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Garmin G1000 Cockpit Reference Guide for the Socata TBM 850

190-00708-00 Rev. A

10-18

SECTION 10 – HAZARD

AVOIDANCE

Radar Signal Attenuation

Attenuation because of distance is due to the fact

that the 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. 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 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 area

on the radar display. Proper use of the antenna tilt control

can also help detect radar shadows.

Radar Signal Reﬂectivity

Precipitation

Precipitation or objects more dense than water, such as

earth or solid structures, are detected by the weather radar.

The weather radar will 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 it reﬂects. 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, dry snow, and

dry hail have low levels of reﬂectivity (Figure 10-28) and

often will not be displayed by the radar. Additionally, a

cloud that contains only small raindrops, such as fog or

drizzle, will not reﬂect enough radar energy to produce a

measurable target return.

Figure 10-28 Precipitation Type and Reﬂectivity

1 2 ... 165 166 167 168 169 170 171 172 173 174 175 ... 227 228

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Garmin Software Version 0719.00 Guía de usuario Pagina 170

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