Garmin G1000 Pilot's Manual page 294

OPTIONAL EQUIPMENT
Weather Data Link Page Softkeys
The following softkeys perform the XM Weather func-
tions on the Weather Data Link Page:
NEXRAD – press the NEXRAD softkey to show
NEXRAD weather and radar coverage information
(both are activated at the same time). Composite
data from all of the NEXRAD radar sites in the
United States is shown. This data is composed
of the maximum reflectivity from the individual
radar sweeps. The display of the information is
color-coded to indicate the weather level severity.
The update rate is every five minutes. Refer to the
legend for a description of the color code.
NOTE: WSR-88D weather surveillance radar
or NEXRAD (NEXt generation RADar) is a Dop-
pler radar system that has greatly improved
the detection of meteorological events such
as thunderstorms, tornadoes, and hurricanes.
An extensive network of NEXRAD stations
provides almost complete radar coverage of the
continental United States, Alaska, and Hawaii.
The unobstructed range of each NEXRAD is 124
nautical miles.
NEXRAD Abnormalities
There are possible abnormalities regarding displayed
NEXRAD images. Some, but not all, of those include:
• Ground Clutter
• Strobes and spurious radar data
• Sun strobes, when the radar antenna points directly
at the sun
• Interference from buildings or mountains, which
may cause shadows
• Military aircraft deploy metallic dust which can
cause alterations in radar scans
8-16
NEXRAD Limitations
Certain limitations exist regarding the NEXRAD radar
displays. Some, but not all, are listed here for the user' s
awareness:
NEXRAD Intensity
Colors are used to identify the different NEXRAD
echo intensities (reflectivity) measured in dBZ
(decibels of Z).
transmitted power returned to the radar receiver.
Reflectivity (designated by the letter Z) covers a wide
range of signals (from very weak to very strong). So, a
more convenient number for calculations and compari-
son, a decibel (or logarithmic) scale (dBZ), is used. The
dBZ values increase as the strength of the signal returned
to the radar increases.
Garmin G1000 Pilot's Guide for Cessna Nav III
• NEXRAD base reflectivity does not provide sufficient
information to determine cloud layers or precipita-
tion characteristics (hail vs. rain, etc).
• NEXRAD base reflectivity is sampled at the minimum
antenna elevation angle. An individual NEXRAD site
cannot depict high altitude storms at close ranges,
and has no information about storms directly over
the site.
• The resolution of displayed NEXRAD data is 2 kilo-
meters. Therefore, when zoomed in on the display,
each square block is 2 kilometers. The intensity
level reflected by the square will be the highest level
sampled within the 2 kilometer square area.
"Reflectivity" is the amount of
190-00498-00 Rev. A