Radar System Engineering

100 PROPERTIES OF RADAR TARGETS [SEC. 3.15
periphery of the picture, the buildings comprising Edgewood Arsenal
are clearly visible; a similar group southeast of the aircraft corresponds to
Aberdeen Proving Ground. A third bridge over the Susquehanna
River, just north of the two visible in the picture, is barely suggested
by two bright dots at the upper end of the small island in the river.
In general, the three bridges appear as equally strong signals; in the
particular sweep corresponding to this photograph the third bridge
returned a poor signal.
Many structures stand out against ground return because they
project vertically above the surrounding level ground and intercept
energy intcmlcd for target areas behind them. The central portion of
the railroad in Fig. 3.25 is an example of a structure which casts a shadow.
Seen broadside from a relatively low angle, the wires and supports of
the overhead electrification system and the embankment upon which
the tracks are laid intercept energy and cast their shadows in the same
k
1:1IJ.3.26.—Increasednngleintercepted by inclined target area. The length in range of a
PUISCpacket, AR, is ~cr.
way as would a small hill or ridge. This type of contrast is less effective
\vhcn the target is viefvcd from a higher angle, for the same reasons that
make mountain contrast Icss effective at high angles of incidence.
Natural structures also show radar contrast. Examples are the rows
of trees that line canals and streams in otherwise treeless regions, the
hedgerows of Normandy and the cliffs of Dover, or the edges of forestsl
:md the banks of rivers. As an illustration, Fig. 3.26 shows the increased
angle intcrccptcd by a target area which spans a river bank. In Fig.
3,25, the bright line marking the east shore of the Susquehanna River is
characteristic of the strong reflections returned by sharply inclined river
embankments.
The radar signals received from man-made structures are often too
strong to bc fully explained in terms of the solid angle intercepted by the
targct. Figure 3.25 sholrs several examples of the bright signals from
groups of buildings, without accompanying shadows. In order to
account for contrasts as strong as those in Fig. 3.25 a certain amount of
rctrodircctivity in the target objects themselves must be present.
This rctrodirectivity can arise in several }vays. Strong specular
1“~lIc llcur edge of a forestor groupof treesgivesa brightsignal; the far edgegives
i wcikcncd signal or cnstsa shadoJv.