Dictionary:Global Positioning System (GPS)

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The GPS Navstar positioning system of the U.S. Defense Dept. provides location determination by observation of 24 satellites (4 each in 6 different 55° orbital planes) at 20 200 km altitude; they orbit the Earth in about 12 hours. Each satellite broadcasts on two frequencies, L1 (1575.42 MHz) and L2 (1227.60 MHz); they broadcast a hand-over word (q.v.) and almanac (q.v.) information that are superimposed on the carrier by biphase phase-shifting using C/A-, P-, and Y-codes. The system is available worldwide with six or seven satellites continuously observable. A receiver determines its distance (pseudorange) from a satellite by correlating the satellite’s phase-encoded signal with one it generates; the pseudorange differs from actual range because the receiver’s and satellite’s clocks are not synchronized. From simultaneous observation of the various satellites, a stationary or moving observer can calculate precise time, 3-D position, and 3-D velocity. When combined with similar information received at a nearby fixed station (differential GPS or translocation), accuracy of less than a meter can be achieved. Provisions are made to degrade the accuracy if required by the military.