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6/17/21 

AIM 

Section 5.  Surveillance Systems 

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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

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1.) 

FIG 4

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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 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 historical 
solution to screening has been the installation of 
strategically placed multiple radars, which has been 
done in some areas, but ADS

B now provides ATC 

surveillance in some areas with challenging terrain 
where multiple radar installations would be impracti-
cal. 

(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 

Surveillance Systems 

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