Page 330 - Fundamentals of Radar Signal Processing

while the clutter region is a larger percentage. The target originally at velocity

v is still unaliased and so still appears at Doppler shift 2v /λ. The Doppler shift
1
1
of the target originally at the (negative) velocity v was outside the new
2
spectrum range ±PRF′/2 and so aliases to a new apparent Doppler shift of 2v /λ
2
+ PRF′.

FIGURE 5.2 Effect of a 40 Percent PRF reduction on the Doppler spectrum of
Fig. 5.1c.

Now consider the effect of platform motion on the range-Doppler map
observed from an airborne or spaceborne platform. As described in Chap. 3, the
Doppler shift observed for any target or clutter scatterer is increased by the
platform-induced Doppler shift component of F = 2v cosψ/λ Hz, where ψ is the
D
cone angle between the platform velocity vector and the line of sight (LOS) to
the scatterer. Figure 5.3a shows an aircraft in level flight with a forward but
down-looking radar illuminating ground clutter. In this case the cone angle ψ is

the depression angle ψ MLC shown. While most of the echo energy will be in the
radar mainbeam, it is common to have measurable (above the noise floor)
clutter echo through both forward-and rear-looking sidelobes as well. Ground
clutter echoes received from different directions will appear at both different
Doppler shifts due to different cone angles, and at different ranges due to

different distances to the ground. Target echoes will again appear at the
appropriate Doppler shift and range bin, where the Doppler shift now is
proportional to the total radial velocity due to both the platform motion and the
target motion.