Embraer Prodigy® Flight Deck 100 Pilot’s Guide
190-00728-04 Rev. A
358
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.
Figure 6-50 Radar Beam from a 12 inch Antenna
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°
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 6-51 Radar Beam in Relation to the Curvature of the Earth