A Global Positioning System (GPS) is a space-based radio-navigation system. GPS provides users with accurate information about their position and velocity, as well as the time, anywhere in the world and in all weather conditions.

GPS, formally known as the Navstar Global Positioning System, was initiated in 1973 to reduce the proliferation of navigation aids. GPS is operated and maintained by the United States Department of Defense. By creating a system that overcame the limitations of many existing navigation systems, GPS became attractive to a broad spectrum of users. GPS has been successful in classical navigation applications, and because its capabilities are accessible using small, inexpensive equipment, GPS has also been used in many new applications.

GPS determines location by computing the difference between the time that a signal is sent by a satellite and the time it is received by a GPS receiver. GPS satellites carry atomic clocks that provide extremely accurate time. The time information is placed in the codes relayed by a satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. A receiver uses the time difference between the time of signal reception and the broadcast time to compute the distance, or range, from the receiver to the satellite. A receiver must account for propagation delays, or decreases in the signal's speed caused by the ionosphere and the troposphere. With information about the ranges to three satellites and the location of satellites when signals are sent, a receiver can compute its own three-dimensional position.

• Datums and Projections
• Contacts

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