User Guide

190-00494-00 Rev. B
Garmin G1000 Pilot’s Guide for Cessna Citation Mustang
6-31
HAZARD AVOIDANCE
RADAR SIGNAL ATTENUATION
The phenomenon of radar signal attenuation needs to be kept in mind whenever operating the weather
radar. When the radar signal is transmitted, it is progressively absorbed and scattered, making the signal
weaker. This weakening, or attenuation, is caused by two primary sources, distance and precipitation.
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. The reflected radar energy from a target 40 miles away that fills
the radar beam will be one fourth the energy reflected from an equivalent target 20 miles away. This would
appear to the operator that the storm is gaining intensity as the aircraft gets closer. Internal circuitry within
the GWX 68 system will compensate for much of this distance attenuation.
Attenuation due to precipitation is not as predictable as distance attenuation. It is also more intense. As the
radar signal passes through moisture, a portion of the radar energy is reflected back to the antenna. However,
much of the energy is absorbed. If precipitation is very heavy, or covers a large area, the signal may not
reach completely through the area of precipitation. The weather radar system cannot distinguish between an
attenuated signal and area of no precipitation. If the signal has been fully attenuated, the radar will display
a “radar shadow”. This appears as an end to the precipitation when, in fact, the heavy rain may extend much
further. A cell containing heavy precipitation may block another cell located behind the first, preventing it
from being displayed on the radar. Never fly into these shadowed areas and never assume that all of the heavy
precipitation is being displayed unless another cell or a ground target can be seen beyond the heavy cell. The
WATCH™ feature of the GWX 68 Weather Radar system can help in identifying these shadowed areas. Areas
in question will appear as ‘shadowed’ or gray area on the radar display. Proper use of the antenna tilt control
can also help detect radar shadows.
Attenuation can also be due to poor maintenance or degradation of the radome. Even the smallest amount
of wear and tear, pitting, and pinholes on the radome surface can cause damage and system inefficiency.