Radar System Engineering

SEC. 167] FL UCTCA TIONS DUE TO SCANNING 645
beamwidth (as measured between half-power points, one way). For
example, in the case of a 1° beam rotating at 6 rpm with a PRF of 1000, n
is 28 and Ag is therefore 5 per cent of yO. This represents the residue that
would be left after cancellation if a linear receiver were used.
In the next section it will be shown that a nonlinear receiver is necessary
in order to remove all the clutter from the PPI. Figure 16. 19b
shows the output of a simple limiting receiver; the maximum pulse-topulse
variation Ayl is less th,an before and occurs farther out on the
antenna pattern. The fluctuation is now given by the expression
Ay, = k
yl ~’
wherek is no longer a constant but depends on the ratio of maximum input
signal yOto the limit level vI, as shown in Table 16.3.
yo
TABLE 16.3
;1 k
10 5.1
100 7.2
1000 8.7
It will be noticed that k does not change much as the input signal strength
is varied over a wide range. This makes it easy to obtain removal of
clutter of varying size.
Now let us consider the fluctuations in the
r
case of extended ground clutter. The echo
from such clutter, as we have already seen in
Sec. 16.6, consists of the vector sum of the contributions
R’
of a large number of scattering ele-
ments. As the beam sweeps over the ground,
R
new elements are illuminated and the old ones
pass out of the beam. Thus the signal fluctuates
in both amplitude and phase, as shown in /
Fig. 16.20, where R is the signal voltage at a F I G 113.20 .-f5canning
given instant and R’ the value after one repetition
fluctuation of the echo from
period. The change from pulse to pulse is
an extended target.
represented by the vector r. If it is assumed that the scattering elements
are equal in size and randomly distributed, it is not hardl to calculate the
ratio of the rms values of R and r. The result is
(8)
(9)
1A. G. Emslie, “ Moving Target Indication on ME~T, ” RL Report No. 1080,
Feb. 19, 1946.