Radar System Engineering

SF,C. 3.16] CITIES 101
reflection will result whenever a flat surface happens to be oriented normal
to the line of sight; yet the mere presence of flat surfaces is not enough to
guarantee a strong reflection. If these surfaces were oriented in random
directions, the probabilityy of finding one at just the right orientation
would be so low that the average signal from such a group of flat surfaces
would be no stronger than the average signal from a collection of isotropic
scatterers filling about the same volume. Therefore the flat surfaces
must be so oriented that the reflection is concentrated in the direction
of the radar receiver. In a group of buildings, a large proportion of the
flat surfaces will be vertical walls, while many others are smooth pavements
or flat roofs. There are many opportunities for combinations of
three flat surfaces at right angles to form corner reflectors (Sec. 3.5),
which are highly retredirect ive targets. The full potentialities of these
tremendous corner reflectors are never realized in practice because of
the strict tolerances imposed by the short wavelengths of microwave
radar. Insufficient flatness in the walls makes a huge, imperfect corner
reflector behave like one which is smaller but perfect. Inadequate
perpendicularity results in several return beams in the vicinity of the
aircraft instead of a single return beam pointed directly at the aircraft.
Nevertheless, the average effect of many triple corners is to provide
retrodirectivity in the radar target.
Vertical and horizontal surfaces can combine into double (rather
than triple) corners, giving directivity in elevation under certain conditions.
It was shown in Sec. 2.12 that rough surfaces like the ground
can serve as satisfactory mirrors for more distant targets when the angle
of incidence is sufficiently low. Because double-corner directivity
depends on the mirror-like properties of a horizontal surface in front of
vertical structures, we should expect strongest signals from these structures
when they are seen from low angles. Many buildings or groups of
buildings return strong signals at long ranges but tend to fade at shorter
ranges when the higher angle of incidence reduces their retrodirectivity.
If the line of sight is nearly horizontal, strong signals are sometimes
observed by direct specular reflection from vertical surfaces without the
benefit of mirror reflection from’ an intermediate horizontal surface.
Special cases of target directivity in azimuth arise when, for instance,
large groups of buildings have parallel walls. The signals in directions
perpendicular to these walls are often intensified, as can be seen in
Fig. 3.35.
3.16. Cities.-The brightest signals within a built-up area (Boston,
Mass. ) are presented in Fig. 3.27. This particular photograph was
obtained with a so-called “ three-tone” presentation (Sec. 13.21), in
which gain and limit level are electronically switched back and forth
from levels most suitable for land-water contrast to levels giving the