Page 119 - Fundamentals of Radar Signal Processing

decorrelate to a specified degree. If a rigid target such as a building is

illuminated with a series of identical radar pulses and there is no motion
between the radar and target, one expects the same received complex voltage y
from each pulse (ignoring receiver noise). If motion between the two is
allowed, however, the relative path length between the radar and the various
scatterers comprising the target will change, causing the composite echo
amplitude to fluctuate as in Fig. 2.9. Thus, for rigid targets, decorrelation of the

RCS is induced by changes in range and aspect angle. On the other hand, if
natural clutter such as the ocean surface or a stand of trees is illuminated, the
signature will decorrelate even if the radar and target do not move relative to
each other. In this case the decorrelation is caused by the “internal motion” of
the clutter, such as the wave motion on the sea surface or the blowing leaves and
limbs of the trees. The rate of decorrelation is influenced by factors external to
the radar such as wind speed. Range or aspect changes also induce

decorrelation of clutter signatures.
Although the behavior of real targets can be quite complex, a useful
estimate of the change in frequency or angle required to decorrelate a target or
clutter patch can be obtained by the following simple argument. Consider a
target consisting of a uniform line array of point scatterers tilted at an angle θ
with respect to the antenna boresight and separated by Δx from one another, as

shown in Fig. 2.12. Assume an odd number 2M + 1 of scatterers indexed from –
M to +M as shown. The total target extent is then L = (2M + 1)Δx. If the
nominal distance to the radar R is much larger than the target extent, R L,
0
0
then the incremental distance an EM plane wave must travel from one scatterer
to the next is Δx · sinθ. If the target is illuminated with the waveform Aexp(jΩt),
the received signal will be