Global Positioning System (GPS)

The control segment consists of one Master Control Station (MCS) and five monitoring stations. The five monitor stations are used to track the flight paths of the satellite, which located at Hawaii, Kwajalein, Diego Garcia, Ascension Island and Colorado Springs, Colorado. These tracking information is sent to the Air Force Space Command’s master control station at Schriever Air Force Base in Colorado Springs, which is operated by the 2^nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF). Generally, the MCS will compare the tracking data from the monitor stations and calculates the satellite orbit and clock parameters.

The 2 SOPS contacts each GPS satellite regularly with a navigational update (using the ground antennas at Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite’s internal orbital model. The updates are created by a Kalman filter which uses inputs from the ground monitoring stations, space weather information, and various other inputs.

Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked ‘unhealthy’, so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.