Radar System Engineering

SEC. 3.10] COMPOUND TARGETS 81
effect on the received power of the variation of illumination for a vertical
rectangular sheet and uses the formula thus obtained as a definition of
cross section.
3.10. Compound Targets Extended through Space.—Targets such as
rain, vegetation, 6’window,’” or the surface of the sea are much more
complicated than the targets considered in the preceding sections. It is,
however, easier to predict and verify certain features of the return from
such targets because the compound targets here considered are composed
of large numbers of independent individual scatterers, and for that
reason statistical considerations can be used in their treatment. The
aim of this statistical treatment is to predict the probability distribution
of the returned power and of the correlat ion of successive measurements
of the return power.
It will be useful to distinguish between compound targets distributed
throughout a volume (rain, “ window”) and those distributed over a
surface (vegetation, waves). In this section the first class is considered,
the second being deferred to Sec. 3.11.
The Rain-echo Problem.—Echoes fmm rain are frequently observed
on microwave radar (Fig. 3012). Under some circumstances, the ability
to map out storm areas by radar may be put to good use. In many
applications, however, the presence on the radar screen of storm echoes
(that is, echoes from rain drops within a storm area) is objectionable for
either of two reasons. (1) A small isolated storm cloud at long range
may be mistaken for a legitimate target; airborne sea-search radar is
especially vulnerable to this kind of confusion. (2) Echoes from a storm
area may mask or confuse the echoes from targets at the same range and
azimuth.
The nuisance of “ rain clutter,” which the latter effect is sometimes
called, is most severe when the radar cross section of the desired target
is small. Other factors that determine the intensity of the rain echo
relative to that of the target echo are the beamwidth, pulse length, and
}ravelength of the radar, the distance to the target, and the number and
size of the water drops in the neighborhood of the target. In the case of
a radar set using a pencil beam or a simple fan beam, these factors enter
the problem as follows:
R’A~
-( )
Average rain-echo intensity = _ CT _ .Voo
Target-echo intensity .1” 2 T’
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foil or sheetcut into piecesof sucha sizethat eachpieceresonatesas a dipoleat enemy
radar frequency. When this matermlis dispensedfrom aircraft, large volumes of
space can be filled with it. It falls at a speed of only a few malesper hour, Tbe
strong signalsit returnsso effectively mask the radar signalsfrom aircraft that arc
in tbe midst of a cloud of windowthat severaltons of aluminumusedto he dispensed
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