Radar System Engineering

24 THE RADAR EQUATION [SEC.2.5
time, a narrow radial strip extending outward from, for example, the
point directly beneath the aircraft and rotating or “scanning” this strip
about a vertical axis (see Fig. 2.2). Evidently some sort of fan beam, as
narrow as possible in the horizontal direction but spread out vertically, is
demanded. To find the shape which the beam should have, in a vertical
plane, the properties of the target must be taken into account. Unlike
the targets previously discussed, which were assumed to be small tom.
pared to the cross section of the beam and to be characterized by a fixed
radar cross section u, the ground is an extended, or compound target.It
consists of a multitude of small scattering or reflecting objects, many of
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0
FIG.2.2.—Theproblemof theground-mappingantenna.
which contribute to the echo received at any instant. It is easy to see
that the area of the patch on the ground, such as C in Fig. 2.2, from which
echoes can be received at one instant is proportional to the width of the
radar beam at the range in question and to the pulse duration. Also, the
effectiveness of such a patch undoubtedly depends on the angle from
which it is viewed. A detailed discussion of this matter must be reserved
for Chap. 3, but the foregoing remarks should make plausible the following
hypothesis:
v = LRa sin 0, (6a)
in which a is the beamwidth in azimuth and L is a factor having the
dimensions of a length that contains the pulse length and otherwise
depends only on the characteristics of the terrain. The factor sin 0