2.3 Control Segment

• It is the part of the GPS with 5 observations stations that:
  • Monitor and control the satellite system continously
  • Determine the GPS system time
  • Predict the satellite ephemerides and the behavior of the satellite clocks
  • Update periodically the navigation message for each particular satellite.
• There are one Master Control Station (Colorado Springs), the five are monitor stations and there are three stations with ground antennas for uploading data into the satellites.
• Recently an additional station has been added in Cape Cañaveral (FL, USA) and an additional military network (NIMA) is used for near real time performance evaluation and additional real-time data as well.

• The control segment observes the GPS constellations with GPS receivers and determine orbits, clocks and forms the navigation data (message). The GPS system time is referred to a given cesium oscillator.