BendixKing IN-862A System RDR-2100 - Manual (Page 17)

Effective Date: 9/98 RDR 2100 Pilot's Guide: Rev 1
13
Theory of Operation
WEATHER ATTENUATION COMPENSATION
An extremely important phenomena for the weather avoidance radar
operator to understand is that of attenuation. When a radar pulse is trans-
mitted into the atmosphere, it is progressively absorbed and scattered so
that it loses its ability to return to the antenna. This attenuation or weak-
ening of the radar pulse is caused by two primary sources, distance and
precipitation. The RDR 2100 has several advanced features which signif-
icantly reduce the effects of attenuation (no airborne weather radar can
eliminate them completely). It is therefore up to the operator to under-
stand the radar’s limitations in dealing with attenuation.
Attenuation because of distance is due to the fact that the radar energy
leaving the antenna is inversely proportional to the square of the distance.
For example, the reflected radar energy from a target 60 miles away will
be one fourth (if the target is beam filling) of the reflected energy from an
equivalent target 30 miles away. The displayed effect to the pilot is that
as the storm is approached, it will appear to be gaining in intensity. To
compensate for distance attenuation both Sensitivity Timing Control
(STC) and Extended STC circuitry are employed. The RDR 2100 has an
STC range of 0 to approximately 40 nautical miles. Additionally, the
radar will electronically compensate for the effects of distance attenuation
with the net effect that targets do not appear to change color as the dis-
tance decreases.
Outside the STC range the Extended STC circuitry increases the dis-
played intensity to more accurately represent storm intensity. The
Extended STC will not, however, totally compensate for distance attenua-
tion and, therefore, targets in this range can be expected to show more
detail as the distance decreases until reaching the STC range.
Attenuation due to precipitation is far more intense and is less predictable
than attenuation due to distance. As the radar pulses pass through mois-
ture, some radar energy is reflected. But much of that energy is
absorbed. If the rain is very heavy or extends for many miles, the beam
may not reach completely through the area of precipitation. The weather
radar has no way of knowing if the beam has been fully attenuated or has
reached the far side of the precipitation area. If this beam has been fully
attenuated the radar will display a “radar shadow” which appears as an
end to the precipitation when, in fact, the heavy rain may extend for many
more miles. In the worst case, precipitation attenuation may cause the
area of heaviest precipitation to be displayed as the thinnest area of
heavy precipitation. It may cause one cell containing heavy precipitation
to totally block or shadow a second heavy cell located behind the first cell
and prevent it from being displayed on the radar. Never fly into radar
shadows and never believe that the full extent of heavy rain is being
seen on radar unless another cell or a ground target can be seen beyond
the heavy cell. Proper use of the antenna tilt control can help detect radar
shadows.
RDR 2100 PG 3/12/07 3:45 PM Page 13