4-65190-01102-02 Rev. B Garmin G500 Pilot’s GuideForewordSec 1SystemSec 2PFDSec 3MFDSec 4HazardAvoidanceSec 5AdditionalFeaturesSec 6Annun.& AlertsSec 7SymbolsSec 8Glossary Appendix AAppendix BIndex NMFigure 4-56 Radar Beam in Relation to the Curvature of the Earth4.8.1.3 Radar Signal AttenuationThe phenomena of weather attenuation needs to be kept in mind wheneveroperating the weather radar. When the radar signal is transmitted, it is progres-sively absorbed and scattered, making the signal weaker. This weakening, orattenuation, is caused by two primary sources, distance and precipitation.Attenuation because of distance is due to the fact that the amount of radarenergy at a distance from the antenna is inversely proportional to the squareof the distance. The reflected radar energy from a target 40 miles away thatfills the radar beam will be one fourth the energy reflected from an equivalenttarget 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 68system 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 ofthe radar energy is reflected back to the antenna. However, much of the energy isabsorbed. If precipitation is very heavy, or covers a large area, the signal may notreach completely through the area of precipitation. The weather radar systemcannot 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 mayextend much further. A cell containing heavy precipitation may block anothercell 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 heavyprecipitation is being displayed unless another cell or a ground target can beseen beyond the heavy cell. The WATCH™ feature of the GWX 68 WeatherRadar system can help in identifying these shadowed areas. Areas in questionwill appear as “shadowed” or gray area on the radar display. Proper use of theantenna tilt control can also help detect radar shadows.
