Garmin G900X Pilot's Manual page 417

RADAR SIGNAL ATTENUATION
The phenomenon of radar signal attenuation affects the operation of weather radar. When the radar
transmits the signal, the signal is progressively absorbed and scattered, making the signal weaker. This
weakening, or attenuation, is caused by two primary sources, distance and precipitation.
Radar energy leaving the antenna is inversely proportional to the square of the distance. The reflected
radar energy from a target 40 miles away that fills the radar beam is one fourth the energy reflected from
an equivalent target 20 miles away. This would appear to the operator that the storm is gaining intensity as
the aircraft gets closer. Internal signal processing within the GWX 68 system compensates for much of this
distance attenuation.
Attenuation due to precipitation is not as predictable as distance attenuation. It is also more intense. As the
radar signal passes through moisture, a portion of the radar energy is reflected back to the antenna. However,
much of the energy is absorbed. If precipitation is very heavy, or covers a large area, the signal may not
reach completely through the area of precipitation. The weather radar system cannot distinguish between an
attenuated signal and an area of no precipitation. If the signal has been fully attenuated, the radar displays
a radar shadow. This appears as an end to the precipitation when, in fact, the heavy rain may extend much
further. A cell containing heavy precipitation may block another cell located behind the first, preventing it
from being displayed on the radar. Never fly into these shadowed areas and never assume that all of the heavy
precipitation is being displayed unless another cell or a ground target can be seen beyond the heavy cell. The
WATCH ® feature of the GWX 68 Weather Radar system can help in identifying these shadowed areas. Areas
in question appear as shadowed or gray on the radar display. Proper use of the antenna tilt control can also
help detect radar shadows.
Attenuation can also be due to poor maintenance or degradation of the radome. Even the smallest amount of
wear and scratching, pitting, and pinholes on the radome surface can cause damage and system inefficiency.
RADAR SIGNAL REFLECTIVITY
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ReciPitation
Weather radar detects precipitation or objects more dense than water, such as the surface of the earth
or solid structures. The weather radar does not detect clouds, thunderstorms, or turbulence directly.
It detects precipitation associated with clouds, thunderstorms, and turbulence. The best radar signal
reflectors are raindrops, wet snow, or wet hail. The larger the raindrop, the better the reflectivity. The
size of the precipitation droplet is the most important factor in radar reflectivity. Because large drops in a
small concentrated area are characteristic of a severe thunderstorm, the radar displays the storm as a strong
return. Ice crystals, dry snow, and dry hail have low levels of reflectivity as shown in the illustration, and
are often not displayed by the radar. Additionally, a cloud that contains only small raindrops, such as fog
or drizzle, does not reflect enough radar energy to produce a measurable target return.
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Garmin G900X Pilot's Guide
HAZARD AVOIDANCE
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