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The Global Positioning System (GPS) is a navigation system designed by the U.S. Department of Defense that makes use of satellites orbiting the earth and was primarily used in vital military applications. It was developed in 1973 as a method to overcome old navigation systems. It became fully operational in 1994, at which time it was also made available to civilians.
There are 24 functional solar-powered satellites orbiting the planet twice in a day, 21 of which are always active. Three other satellites act as spares. Each satellite contains an atomic clock, a computer and a radio, which is used to broadcast the current time and its continuously changing location. Every satellite is operationally synchronized to send data at the same time. They also make corrections once a day by checking their own sense of location and time against that of a ground station's. When the data is broadcasted, GPS receivers get the data and use it to estimate their own location by triangulating the distance using at least three satellites. A GPS device determines the distance from each satellite and uses this information to pinpoint a specific location. This is also known as trilateration.