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7.6. Typical Radar Returns:
7.6.1. Returns From Land. All land surfaces present minute irregular parts of the total surface for
reflection of the radar beam; thus, there is usually a certain amount of radar return from all land areas.
The amount of return varies considerably according to the nature of the land surface scanned. This
variance is caused by the difference in reflecting materials of which the land area is composed and the
texture of the land surface. These are the primary factors governing the total radar return from specific
land areas.
7.6.2. Flat Land. A certain amount of any surface, however flat in the overall view, is irregular enough
to reflect the radar beam. Surfaces which are apparently flat are actually textured and may cause returns
on the scope. Ordinary soil absorbs some of the radar energy and, thus, the return that emanates from
this type of surface is not strong. Irregularly textured land areas present more surface to the radar beam
than flat land and, thus, causes more return. The returns from irregularly textured land areas are most
intense when the radar beam scans the ridges or similar features at a right angle. This effect is
particularly helpful in detecting riverbeds, gullies, or other sharp breaks in the surface height. At times,
in desolate areas that are flat, these occasional surface changes are apparent where it would not have
appeared in more irregular topography. Such returns provide recognizable targets in otherwise sparse
circumstances. In other cases, especially at low-level over broken terrain, this effect could complicate
scope interpretation.
7.6.3. Hills and Mountains. Hills and mountains will normally give more radar returns than flat land
because the radar beam is more nearly perpendicular to the sides of these features. The typical return is a
bright return from the near side of the feature and an area of no return on the far side. The area of no
return, called a mountain shadow, exists because the radar beam cannot penetrate the mountain and its
AFPAM11-216 1 MARCH 2001 179
LOS transmission does not allow it to intercept targets behind the mountain (Figure 7.6). The shadow
area will vary in size, depending upon the height of the aircraft with respect to the mountain. As an
aircraft approaches a mountain, the shadow area becomes smaller at higher altitudes. Furthermore, the
shape of the shadow area and the brightness of the return from the peak will vary as the aircraft's
position changes. As the aircraft closes on the mountainous area, shadows may disappear completely as
the beam covers the entire surface area. At this point, a great deal of energy is reflected back at the
antenna and recognizable features in that area will be rare.
Figure 7.6. Mountain Shadows.
7.6.3.1. Recognition of mountain shadow is important because any target in the area behind the
mountain cannot be seen on the scope. In areas with isolated high peaks or mountain ridges, contour
navigation may be possible because the returns from such features assume an almost three-dimensional
appearance. This allows specific peaks to be identified.
7.6.3.2. In more rugged mountainous areas, however, there may be so many mountains with resulting
return and shadow areas that contour navigation is almost impossible. But these mountainous areas are
composed of patches of mountains or hills, each having different relative sizes and shapes and relative
positions from other patches. By observing these relationships on a chart, general aircraft positioning is
feasible.
7.6.4. Coastlines and Riverbanks. The contrast between water and land is very sharp, so that the
configuration of coasts and lakes are seen with map-like clarity in most cases (Figure 7.7). When the
radar beam scans the banks of a river, lake, or larger body of water, there is little or no return from the
water surface itself, but there is usually a return from the adjoining land. The more rugged the bank or
180 AFPAM11-216 1 MARCH 2001
coastline, the more returns will be experienced. In cases where there are wide, smooth mud flats or
sandy beaches, the exact definition of the coastline will require careful tuning.
Figure 7.7. Radar Returns.
7.6.4.1. Since both mountains and lakes present a dark area on the scope, it is sometimes easy to mistake
a mountain shadow for a lake. This is particularly true when navigating in mountainous areas that also
contain lakes.
7.6.4.2. One difference between returns from mountain areas and lakes is that returns from mountains
are bright on the near side and dark on the far side, while returns from lakes are of more uniform
brightness all around the edges. Another characteristic of mountain returns is that the no-show area
changes its shape and position quite rapidly as the aircraft moves; returns from lakes change
　
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