Kwajalein, Diego Garcia, and Ascension.  All monitor stations except Hawaii and Falcon AFB are

also equipped with ground antennas (see Figure 1 3).  The Control Segment includes a Prelaunch

Compatibility Station (PCS) located at Cape Canaveral, USA, and a back up MCS capability. 

Figure 1 3.  GPS Control Segment Locations

The MCS is the central processing facility for the Control Segment and is responsible for

monitoring and managing the satellite constellation.  The MCS functions include control of

satellite station keeping maneuvers, reconfiguration of redundant satellite equipment, regularly

updating the navigation messages transmitted by the satellites, and various other satellite health

monitoring and maintenance activities.  The monitor stations passively track all GPS satellites in

view, collecting ranging data from each satellite.  This information is transmitted to the MCS

where the satellite ephemeris and clock parameters are estimated and predicted.  The MCS uses

the ground antennas to periodically upload the ephemeris and clock data to each satellite for

retransmission in the navigation message.  Communications between the MCS the MS and GA are

typically accomplished via the U.S. Defense Satellite Communication System (DSCS).  The

navigation message update function is graphically depicted in Figure 1 4.

1 4