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4-65190-01102-02 Rev. B
Garmin G500 Pilot’s Guide
Foreword
Sec 1
System
Sec 2
PFD
Sec 3
MFD
Sec 4
Hazard
Avoidance
Sec 5
Additional
Features
Sec 6
Annun.
& Alerts
Sec 7
Symbols
Sec 8
Glossary Appendix A
Appendix B
Index
NM
Figure 4-56 Radar Beam in Relation to the Curvature of the Earth
4.8.1.3 Radar Signal Attenuation
The phenomena of weather attenuation needs to be kept in mind whenever
operating the weather radar. When the radar signal is transmitted, it is progres-
sively 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 amount of radar
energy at a distance from 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 gain-
ing intensity as the aircraft gets closer. Internal circuitry within the GWX 68
system compensates 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.