Global positioning system (GPS) is a worldwide timing and navigation system that utilizes 24 satellites positioned in orbit approximately 12,000 miles above the Earth's surface. These satellites continuously transmit digital radio signals pertaining to the exact time (using atomic clocks) and exact location of the satellites. By referring to signals transmitted by four satellites, the GPS can be used to calculate longitude, latitude, and altitude. The Department of Defense originally developed the GPS for all-weather military applications, but the system is available at no additional charge for civilians and commerce. Boeing Intelligence and Security Systems Mission Operations controls the satellite telemetry, system health and troubleshooting.
Link: http://www.boeing.com/defense-space/space/gps/m_gen.html