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air traffic.
ARSR is a long-range radar system designed primarily to cover
large areas and provide a display of aircraft while en route
between terminal areas. The ARSR enables air route traffic
control center (ARTCC) controllers to provide radar service
when the aircraft are within the ARSR coverage. In some
instances, ARSR may enable ARTCC to provide terminal radar
services similar to but usually more limited than those provided
by a radar approach control.
ASR is designed to provide relatively short-range coverage in
the general vicinity of an airport and to serve as an expeditious
means of handling terminal area traffic through observation
of precise aircraft locations on a radarscope. Nonprecision
instrument approaches are available at airports that have an
approved surveillance radar approach procedure. ASR provides
radar vectors to the final approach course and then azimuth
information to the pilot during the approach. In addition to
range (distance) from the runway, the pilot is advised of MDA,
when to begin descent, and when the aircraft is at the MDA. If
requested, recommended altitudes will be furnished each mile
while on final.
PAR is designed to be used as a landing aid displaying range,
azimuth, and elevation information rather than as an aid for
sequencing and spacing aircraft. PAR equipment may be
used as a primary landing aid, or it may be used to monitor
other types of approaches. Two antennas are used in the PAR
array, one scanning a vertical plane, and the other scanning
horizontally. Since the range is limited to 10 miles, azimuth to
20°, and elevation to 7°, only the final approach area is covered.
The controller’s scope is divided into two parts. The upper half
presents altitude and distance information, and the lower half
presents azimuth and distance.
PAR is a system in which a controller provides highly accurate
navigational guidance in azimuth and elevation to a pilot. Pilots
are given headings to fly to direct them to and keep their aircraft
aligned with the extended centerline of the landing runway.
They are told to anticipate glide path interception approximately
10–30 seconds before it occurs and when to start descent.
The published decision height (DH) is given only if the pilot
requests it. If the aircraft is observed to deviate above or below
the glide path, the pilot is given the relative amount of deviation
by use of terms “slightly” or “well” and is expected to adjust
the aircraft’s rate of descent/ascent to return to the glide path.
Trend information is also issued with respect to the elevation
of the aircraft and may be modified by the terms “rapidly” and
“slowly” (e.g., “well above glide path, coming down rapidly”).
Range from touchdown is given at least once each mile. If an
aircraft is observed by the controller to proceed outside of
specified safety zone limits in azimuth and/or elevation and
continue to operate outside these prescribed limits, the pilot will
be directed to execute a missed approach or to fly a specified
course unless the pilot has the runway environment (runway,
approach lights, etc.) in sight. Navigational guidance in azimuth
and elevation is provided to the pilot until the aircraft reaches
the published decision altitude (DA)/DH. Advisory course and
glide path information is furnished by the controller until the
aircraft passes over the landing threshold, at which point the
pilot is advised of any deviation from the runway centerline.
Radar service is automatically terminated upon completion of
the approach.
Airport Surface Detection Equipment
Radar equipment is specifically designed to detect all principal
features on the surface of an airport, including aircraft and
vehicular traffic, and to present the entire image on a radar
indicator console in the control tower. It is used to augment
visual observation by tower personnel of aircraft and/or
vehicular movements on runways and taxiways.
Radar 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.
2. The characteristics of radio waves are such that they
normally travel in a continuous straight line unless
they are “bent” by abnormal atmospheric phenomena
such as temperature inversions; reflected or attenuated
by dense objects such as heavy clouds, precipitation,
　
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