Radar System Engineering

SEC.5.11] PULSE-MODULATED DOPPLER SYSTEM 153
Design Procedure.—The leading requirement in the design of a system
such as that described above was a range of 75 miles on single targets
movingat speeds upto400 mph, even in the presence of extreme ground
clutter.
First, since ability to deal with multiple targets was not required,
the modulation cycle was taken to be half on and half off. Next, the
repetition frequency was chosen to be as high as possible, consistent
with getting a reasonable fraction of the extreme range signals to return
during the time the receiver is on. Thus the value of j, = 1000 CPS,
which was selected, lets the entire return signal into the receiver at
r = 46.5 miles and none at 93 miles, so that at 75 miles about half is
lost. The advantage of a high value off, is the reduction in number of
stop bands required in the filter.
The dish size and wavelength were chosen—on the basis of range,
propagation factors, and beamwidth-to be 12 ft and 40 cm, respectively.
This latter value also determines the doppler band, which extends from
zero to about 1000 cps for 400-mph targets. Thus we need carry only
one harmonic of the repetition frequency.
The transmitter power depends on geometrical factors in the usual
way, and on the effective bandwidth of the receiver. It might be thought
that this would be the total width of the pass bands in the second filter
of Fig. 5.15, but actually, because of the rectifier between the two filters,
the noise depends on the bandwidths, both before and after this detector.
In fact, the effective bandwidth is the geometric mean of the two bandwidths.
The last bandwidth was chosen at about 4 cps, this being the
smallest consistent with the time during which the beam remains on the
target; and the first bandwidth was about twice the maximum doppler
frequency. Thus the effective bandwidth, for noise-computation purposes,
was about 100 cps. With this bandwidth, a transmitter power of
100 watts should give the required range.
Apparatus Considerations. —The leading design parameters having
been chosen, the next subject of discussion is the technical difficulties
involved. There are two chief ones, both having to do with elimination
of ground clutter: (1) frequency, and also amplitude, modulation in the
transmitter must be held to very low values; and (2) the filters, which
reject frequencies O,j,, 2~,, etc., present a considerable problem.
As to the first point, the entire operation of the system is predicated
on the assumption that the ground returns are periodic. This assumption
would be completely falsified, if the range of a ground target, measured in
wavelengths, were to change by as little as x/2 in one repetition cycle.
Since, in round numbers, there are 10’ wavelengths contained in twice the
maximum range, it is apparent that the operation will not even approximate
the above description unless the short-time frequency stability