Pilot's Guide

Garmin G1000 Pilot’s Guide for the Hawker Beechcraft C90A/GT/GTi
190-00663-02 Rev. B330
HAZARD AVOIDANCE
NEXRAD AND AIRBORNE WEATHER RADAR
BothAirborneWeatherRadarandNEXRADmeasureweatherreectivityindecibels(dB).Adecibelisa
logarithmicexpressionoftheratiooftwoquantities.AirborneWeatherRadarmeasurestheratioofpower
againstthegainoftheantenna,whileNEXRADmeasurestheenergyreectedbacktotheradar,ortheradar
reflectivity ratio.
Bothsystemsusecolorstoidentifythedifferentechointensities,butthecolorsarenotinterchangeable.
AirbornecolorradarvaluesusedbyGarminAirborneColorWeatherRadarshouldnotbeconfusedwith
NEXRADradarvalues.
ANTENNA BEAM ILLUMINATION
Theradarbeamismuchlikethebeamofaspotlight.Thefurtherthebeamtravels,thewideritbecomes.
Theradarisonlycapableofseeingwhatisinsidetheboundariesofthebeam.Thegurebelowdepictsaradar
beam’scharacteristics.Thegureillustratesverticaldimensionsoftheradarbeam,althoughthesameholds
trueforthehorizontaldimensions.Inotherwords,thebeamisaswideasitistall.Notethatitispossibleto
missareasofprecipitationontheradardisplaybecauseoftheantennatiltsetting.Withtheantennatiltset
tozerointhisillustration,thebeamovershootstheprecipitationat15nauticalmiles.
0
80
Altitude (x1000 ft.)
30 0 45 60 75 90
Range (nautical miles)
Half Power at Beam Sidelobes
Antenna at Zero Tilt
18,000 ft.
18,000 ft.
Max Power at Beam Center
15
8°
Figure 6-53 Radar Beam from a 12 inch Antenna
Thecurvatureoftheearthcanalsobeafactorinmissingareasofprecipitation,especiallyatrangesettings
of150nauticalmilesormore.Herethebeamovershootstheprecipitationatlessthan320nauticalmiles.
320 nm
Figure 6-54 Radar Beam in Relation to the Curvature of the Earth