Cockpit Reference Guide

190-00763-02 Rev. A
Garmin G1000 Pilot’s Guide for the Piper PA-46 Meridian
331
HAZARD AVOIDANCE
SYSTEM
OVERVIEW
FLIGHT
INSTRUMENTS
EICAS
AUDIO PANEL
& CNS
FLIGHT
MANAGEMENT
HAZARD
AVOIDANCE
AFCS
ADDITIONAL
FEATURES
APPENDICES INDEX
SYSTEM
OVERVIEW
FLIGHT
INSTRUMENTS
EICAS
AUDIO PANEL
& CNS
FLIGHT
MANAGEMENT
HAZARD
AVOIDANCE
AFCS
ADDITIONAL
FEATURES
APPENDICES INDEX
RADAR SIGNAL ATTENUATION
The phenomenon of radar signal attenuation affects the operation of 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reectedradarenergyfromatarget40milesawaythatlls
theradarbeamisonefourththeenergyreected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signalprocessing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reected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anareaofnoprecipitation.Ifthesignalhasbeenfullyattenuated,theradardisplays
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rst,preventingit
frombeingdisplayedontheradar.Nevery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appearasshadowedorgray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 scratching, pitting, and pinholes on the radome surface can cause damage and system inefficiency.