Part Two: Section 2
XM Weather
is "METAR" (rather than "NEXRAD"), press
the small right (CRSR) knob to highlight
that word. Then turn the small right knob to
change "METAR" to "NEXRAD."
3.
Press the small right knob again to bring
down the cursor (that is, to stop the blinking
highlighting of "NEXRAD") and retain the
selection.
NEXRAD Intensity
Colors are used to identify the different NEXRAD
echo intensities (reflectivity) measured in dBZ (deci-
bels of Z). "Reflectivity" is the amount of 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 comparison, a
decibel (or logarithmic) scale (dBZ), is used. The dBZ
values increase as the strength of the signal returned
to the radar increases. There are seven gradations for
rain, two gradations for mixed rain and snow, and two
gradations for snow.
To display the Nexrad Intensity Legend:
1.
While viewing the XM Weather page, press the
MENU key to display the Page Menu.
Turn the large or small knob to select "Display
2.
legend?"
14
Press ENT to display the Nexrad Intensity
3.
Legend.
4.
Turn the large or small knob to scroll through
the full table. Press CLR to return to the normal
view.
NEXRAD Abnormalities
There are possible abnormalities regarding dis-
played NEXRAD images. Some, but not all, causes of
abnormal displayed information include:
• Ground Clutter
• Strobes and spurious radar data
• Sun strobes, when the radar antenna points
directly at the sun
• Military aircraft deploy metallic dust which can
cause alterations in radar scans
• Interference from buildings or mountains,
which may cause shadows
• Scheduled maintenance may put a radar off-line
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