Overview  

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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