Antenna Beam Illumination - Garmin G500 Pilot's Manual

of target range. It takes 123.6 micro-seconds for a transmitted pulse to make
the round trip if a target is 10 NM away.
The GWX 68 weather radar should be used to avoid severe weather, not
for penetrating severe weather. The decision to fly into an area of radar targets
depends on target intensity, spacing between the targets, aircraft capabilities
and pilot experience. Pulse type weather radar detects only precipitation, not
clouds or turbulence. The display may indicate clear areas between intense
returns, but this does not necessarily mean it is safe to fly between them. Only
Doppler radar can detect turbulence.
Airborne weather radar has other capabilities beyond weather detection.
It also has the ability to detect and provide distance to objects on the ground,
such as, cities, mountains, coastlines, rivers, lakes, and oceans.
4.8.1.2

Antenna Beam Illumination

It is important to understand the concept of the antenna beam illumina-
tion. 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.
The vertical dimensions of the radar beam are shown in the figure above
and 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 precipita-
tion at 15 NM. The curvature of the earth can also be a factor, especially at
range settings of 150 NM or more.
4-64
Figure 4-55 Radar Beam from 12 inch Antenna
Garmin G500 Pilot's Guide 190-01102-02 Rev. B