Garmin Software Version 0370.17 Podręcznik Użytkownika Pobierz pdf (Strona 103)

Garmin G1000 Cockpit Reference Guide for the DA42

190-00406-05 Rev. B

10-13

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 will be 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 will compensate

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 will display 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 will 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, will be 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 10-21) 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 10-21 Precipitation Type and Reﬂectivity

1 2 ... 98 99 100 101 102 103 104 105 106 107 108 ... 141 142

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