Control Segment (Monitor Stations)

The GPS-System is controlled by the United States Air Force. The “master control station” (Schriever AFB) and four additional monitoring stations (on Hawaii, Ascension Islands, Diego Garcia and Kawajalein) were set up for monitoring the satellites.
During August and September 2005, six more monitor stations of the NGA (National Geospatial-Intelligence Agency) were added to the grid. Now, every satellite can be seen from at least two monitor stations. This allows to calculate more precise orbits and ephemeris data. For the end user, a better position precision can be expected from this. In the near future, five more NGA stations will be added so that every satellite can be seen by at least three monitor stations. This improves integrity monitoring of the satellites and thus the whole system.

Position of the monitor stations and the master control station
Satellite-tracking-station on Hawaii
(Source: Schriever Air Force Base Satellite Flyer Vol. 6; No.12)

The passive monitor stations are GPS receivers which track all satellites in their range and collect data of the satellite signals. The raw data are then sent to the mater control station where the data are processed. The stations on
Ascension Islands, Diego Garcia and Kwajalein are also transmitting stations for correction data.The “master control station” is located on the Schriever Air Force Base (formerly Falcon AFB), about 20 km south of Colorado Springs.

The “50th Space Wing’s 2nd Space Operations Squadron” is responsible for the operation of the GPS system. Here the data from the monitor stations are processed 24 h a day in real time. As results, information about orbits and clocks of the satellites are obtained. Doing this, possible malfunctions can quickly be detected.
Additionally, from the raw data new ephemeresis data are calculated. Once to twice a day, theses data and other commands are sent back to the satellites via the transmitting antennae on Ascension Islands, Diego Garcia or Kwajalein by means of a S-band signal (S-band: 2000 – 4000 MHz).
Block IIR satellites are capable of exchanging data with other satellites and can correct their orbit data on their own. In theory they only need a contact to a ground station every 180 days. Latest information about the status of the GPS system can be obtained here and here.