
echoes are sometimes interference and sometimes the desired signal. For
instance, synthetic aperture imaging radars are designed to image the earth
surface, thus the terrain clutter is the target in a SAR. For an airborne or space-
borne surveillance radar trying to detect moving vehicles on the ground, clutter
echo from the surrounding terrain is an interference signal.

From a signal processing point of view, the major concern is how to model
clutter echoes. As with man-made targets, clutter is a complex target with many
scatterers per resolution cell so that the echoes are highly sensitive to radar
parameters and encounter geometry. Like complex targets, clutter is therefore
modeled as a random process. In addition to temporal correlation, clutter can