Garmin GPS 400 Pilot's Manual And Reference page 136

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AUX PAGES
Setup 1
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Position / Map Datum— configures position readout information to the desired
position format and datum. The NavData® card contains over 100 map datums
to adjust your position reading, making it conform to specific paper charts. If
you are using a chart based on another datum, you must set the GPS 400 to use
the same datum. Using a map datum that does not match the charts you are
using can result in significant differences in position information. If you are
using the paper charts for reference only, the GPS 400 will still provide correct
navigation guidance to the waypoints contained in the database, regardless of
the datum selected.
Date / Time— provides settings for time format (local or UTC; 12- or 24-hour)
and time offset. The time offset is used to define current local time. UTC (also
called "GMT" or "zulu") date and time are calculated directly from the GPS
satellites' signals and cannot be changed. If you prefer to use local time, simply
designate the offset by adding or subtracting the correct number of hours.
Setup 1 Page: Airspace Alarms
To set the airspace warning messages or change the altitude buffer:
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When an approach has been loaded into the active flight plan, airspace
alert messages will be disabled within 30 nm of the destination airport.
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