DISPLAY
SCREEN
FUNCTION
KEYS
ALPHANUMERIC
KEYPAD
ANTENNA
GPS RECEIVER
1-4
Monitor stations and ground antennas throughout the world passively
track the GPS satellites and relay data to the master control station.
Exact satellite position and signal-data accuracy can therefore be
constantly updated and maintained. Minor discrepancies between
where the satellite "thinks" it is and where the monitor station "knows" it
is can also be adjusted.
lf any satellite emits erroneous data or is otherwise not operating
properly, a ground station marks it "unhealthy." The affected satellite
broadcasts its status to the GPS receiver, which is programmed to
ignore an unhealthy satellite and use the next best satellite to obtain a
position fix.
The master control station can selectively degrade satellite data. This
degradation, or Selective Availability (SA), can cause positioning errors
of 100 meters (2D RMS).
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