Radar System Engineering

SEC. 44] CLUTTER 125
How can a low-flying plane be tracked across a radar map filled with
“permanent” echoes?
The only basis that is known for a distinction between the echoes
from a particular target and equally strong echoes from its immediate
surroundings is the motion, if any, of the target relative to its surroundings.
1 The reflection of a wave by a moving object gives rise to a signal
as received in the neighborhood of the (stationary) transmitter, which
differs slightly in frequency from the outgoing wave. This is the familiar
doppler effect. The frequency difference Aj is but a small fraction of
the original frequency j., in the case of electromagnetic waves, being in
fact (2u/c)fo where v is the velocity of the target along the line of sight
and c the velocity of light.
Doppler detection systems, or “c-w radar” as they are sometimes
loosely called, utilize this principle. In fact the doppler effect and c-w
radar were from the beginning so closely identified as to create a rather
widespread impression that pulse radar was inherently incapable of
capitalizing on this essential difference between fixed and moving targets.
This is not true, fortunately, as has been vividly demonstrated by the
development of methods, described in Chap. 16, which make possible a
distinction between moving targets and their surroundings. In some
cases the echoes from the latter are automatically rejected.
This is a spectacular advance in pulse-radar technique but it does
not entirely eliminate the clutter problem. It is applicable only to
moving targets, and then only when the unwanted echoes from the surroundings
do not fluctuate so rapidly as to defeat the pulse-to-pulse
comparison which is an essential part of the scheme.
Another direction from which the clutter problem can be attacked
is that of resolution. To just this quality is due the great advantage of
microwave radar over long-wave radar in respect to clutter. As the
radar beam is made narrower and the pulse shorter, the amount of
clutter signal superimposed on the target signal decreases. Also,
improved resolution often allows recognizable forms to be distinguished
in the radar picture. A road or a river may be plainly discernible on a
radar map even though any one of the many traces making up the picture
}~ould be undecipherable. How far we can hope to go in improving
resolution depends on factors already discussed, namely the beamwidthaperture-wavelength
relation, considerations of scanning speed, and the
practical limits on pulse length and range resolution. Within these
limitations, however, there is still much room for development.
1One exception to this statementmust be made. Echoes from sphericalwater
drops (rain clutter) accuratelypreservethe polarizationof the incident wave, and it
has been demonstratedthat rain cluttercan be almost entirelyeliminatedby making
use of this fact (Sec. 3.10),