Garmin GTN 725 Pilot's Manual page 337

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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.

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.

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 precipitation at 15 NM. The

curvature of the earth can also be a factor, especially at range settings of 150 NM

Figure 12-45 Radar Beam in Relation to the Curvature of the Earth

190-01007-03 Rev. D

Figure 12-44 Radar Beam from 12 inch Antenna

Garmin GTN 725/750 Pilot's Guide

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