Garmin GNC 420W Addendum au manuel du pilote - Page 18

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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."

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:

While viewing the XM Weather page, press the

MENU key to display the Page Menu.

Turn the large or small knob to select "Display

legend?"

Press ENT to display the Nexrad Intensity

Legend.

Turn the large or small knob to scroll through

the full table. Press CLR to return to the normal

view.

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