Global Positioning System
Global Positioning System (GPS) is a United States satellite system that lets those on the ground, on the water or in the air determine their position with extreme accuracy based on timing signals sent from satellites.
The system was developed by the US Department of Defense and made available to the general public in 1983 under orders of President Reagan. GPS for civilian use used to include a mechanism called Selective Availability that intentionally limited the accuracy of GPS, a mechanism originally intended to confuse potential enemies who might launch a missile attack, but Selective Availability was disabled in 2000, and appears unlikely to be reinstated in the foreseeable future.
Prosecutors are now using data from GPS units to convict criminals. Authorities can analyse data from the GPS units to establish a defendant's whereabouts or to track suspects. 
When the Soviet Union launched the satellite Sputnik in 1957, American scientists of the Johns Hopkins University Applied Physics Laboratory used Sputnik's radio-signal's Doppler Effect to track its position. They realized that a similar procedure for satellites with well known orbits would allow to calculate an unknown position on earth, and so the idea for the Transit was born: the first operational satellite-based navigation system. It consisted from seven low-altitude polar-orbiting satellites, and one of its main purposes was to provide navigation data for the Polaris missile submarines. Though it had a couple of disadvantages, the Transit system was in use until 1996.
Another project was United States Air Force's System 621B, which - other than Transit - allowed for three-dimensional navigation. The army had its own project: Army-SECO, while navy's Timation program used the most accurate clocks, deploying its first satellite in 1967 with quartz-crystal oscillators, but developing atomic clocks for its last two satellites.
In 1968 the Department of Defense coordinated these efforts by creating a tri-service committee - the navigation Satellite Executive Group (NAVSEG): the technologies of System 621B and Timation were combined under the name NAVSTAR GPS - and the last two satellites of the Timation program became the first NAVSTAR satellites: NTS-1 (Feb 22, 1978) and NTS-2 (May 13, 1978).
In 1978, the Naval Research Lab in Washington published its report on the Initial Results of the NAVSTAR GPS NTS-2 Satellite. It listed the following GPS objectives that have been achieved to date:
- launch insertion into GPS constellation position
- demonstrated orbit stability and controllability
- first cesium frequency standard in space
- verification of relativity theory
Until 1985, the eleven Block I satellites were built. This number was increased until 1989 by the Block II satellites. GPS today uses 24 satellites and numerous ground stations.
- Timation, U.S. Naval Research Laboratory, Space Systems Development Department (SSDD)
- The Global Positioning System, Space-Based Positioning Navigation & Timing, National Executive Comittee
- An Introduction to the Global Positioning System, Charlie Leonard, 1999 (revised 2001, 2002), PowerPointPresentation
- Small GPS Devices Help Prosecutors Win Convictions, Associated Press, Fox News, August 28, 2008
- National Space Science Data Center 1978-020A
- National Space Science Data Center 1978-047A
- Initial Results of the NAVSTAR GPS NTS-2 Satellite, Naval Research Lab Washington, D.C., 25 May 1978 (Full text: pdf)