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AIM

10/12/17

4−5−1

Surveillance Systems

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

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

AREA BLACKED OUT
BY ATTENUATION

NOT  OBSERVED

OBSERVED

ECHO

(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 phe-

nomena 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.

(c) Radar energy that strikes dense objects

will be reflected and displayed on the operator’s

scope thereby blocking out aircraft at the same range

and greatly weakening or completely eliminating the

display of targets at a greater range. Again, radar

beacon and MTI are very effectively used to combat

ground clutter and weather phenomena, and a method

of circularly polarizing the radar beam will eliminate

some weather returns. A negative characteristic of

MTI is that an aircraft flying a speed that coincides

with the canceling signal of the MTI (tangential or

“blind” speed) may not be displayed to the radar

controller.

(d) Relatively low altitude aircraft will not be

seen if they are screened by mountains or are below

the radar beam due to earth curvature. The only

solution to screening is the installation of strategi-

cally placed multiple radars which has been done in

some areas.

(e) There are several other factors which

affect radar control. The amount of reflective surface

of an aircraft will determine the size of the radar

return. Therefore, a small light airplane or a sleek jet

fighter will be more difficult to see on radar than a

large commercial jet or military bomber. Here again,

the use of radar beacon is invaluable if the aircraft is