The Control Segment of the Global Positioning System is a network of ground stations that monitors the shape and velocity of the satellites' orbits.
The accuracy of GPS data depends on knowing the positions of the satellites at all times.
The orbits of the satellites are sometimes disturbed by the interplay of the gravitational forces of the Earth and Moon.
The GPS Ground Segment (also referred to as Control Segment or Operational Control System) is the responsible for the proper operation of the GPS system.
The GPS Control Segment is composed by a network of Monitor Stations (MS), a Master Control Station (MCS), a backup of the MCS and the Ground Antennas (GA).

The Master Control Station (MCS) processes the measurements received by the Monitor Stations (MS) to estimate satellite orbits (ephemerides) and clock errors, among other parameters, and to generate the navigation message.
These corrections and the navigation message are uploaded to the satellites through the Ground Antennas, which are co-located in four of the Monitor stations (Ascension Island, Cape Canaveral, Diego Garcia, and Kwajalein).

The main tasks performed by the CS are the following:

• Monitoring and control of satellite orbital parameters;
• Monitoring health and status of the satellite subsystems (solar arrays, battery power and the level of propellant used for maneuvers);
• Activation of spare satellites;
• Update of parameters in the navigation message (ephemeris, almanac and clock corrections);
• Resolving satellite anomalies;
• Controlling Selective Availability (SA) and Anti-Spoofing (A/S)
• Passive tracking of the satellites.