Radar System Engineering

SEC.1610] CHOICE OF SYSTEM CONSTANTS 652
As is pointed out at the end of Sec. 16.6, the dependence on x is only
approximate.
The ratio Ro/s, which can be called the ‘‘ subclutter visibility,” gives
a measure of the performance of an MT1 system. When scanning and
wind effects are present together, along with the effect of system instability,
we have to go back to Eq. (24) and write it as
S = 2.4(r&c.m,,,n.+ ~~w,nd + &.m.rn); ‘, (27)
since the three fluctuations are independent of each other. A-ow each
fluctuation is proportional to the size of the clutter. Thus we have
— R, = 1
),~ (28)
s
1
~+~
( K~n’ K;A2j~ < + E: )
where the terms in the denominator refer respectively to the effects of
scanning, wind, and system instability. The quantities Kl and K2 are
constants, but Ks may depend somewhat on A and f,. For example, it is
probably harder to stabilize the local oscillator at 3 cm than at 10 cm.
Again, Eq. (3) shows that the repetition rate enters into the stability
condition. However, we shall assume, for simplicity, that the over-all
system instability is independent of A and jr.
16s10. Choice of System Constants.-The following are the objectives
in the design of a ground radar set with MT1:
1. Ylaxlmum sensitivity to moving targets in the clutter.
2. Maximum sensitivity to moving targets in the clear.
3. Maximum cancellation of undesirable moving objects, such as
clouds.
Some of these objectives are in direct conflict with the normal aims of
a radar set. For example, a broad beam and slow rate of scan will reduce
scanning fluctuations and therefore help the first obfective, but will
decrease resolution and hamper the rate of flow of information. Because
of this, MT1 should be integrated into the radar set from the start and
not added as an afterthought.
S’ubchdter Vi.sibik@-From Eq. (28) we see that the subclutter
visibility can be increased by increasing n and A.f,,but that nothing much
is gained by going greatly beyond the point where the scanning and wind
fluctuations are equal to those due to system instability. A well-designed
system might have an rms instability of 2 per cent, in which case,
from Eq. (24), the greatest possible subclutter visibility is 26 db. There
is then no need to make the individual subclutter risibilities for scanning
and wind greater than 30 db. For the case of scanning, that means, from
Eq. (25), that n does not have to be larger than 120 pulses per beamwidth.
Again, from Eq. (24), therms wind fluctuation should be less than 22 db.
This happens to be the value for wooded terrain when A = 3.2 cm and