To provide satellite ranging, a system of 21 US satellites orbit earth twice a day at an altitude of 10,900 miles. Accurate to within a yard if supplemented by a ground-based transmitter. Any point on earth can be identified by measuring its distance from a group of these satellites. A navigation device in a boat, car, ship, airplane, or mobile phone locks on to at least four satellites. Each of these satellites measure a precise location of longitude, latitude and, in some cases, altitude and velocity.

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