Pilot's Guide

190-00663-02 Rev. B
Garmin G1000 Pilot’s Guide for the Hawker Beechcraft C90A/GT/GTi
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HAZARD AVOIDANCE
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 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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.
RADAR SIGNAL REFLECTIVITY
PreciPitation
Precipitation or objects more dense than water, such as the surface of the earth or solid structures, are
detectedbytheweatherradar.Theweatherradardoesnotdetectclouds,thunderstorms,orturbulence
directly.Itdetectsprecipitationassociatedwithclouds,thunderstorms,andturbulence.Thebestradar
signalreectorsareraindrops,wetsnow,orwethail.Thelargertheraindrop,thebetterthereectivity.The
sizeoftheprecipitationdropletisthemostimportantfactorinradarreectivity.Becauselargedropsina
small concentrated area are characteristic of a severe thunderstorm, the radar displays the storm as a strong
return.Icecrystals,drysnow,anddryhailhavelowlevelsofreectivityasshownintheillustration,and
oftennotdisplayedbytheradar.Additionally,acloudthatcontainsonlysmallraindrops,suchasfogor
drizzle, does not reflect enough radar energy to produce a measurable target return.