GPS – Control Segment

If you’ve already read articles about the Space Segment and gps User Segment, you’ve come to the end of the engineering presentation of the system that revolutionized maritime, air and land transportation.

The control segment consists of a network of stations spread across the globe and at various latitudes that perform:

  • Satellite tracking (they chase them into orbit to verify that the real and expected positions are identical net of small orbit disturbances).
  • Signal monitoring.
  • Performance analysis (in terms of accuracy).
  • Sending updates to satellites.
  • Corrections of positions in orbit.

The current control segment configuration, OCS – Operational Control Segment, includes a single Master Station, an Alternative Master Station, 11 command and control antennas, 16 monitoring sites.

icaptain, GPS - Segmento di Controllo
Distribution of the GPS System Control Segment

The Master Station is located in Colorado and is the only station capable of communicating with satellites to modify their orbital parameters or update ephemerides as a result of processing data from tracking and/or monitoring stations (alternatively the California Master of Respect can intervene).

Monitor Stations:

  • They monitor satellites when they are in sight.
  • They collect navigation data.
  • They calculate the position of the receiver that will allow to carry out checks on the operation of the satellites.
  • Signals are received and processed with advanced GPS receivers and all collected data is sent to the Master Station.

Of the 16 monitoring stations, 6 are military sites operated entirely by the AIR Force.

The other 10 additional stations are the result of the project L-ALL Legacy Accuracy Improvement Initiative completed in 2008 and created to increase the accuracy of the system through the expansion of the control segment; This expansion has improved the overall accuracy of the system from 10 to 15; The result was made possible by monitoring on a global scale previously limited to military bases only.

Stations introduced by L-ALL are operated by the NGA – National Geospatial-Intelligence Agency

The Ground Antennas complex consists of 4 dedicated GPS antennas plus 7 Air Force Satellite Control Network for satellite tracking.

These antennas are capable of:

  • Send signals in order to update satellite navigation data.
  • Process and program satellite data.
  • Collect telemetry.
  • Communicate via Band S with the space segment to resolve any anomalies related to the orbits.

The Master Control Station is the only ground station that can make changes to the entire constellation using data computed from the various control stations; In addition:

  • Generates navigation messages to update on satellites.
  • It monitors transmissions and system integrity in order to keep the space segment in good condition even in terms of accuracy.
  • Plans the maintenance and resolution of anomalies of any satellites including repositioning in orbit in order to keep the constellation in the best configuration.
Master Station Operations Centre

The Master Station, in order to manage operational and decommissioned satellites, uses the AEP system – Architecture Evolution Plan and LADO – Launch/Early Orbit, Anomaly Resolution, and Operations Disposal.

In concluding these first three articles dedicated to GPS, we recommend that you stay up to date on the evolution of the system to visit exclusively the US Government Portal on which the official and up-to-date data are present.