Radar System Engineering

652 MOVING-TARGET INDICATION [SEC 16.9
clutter has to compete with the clutter-noise in much the same way as a
target intheclear competes with ordinary receiver noise. Figure 16.23is
a vector diagram for a moving target in the clutter. The pulse-to-pulse
variation r in the resultant vector has both amplitude and phase components
just like the clutter fluctuations. The magnitude of r can easily
be obtained from the fact that s rotates with the doppler frequency jd.
Thus the angle betweens and s’ is 2rf,T, and accordingly
r = 2s sin (mf~T). (22)
This equation, which is the same as Eq. (14), shows that blind speeds
also exist for targets in the clutter. If we take the arithmetical average
of Eq. (22) for all speeds, we get
4s
~=—.
7r
(23a)
If, on the other hand, we take the median with respect to velocity, which
is the average that we took in the case of targets in the clear, we get
? = ~2s. (23b)
Since the two values of ? differ by only 10 per cent or about 1 db, it does
not matter which we take. The value given by Eq. (23a) is also the
arithmetical average for the case of a noncoherent signal from a signal
generator. Measurements can therefore be made without appreciable
error by using the signal from a signal generator instead of that due to an
actual target.
A moving-target or signal-generator pip can be expected to be visible
on the PPI if ? is at least three times the rms clutter fluctuation r~.
Using Eq. (23a), this condition can therefore be written as
37r
s = — rO = 2.4r0.
4
That is, the signal amplitude should be 8 db larger than the rms clutter
fluctuation.
It is convenient to express the visibility condition in terms of the rms
clutter amplitude RO instead of the rms fluctuation. In the case of
scanning effects this leads, with the help of Eq. (10), to the following simple
relationship:
(24)
Lo=: (scanning), (25)
s
where n is the number of pulses per beamwidth. In the case of wind
effects, we have, from Eq. (6),
(26)