Radar System Engineering

SEC,,16.9] TARGET i71SIBILITY 649
Consider howalimitingr eceivershouldbe adjusted in each of these
cases. Since only thephase part of the fluctuation isselected, the fluctuation
of the video output is 1/v’2 or 3 db less than the total input fluctuation,
Thus the figure – 22 db becomes – 25 db at the output, which
means that the rms fluctuation in the first case is about ~ of the limit
level, Therefore, in order to obtain a PPI with uniform background, the
gain control of the receiver, ahead of the limiting stages, should be
adjusted so that rms noise is also ~ of limit level. “ Peak” noise is
then about $ of limit level. This adjustment of noise relative to limit
level is made while viewing the output of the limiting receiver on an
A-scope. JVhen this has been done, the video gain control is adjusted
so that the noise and canceled residue just show on the PPI. The MT1
photograph in Fig. 16.1 shows everything in correct adjustment so that
the residue of the clutter blends with the noise. The corresponding
photograph in Fig. 162 sho~vs a case of incorrect adjustment: receiver
noise is too large compared with the limit level. The result is that the
residue does not show on the scope and black “holes” appear where the
clutter ~vas. L’nder these conditions there is an excessive loss of sensitivity
for moving targets in the clutter.
In the second case (high PRF and broad beam) the predominating
fluctuations will be due to system instability and maybe of the order of 5
per cent (peak) if all the stability requirements are met. Thus peak
receiver noise should in this case be set at 5 per cent of limit level.
16.9. Target Visibility .—There are two problems to be discussed in
this section. The first has to do with the visibility of moving targets
when they are clear of the clutter; it includes consideration of undesirable
targets, like clouds, as well as desirable targets such as aircraft. The
second problem is concerned with the visibility of moving targets that
occur at the same range and azimuth as ground clutter.
Targets in the Clear.—lYe have seen in Sec. 16.2 that the video pulses
from the 31TI receiver, in the case of a moving target, are amplitudemodulated
at the doppler frequency .f,i. Thus the pulse amplitude at a
given instant t is given by
~1 = yn COS %jd!.
For the next pulse,
y, = ?/0 Cos 2TfJ(f + z’).
The canceled video pulse is therefore
y = yl – YZ = 2yo sin (7rf~T) sin 27rfcf t + ~
()
. (13’,
Thus the canceled pulses also exhibit the doppler modulation frequent}
and have an amplitude
yj = 2yo sin (rj,jT). (14)