interfaces with the FAA National Airspace System Enroute Stage A, ARTS facilities, and
the Advanced Combat Direction System (ACDS).
RADAR Performance Characteristics
When a RADAR system is developed that detects only flying aircraft and nothing else,
the RADAR controller will have a nearly perfect system for controlling traffic. However,
since the perfect RADAR system has yet to be developed, you need to be aware of the
limitations of existing systems. Such limitations include target fades, anomalous
propagation, false targets, jamming, and electronic interference.
Target Fades
A property of all RADAR systems with which the controller should become thoroughly
familiar is target fading. Target fades varies with the type of equipment, antenna height,
tilt angle of the antenna, atmospheric conditions, and the surrounding terrain. Target
fades are clear when an aircraft is over the antenna site. The degree and length of such
a fade is determined by the amount of antenna tilt. The lower the tilt angles of the
antenna, the better the low-angle coverage. Conversely, the higher the tilt angles of the
antenna, the better the high-angle coverage. Most antennas are set to give maximum
coverage for the particular type of control being employed.
The coverage in range, altitude, and azimuth for a particular site is determined by
means of a flight inspection evaluation. An FAA flight inspection team conducts this
evaluation before a facility is commissioned. When a previously unknown fade area is
suspected, another flight inspection should be requested to verify or confirm its
existence. The data obtained from the flight inspection gives the controller an indication
of coverage and target fades built in the type of equipment being used. To understand
the capabilities and limitations of the system, you should become thoroughly familiar
with the coverage pattern and fade areas determined by the flight inspection. For further
information and a detailed description of the procedures used when flight inspections
are performed, refer to the United States Standard Flight Inspection Manual, NAVAIR
16-1-520, and NATOPS Air Traffic Control Facilities Manual, NAVAIR 00-80T-114.
Anomalous Propagation
The atmosphere surrounding earth is not uniform in density or moisture content. It is
possible for local conditions to exist in which RADAR beams are bent upon passage
through the atmosphere. Conditions under which the RADAR beam does not travel a
straight line are called conditions of anomalous propagation. This condition is most apt
to occur on days when there is little wind and when the air temperature is different from
the ground temperature.
Anomalous propagation is most common over water where water evaporation causes a
temperature and moisture gradient. The refraction of dry or dense air is greater than
that of moist or less dense air; therefore, RADAR beams are bent in the direction of the
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