Garmin Flight Deck 100 Marine Instruments User Manual


 
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.