Garmin International Inc 0126000 Manuel D’Utilisation
PRELIMINAR
Y
Garmin G5000
™
Pilot’s Guide (Preliminary)
337
HAZARD AVOIDANCE
capabilities, and pilot experience. Airborne weather radar detects rain or hail, not clouds or mist. The display
may indicate clear areas between intense returns, but this does not necessarily mean it is safe to fly between
them. In addition, Doppler radar measurement of precipitation velocity only occurs when rain or hail is
moving along the radar beam and either toward or away from the antenna. The system cannot detect Clear Air
Turbulence as there are no radar echoes to process.
may indicate clear areas between intense returns, but this does not necessarily mean it is safe to fly between
them. In addition, Doppler radar measurement of precipitation velocity only occurs when rain or hail is
moving along the radar beam and either toward or away from the antenna. The system cannot detect Clear Air
Turbulence as there are no radar echoes to process.
Airborne weather radar has other capabilities beyond weather detection. It also has the ability to detect and
provide distance to cities, mountains, coastlines, rivers, lakes, and oceans.
NEXRAD AND AIRBORNE WEATHER RADAR
Both Airborne Weather Radar and NEXRAD measure weather reflectivity in decibels (dB). A decibel is a
logarithmic expression of the ratio of two quantities. Airborne Weather Radar measures the ratio of power
against the gain of the antenna, while NEXRAD measures the energy reflected back to the radar, or the radar
reflectivity ratio.
against the gain of the antenna, while NEXRAD measures the energy reflected back to the radar, or the radar
reflectivity ratio.
Both systems use colors to identify the different echo intensities, but the colors are not interchangeable.
Airborne color radar values used by Garmin Airborne Color Weather Radar should not be confused with
NEXRAD radar values.
NEXRAD radar values.
ANTENNA BEAM ILLUMINATION
The radar beam is much like the beam of a spotlight. The further the beam travels, the wider it becomes.
The radar is only capable of seeing what is inside the boundaries of the beam. The figure below depicts a radar
beam’s characteristics. The figure illustrates vertical dimensions of the radar beam, although the same holds
true for the horizontal dimensions. In other words, the beam is as wide as it is tall. Note that it is possible to
miss 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 nautical miles.
beam’s characteristics. The figure illustrates vertical dimensions of the radar beam, although the same holds
true for the horizontal dimensions. In other words, the beam is as wide as it is tall. Note that it is possible to
miss 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 nautical miles.
Figure 6-50 Radar Beam from a 12 inch Antenna
0
80
Altitude (x1000 ft.)
30
0
45
60
75
90
Range (nautical miles)
Half Power at Beam Sidelobes
Antenna at Zero Tilt
18,000 ft.
18,000 ft.
18,000 ft.
Max Power at Beam Center
15
8°
The curvature of the earth can also be a factor in missing areas of precipitation, especially at range settings of
150 nautical miles or more. Here the beam overshoots the precipitation at less than 320 nautical miles.
GWX 70 Prelim.indd 337
7/25/2012 3:14:44 PM