Radar System Engineering

SEC. 6.11] HEIGHT-FINDING INVOLVING GROUND REFLECTION 185
is approximately directly toward the station, the range at which the first
fade is seen is also determined by the target height. If several fade
points are determined, the height can be inferred with better accuracy.
This method has been used extensively with the 200-M c/sec aircraftsearch
radar used on naval vessels. It is especially useful ~vith this type
of radar because the elevation pattern can be well known (the ocean is a
good reflector, and the antenna height is determined by the class of ship
and not by the shape of the terrain), and because a radial approach to a
task force is generally chosen by either enemy or friend.
Height-finding by nulls, however, has inherent drawbacks which
seriously reduce its accuracy in operational use. The elevation pattern
of a ground-based set must be carefully calibrated experimentally,
because the reflection coefficient of the surrounding ground will determine
the pattern, and because uneven ground gives different elevation patterns
at different azimuths. Further, changes in the aspect presented to the
radar by the aircraft may cause
great changes in the echo strength. -
Finallyj variations in the atmos- ~ ~ -\
29/
pheric refractive index can cause ~~
large variations in the amount of ~ ~
radiation falling on the gro~d or ~ ~ 10
water surface surrounding the * =
:
radar and thus appreciably change 10
the pattern.’ Despite these diffi- ~ IO. 6.19.—Typicalelevationlobe pattern.
culties, if there is plent y of friendly
air traffic and if radio contact can be used to enable a continual check on
height accuracy, adept radar plotters can give heights accurate to within
one thousand feet more than half the time by the observation of signal
fades. The method is slow; a reading cannot be made in less time than
that required for a plane to fly through several nulls of the pattern. This
drawback, and the elaborate calibration necessary at an overland site,
have led to the development of other height-finding means, even for longwave
radar sets.
Signal Comparison.—A more rapid and convenient means of heightfinding
which is also based on the existence of maxima and minima in the
elevation pattern of a long-wave radar employing ground reflection is
signal comparison. Two or more antenna systems with different elevation
patterns are provided at a single radar station, and the intensity of
the echo received on one antenna is compared with that received on
another. The comparison may be made either by measuring the vector
1Sincerefractionof one-halfdegreeispossible,andsincevariationsof echo strength
dueto aspectcan be 12 db, altitudeerrorsof severalthousandfeet can occur even in
the favorablecase of sea reflection.