Section 5. Surveillance Systems
4-5-1. Radar
a. Capabilities
1. Radar is a method whereby radio waves are transmitted into the air and are then received when they have
been reflected by an object in the path of the beam. Range is determined by measuring the time it takes (at the
speed of light) for the radio wave to go out to the object and then return to the receiving antenna. The direction
of a detected object from a radar site is determined by the position of the rotating antenna when the reflected
portion of the radio wave is received.
2. More reliable maintenance and improved equipment have reduced radar system failures to a negligible
factor. Most facilities actually have some components duplicated, one operating and another which immediately
takes over when a malfunction occurs to the primary component.
b. Limitations
1. It is very important for the aviation community to recognize the fact that there are limitations to radar
service and that ATC controllers may not always be able to issue traffic advisories concerning aircraft which are
not under ATC control and cannot be seen on radar. (See FIG 4-5-1.)
FIG 4-5-1

Limitations to Radar Service
Precipitation Attenuation

AREA BLACKED OUT
BY ATTENUATION

NOT OBSERVED

OBSERVED
ECHO

The nearby target absorbs and scatters so much of the out-going and returning
energy that the radar does not detect the distant target.

(a) The characteristics of radio waves are such that they normally travel in a continuous straight line
unless they are:
(1) "Bent" by abnormal atmospheric phenomena such as temperature inversions;
(2) Reflected or attenuated by dense objects such as heavy clouds, precipitation, ground obstacles,
mountains, etc.; or
(3) Screened by high terrain features.
(b) The bending of radar pulses, often called anomalous propagation or ducting, may cause many
extraneous blips to appear on the radar operator"s display if the beam has been bent toward the ground or may
decrease the detection range if the wave is bent upward. It is difficult to solve the effects of anomalous
propagation, but using beacon radar and electronically eliminating stationary and slow moving targets by a
method called moving target indicator (MTI) usually negate the problem.

Surveillance Systems

4-5-1