Weather Attenuation Compensation - Telephonics RDR-1600 Pilot's Manual

Theory of Operation (Cont.)

5.6 WEATHER ATTENUATION COMPENSATION

Attenuation is an extremely important phenomenon for the weather radar
operator to understand. When a radar pulse is transmitted into the
atmosphere, it is progressively absorbed and scattered so that it loses its
ability to return to the antenna. This attenuation or weakening of the radar
pulse is caused by two primary sources, distance and precipitation. The
RDR-1600 radars have several advanced features which significantly
reduce the affects of attenuation but no airborne weather radar can elimi-
nate them completely. It is therefore up to the operator to understand 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 operator is that
as the storm is approached it will appear to be gaining in intensity. To com-
pensate for distance attenuation, both Sensitivity Timing Control (STC) and
Extended STC circuitry are employed. The RDR-1600 has an STC range of
approximately 40 nautical miles and within this range the radar will elec-
tronically compensate for the effects of distance attenuation with the net
effect that targets do not appear to grow larger as the distance decreases.
Outside the STC range the Extended STC circuitry increases the displayed
intensity to more accurately represent storm intensity. The Extended STC
will not, however, totally compensate for distance attenuation and,
therefore, targets in this range can be expected to grow as the distance
decreases until reaching the STC range.
Figure 5.6-1. STC Electronically Compensates for
Distance Attenuation
16 RDR-1600 Pilot's Guide
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, however, 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 the 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 precip-
itation. Or it may cause one cell containing heavy precipitation to totally
block or shadow a second heavy cell located behind the first cell and pre-
vent 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.
Attenuation can also be a problem when flying in a large area of general
rain. If the rain is moderate, the radar beam may only reach 20 or 30 miles
before it is fully attenuated. The pilot may fly along for many miles seeing
the same 20-30 miles of precipitation ahead on the radar when, actually, the
rain may extend for a great distance. In order to aid in reducing the effects
of precipitation attenuation, the RDR-1600 contains sophisticated weather
attenuation compensation circuitry. The Attenuation Compensation feature
is totally automatic and requires no pilot action to activate it. However, the
Compensation logic cannot operate until echoes are within the Sensitivity
Time Control range of approximately 40 miles. Whenever a level two
level (yellow) or level three (red) echo is displayed within the STC range,
the Compensation circuits cause the receiver gain to increase while the
antenna scans the sector containing heavy rain. The Compensation circuit-
ry allows the radar beam to effectively look deeper into and through heavy
rain to search for possible storm cells beyond. While Attenuation
Compensation does not eliminate precipitation attenuation, it does allow the
radar to see through more rain at short ranges where every bit of weather
information possible is needed. If there is suspicion that the radar is atten-
uating due to precipitation, exercise extreme caution and ask the ATC
Controller what they are showing. Often the ground-based ATC radar will
have a better overall picture of a large rain area and the pilot can compare
the controller's information with his own radar picture to avoid the strongest
cells in a general area of rain.
TM106101(8/01) TM106101(8/01)
Theory of Operation (Cont.)
CAUTION
RDR-1600 Pilot's Guide 17