Garmin Flight Deck 100 Manual

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190-00728-04 Rev. A

Embraer Prodigy

Flight Deck 100 Pilot’s Guide

359

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 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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.