global positioning system (GPS)

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System of 21 US satellites orbiting earth twice a day at an altitude of 10,900 miles to provide satellite ranging. Almost any point on earth can be located by measuring its distance from a group of these satellites. A navigation device in a boat, car, ship, airplane, or mobile phone gets a 'fix' on at least four satellites to measure its precise location (longitude, latitude and, in some cases, altitude and velocity) accurate to within about a meter where the service is supplemented by a ground based transmitter.


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