Radar System Engineering

154 C-W RADAR SYSTEMS [SEC.5.11
exceeds one partin 10’. Actually, much better than this must be done
since the ground returns may be 90 db or thereabouts above the target
returns. Thus, one might expect stabilities of the order of 10’0 to be
needed. Much calculation can be done on this point but, though such
calculation was useful in showing that a workable system was possible,
it will suffice to say here that by very careful attention to detail it was
found possible completely to eliminate all trouble due to frequency
(and amplitude) modulation of the transmitter. The main points were:
crystal control of both transmitter and modulator, very careful regulation
and filtering of power supplies, and operation of important filament
supplies with alternating current (3000 cps) obtained by multiplication
from the repetition frequency.
We next consider the “infinite rejection” filters that are to remove the
ground clutter.
The maximum attenuation required is the quantity of most importance.
Naturally, this is indefinite since it depends on the terrain, and
pertinent measurements are very scanty. But the attenuation needed
may be estimated in several ways, which agree moderately well, and
finally it was measured on the finished system. Data from all sources
indicate that ground returns in mountainous terrain may be much larger
than is tommy-dy realized—in the present case, of the order of 90 db
above noise.
The first method of estimation is to calculate the largest possible
ground return on the assumption that the return is from a mountain so
large as to fill the beam completely. Such a mountain is certainly
possible, and assuming it to be hemispherical its returns will be larger
than those from a target of cross section a in the ratio r2A/k’u with A
the dish area. In the present case, this comes out at about 110 db.
This is certainly an overestimate because the mountain will generally
not have unity reflection coefficient, and because the part of the model
that does the reflecting-namely, the part of the hemisphere normal to
the line of sight—will usually be missing in actuality. Nevertheless,
the calculation is interesting as indicating that oery large ground returns
are possible.
A second estimate can be derived from measurements made at Ellenville,
N. Y. with a more or less normal (l-psec, 100-kw) pulse system.
These measurements showed a ground return 75 db above noise. To
compare these measurements with our present problem, a number of
factors must be taken into account. These are: the difference in pulse
lengths (N 27 db), the different receiver bandwidths (* 30 db), and the
different transmitter powers (*30 db). Taking these factors into
account, the clutter returns at Point 1, Fig. 5.15, may be very crudely