User Guide

Garmin G1000 Pilot’s Guide for Cessna Citation Mustang
190-00494-00 Rev. B6-30
HAZARD AVOIDANCE
NEXRAD AND AIRBORNE COLOR WEATHER RADAR
Both Garmin Airborne Color Weather Radar and NEXRAD measure weather reflectivity in decibels, or dB.
A decibel is a logarithmic expression of the ratio of two quantities. Garmin Airborne Color Weather Radar
measures the ratio of power against the gain of the antenna, while NEXRAD measures the energy reflected
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. Do
not confuse airborne color radar values used by Garmin Airborne Color Weather Radar with NEXRAD radar
values.
ANTENNA BEAM ILLUMINATION
It is important to understand the concept of the antenna beam illumination. The radar beam is much like
the beam of a spotlight. The farther the beam travels, the wider it gets. The radar is only capable of “seeing”
what is inside the boundaries of the beam.
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°
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Figure 6-34 Radar Beam from 12 inch Antenna
Figure 6-34 depicts a radar beam’s characteristics. The figure illustrates vertical dimensions of the radar
beam, although the same holds true for the horizontal dimensions. In other words, the beam will be as wide
as it is tall. Note that it is possible not to see 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. The curvature of the earth can also be a factor, especially at range settings of 150 nautical
miles or more ( Figure 6-35).

Figure 6-35 Radar Beam in Relation to the Curvature of the Earth