Page 334 - Radar Technology Encyclopedia

radar, airborne mapping radar, aircraft tail-protection 324

vate aircraft carry weather-avoidance radar. Commercial air-
craft weather-avoidance radars typically operate at C-band.
The limited space and payload characteristic make X-band
the frequency of choice for most private aircraft and small
commercial “feeder” airlines. A simple and effective digital
X-band weather-avoidance radar can weigh less than 50 lb,
including displays and controls, and require only 4A of 28V
dc power. Figure R6 shows the antenna and cockpit units of a
weather-avoidance radar designed for small commercial air-
craft.

Figure R5 AN/APG-66 AI radar mounted in nose of F-16
fighter.
craft, which includes a surface mapping mode. Since in either
case the surface of the earth (or some other astronomical
body) is the target, the problem is not signal-to-noise ratio,
which is generally a very large value, but resolution. There
are two basic radar techniques used in aerial mapping of the
surface: (1) real-beam mapping and (2) synthetic array map-
ping. In real-beam mapping, the azimuth resolution is limited
i.
by the actual radar beamwidth q (e., at a range R the azimuth
or cross-range resolution will be Rq). Because antenna azi- Figure R6 AVQ-55 weather-avoidance radar components: (1)
muth beamwidth is proportional to the radar wavelength l transmitter-receiver unit, (2) antenna, and (3) display (RCA
photograph).
divided by the width of the antenna, high resolution can be
achieved with long antennas and short wavelengths. Weather
Because the radar return signal intensity is a function of
conditions usually dictate that airborne mapping radar operate
precipitation (rain) drop size, by sensing the rate of change of
at X-band frequencies or lower, and maximum antenna
signal strength with range, most modern weather-avoidance
dimensions are determined by the airborne platform’s physi-
radars provide the pilot with a color-coded display of con-
cal length and configuration. As an example, an X-band, side-
tours where the rain intensity changes abruptly. A typical
looking, real-beam array antenna 5m long, can achieve azi-
weather avoidance radar uses a cone-shaped antenna beam
muth resolutions sufficiently fine so as to resolve small ships
which is scanned in azimuth (e.g., ± 60°) and covers 5° to 8°
at a range of approximately 10 km.
in elevation. Simple pulsed radars, tilted in elevation to avoid
Ground-mapping using the synthetic array radar (SAR)
excessive ground return, are adequate for most weather-
technique can provide much higher azimuth resolution than
avoidance radar applications. More sophisticated MTI and
possible using real-beam techniques, because the antenna
pulsed doppler techniques can be used to accurately measure
“array” is formed by the linear motion of the radar-equipped
wind shear within a rainstorm and reject ground return, but
aircraft. Thus the length L of a synthetic aperture array may
these features are generally not found on the inexpensive
be hundreds of meters long, and the two-way beamwidth of
variety of radar common to private aviation. Figure R7 shows
the array at a position 4 dB down from peak antenna gain is
the cockpit display of data from a typical weather radar. PCH
equal to l/2L, or half the one-way beamwidth of a real-beam
Ref.: Stimson (1983), pp. 41, 127; Allied Signal Technologies Pilot’s Guide,
array of equivalent length. Practical SAR arrays are in use
RDS 86 Digital Weather Radar; Hovanessian (1984), pp. 334–338;
that yield azimuth resolutions in terms of a few meters. Davydov (1988), pp. 176–182.
Both real-beam and SAR mapping radars can take advan-
aircraft control radar (see air traffic control radar;
tage of pulse compression techniques to enhance resolution in
ground-controlled approach radar).
the downrange dimension. (See also sidelooking radar, syn-
thetic aperture radar.) PCH An aircraft tail-protection radar, also referred to as tail-
Ref.: Stimson (1983), pp. 521, 527–548; Currie (1987), Ch. 15. warning radar, is an acquisition radar installed in the tail sec-
tion of an aircraft to warn the crew of the presence of an
An airborne weather-avoidance radar is a radar installed
enemy fighter or missile in the aircraft’s rear hemisphere. The
aboard an aircraft for detecting and avoiding severe weather.
radars are used to warn the aircraft crew of danger, ensuring a
Virtually all large commercial aircraft, military aircraft not
timely aircraft maneuver and weapon employment. To the
already equipped with tactical airborne radar, and many pri-