12-34GTN 725/750 Pilot’s Guide 190-01007-03 Rev. CForewordGettingStartedAudio &Xpdr CtrlCom/NavFPLDirect-ToProcChartsWpt InfoMapTrafflcTerrainWeatherNearestServices/MusicUtilitiesSystemMessagesSymbolsAppendixIndex12.3.1.3 Radar Signal AttenuationThe phenomena of weather attenuation needs to be kept in mind wheneveroperating the weather radar. When the radar signal is transmitted, it isprogressively 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 radarenergy at a distance from the antenna is inversely proportional to the square ofthe distance. The reflected radar energy from a target 40 miles away that fills theradar beam will be one fourth the energy reflected from an equivalent target 20miles away. This would appear to the operator that the storm is gaining intensityas the aircraft gets closer. Internal circuitry within the GWX system compensatesfor 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 portionof the radar energy is reflected back to the antenna. However, much of theenergy is absorbed. If precipitation is very heavy, or covers a large area, thesignal may not reach completely through the area of precipitation. The weatherradar system cannot distinguish between an attenuated signal and area of noprecipitation. 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, theheavy rain may extend much further. A cell containing heavy precipitation mayblock another cell located behind the first, preventing it from being displayedon the radar. Never fly into these shadowed areas and never assume that all ofthe heavy precipitation is being displayed unless another cell or a ground targetcan be seen beyond the heavy cell. The WATCH™ feature of the GWX 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.Attenuation can also be due to poor maintenance or degradation of theradome. Even the smallest amount of wear and tear, pitting, and pinholes on theradome surface can cause damage and system inefficiency.
