Radar System Engineering

SEC, 3.9] ACTUAL COMPLEX TARGETS 79
A more serious modification of the definition of cross section is made
necessary by the reflection of radar waves w-hen the target is observed at
an elevation angle smaller than half the beamwidth and over a -water
surf ace. In such cases, the received power is modified by the interference
of the direct wave with the reflected wave, both for the incoming and for
the outgoing wave (Sec. 2. 12). The concept of cross section in the
FIG, 31 l.—Fractional modulationof returnedsignalfrom a B-26,both propellersrotating.
original sense breaks clown in such cases, because it is then no longer
possible to define a single quantity characterizing the target —namely the
cross section—in such a manner that the received power depends only
on the properties of the radar set (P, G, and .4), on the range, and on the
cross section.
.%slong as the target does not extend in elevation o~.er more than one
lobe (an assumption that is usually correct for airplanes) a modification