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Global Positioning System (GPS)
GPS
is currently the only fully functional Global Navigation Satellite System. More
than two dozen GPS satellites are in medium Earth orbit, transmitting signals
allowing GPS receivers to determine the location, speed and direction.
A GPS receiver calculates its position by measuring the distance between itself
and three or more GPS satellites. Measuring the time delay between transmission
and reception of each GPS radio signal gives the distance to each satellite,
since the signal travels at a known speed. The signals also carry information
about the satellites' location. By determining the position of, and distance
to, at least three satellites, the receiver can compute its position using
trilateration.Receivers typically do
not have perfectly accurate clocks and therefore track one or more additional
satellites to correct the receiver's clock error.
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