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A system allowing its users to determine position accurately anywhere on the Earth\'s surface. A constellation of satellites sends very precisely timed signals to a receiver; by comparing the arrival times of signals from three or more satellites it is possible to determine two-dimensional position; four or more signals can determine three-dimensional position. Two systems were in operation at the time of writing: the US system, known as GPS, and the Russian system, Glonass. GPS consists of a constellation of 24 satellites, and can determine position to 100 m (in 9 cases out of 10) for civilian applications, and 30 m for military applications, which have access to better signals. By comparing the signals received at two stations it is possible to establish relative position much more accurately. GPS is being used in geocoding, to monitor locations of vehicles, to aid navigation, and for many other practical purposes. (MG) |
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