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Encyclopedia entry: Global Positioning System
- Article from:
- The Oxford Companion to the Earth
- Author:
Copyright© The Oxford Companion to the Earth 2000, originally published by Oxford University Press 2000. (Hide copyright information)
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Global Positioning System
(
GPS
) The Global Positioning System (GPS) is operated by the US Department of Defense and can be used worldwide for both navigation and accurate positioning. The system consists of 24 satellites in high (20 000 km altitude) orbits, positioned so that at least four satellites are always visible to an observer at any point on the Earth with a good view of the sky (Fig.1). Each satellite contains a precise atomic clock and transmits two L-band microwave radio signals (on 1.2 and 1.6 GHz), both of which are modulated with codes defining the time at which the signal left the satellite. Observations from four satellites allow the receiver to compute the precise ...