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the aircraft.
2/17/05 AIM
Surveillance Systems 4−5−1
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 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.
(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 strategically
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
AIM 2/17/05
4−5−2 Surveillance Systems
equipped with an airborne transponder. All ARTCCs’
radars in the conterminous U.S. and many airport
surveillance radars have the capability to interrogate
Mode C and display altitude information to the
controller from appropriately equipped aircraft.
However, there are a number of airport surveillance
radars that don’t have Mode C display capability and;
therefore, altitude information must be obtained from
the pilot.
(f) At some locations within the ATC en route
environment, secondary-radar-only (no primary
radar) gap filler radar systems are used to give lower
altitude radar coverage between two larger radar
systems, each of which provides both primary and
secondary radar coverage. In those geographical
　
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