Control Segment
The control or the ground segment consists of the Master Control Station (MCS) at Colorado Springs in the United States and five monitor stations sited at Ascension Island, Diego Garcia, Kwajalein, Hawaii and Colorado Springs. Ground upload antennae are also located at the monitoring stations. The stations track all satellites in view (five degrees or more above the horizon) and accumulate ranging data on them. The data is passed to the MCS, where it is processed to calculate satellite ephemeris (positions in space), clock drifts and propagation delays. This information is uplinked to the satellites at least three times a day so that the navigation data transmitted by the satellites to the users may be updated.
