The GPS control segment consists of a global network of ground facilities that track the GPS satellites, monitor their transmissions, perform analyses, and send commands and data to the constellation.
The current operational control segment includes a master control station, an alternate master control station, 11 command and control antennas, and 15 monitoring sites. The locations of these facilities are shown in the map above. Download as PDF (111 KB)
2nd Space Operations Squadron (2SOPS)
The GPS constellation delivers consistently high performance due to the dedicated efforts of its operators -- men and women of the U.S. Air Force. The 2nd Space Operations Squadron at Schriever Air Force Base in Colorado Springs, Colorado, is responsible for the daily command and control of the GPS constellation. The squadron ensures continuous GPS availability and high accuracy to millions of users, both military and civilian, on a 24/7 basis.
Control Segment Elements
Master Control Station (MCS)
The master control station in Colorado is where 2SOPS performs the primary control segment functions, providing command and control of the GPS constellation. The MCS generates and uploads navigation messages and ensures the health and accuracy of the satellite constellation. It receives navigation information from the monitor stations, utilizes this information to compute the precise locations of the GPS satellites in space, and then uploads this data to the satellites.
The MCS monitors navigation messages and system integrity, enabling 2SOPS to determine and evaluate the health status of the GPS constellation. 2SOPS uses the MCS to perform satellite maintenance and anomaly resolution. In the event of a satellite failure, the MCS can reposition satellites to maintain an optimal GPS constellation.
Monitor stations track the GPS satellites as they pass overhead and channel their observations back to the master control station. Monitor stations collect atmospheric data, range/carrier measurements, and navigation signals. The sites utilize sophisticated GPS receivers and are operated by the MCS.
There are 15 monitoring stations located throughout the world, including six from the Air Force and nine from the National Geospatial-Intelligence Agency (NGA).
Ground antennas are used to communicate with the GPS satellites for command and control purposes. These antennas support S-band communications links that send/transmit navigation data uploads and processor program loads, and collect telemetry. The ground antennas are also responsible for normal command transmissions to the satellites. S-band ranging allows 2SOPS to provide anomaly resolution and early orbit support.
There are four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape Canaveral. In addition, the control segment is connected to the seven Air Force Satellite Control Network (AFSCN) remote tracking stations worldwide, increasing visibility, flexibility, and robustness for telemetry, tracking, and command.
Control Segment Modernization
As part of the GPS modernization program, the Air Force has continuously upgraded the GPS control segment over the past few years and will keep doing so in the years to come.
Legacy Accuracy Improvement Initiative (L-AII)
The Legacy Accuracy Improvement Initiative, completed in 2008, expanded the number of monitoring sites in the operational control segment from six to 16. This tripled the amount of data collected on GPS satellite orbits, enabling a 10% to 15% improvement in the accuracy of the information broadcast from the GPS constellation.
The L-AII effort added 10 operational GPS monitoring sites owned and operated by the National Geospatial-Intelligence Agency (NGA). NGA originally fielded these sites to help it define the Earth reference frame used by GPS.
Architecture Evolution Plan (AEP)
In 2007, the Air Force implemented the Architecture Evolution Plan, replacing the original, mainframe-based master control station with an entirely new one built on modern IT technologies. The AEP system improves the flexibility and responsiveness of GPS operations and paves the way forward for the next generation of GPS space and control capabilities.
Utilizing commercial off-the-shelf products, AEP also improved GPS monitor stations and ground antennas, substantially enhancing sustainability and accuracy. AEP is capable of managing all satellites in the constellation, including the new Block IIF satellites. AEP features an alternate master control station, a fully operational backup for the MCS.
The AEP system received several upgrades, with the final version declared fully operational in April 2011.
Lockheed Martin is currently under contract to sustain the AEP system through June 2019. Learn more at lockheedmartin.com
Launch and early orbit, Anomaly resolution, and Disposal Operations (LADO)
The GPS master control station can command and control a constellation of up to 32 satellites. In 2007, 2SOPS fielded the LADO system to handle GPS satellites outside the operational constellation. These include newly launched satellites undergoing checkout, satellites taken out of service for anomaly resolution, residual satellites stored in orbit, and satellites requiring end-of-life disposal.
The LADO system serves three primary functions. The first is telemetry, tracking, and control. The second is the planning and execution of satellite movements during LADO. The third function is LADO simulation of different telemetry tasks for GPS payloads and subsystems. The LADO system uses the AFSCN remote tracking stations only, not the dedicated GPS ground antennas.
The LADO system has been upgraded several times since 2007. In October 2010, the Air Force operationally accepted a new version adding GPS Block IIF capability, following testing during the launch of the first GPS IIF satellite.
Next Generation Operational Control System (OCX)
In 2008, the Air Force awarded a contract to Raytheon for development of the Next Generation Operational Control System.
OCX will add many new capabilities to the GPS control segment, including the ability to fully control the modernized civil signals (L2C, L5, and L1C). Learn more about these signals
OCX will be delivered in increments, as follows:
- OCX Block 0: launch & checkout for GPS III
- OCX Block 1: operate & manage GPS constellation, replaces AEP, adds modern features
- OCX Block 2: operate advanced M-Code features and Civil Signal Performance Monitoring
- June 2016: The Air Force announced a critical Nunn-McCurdy breach (cost overrun) in the OCX budget, triggering a mandatory process to either cancel or restructure the program. The Nunn-McCurdy process is expected to conclude in mid-October 2016.
Launch Checkout Capability (LCC)
The Launch Checkout Capability is a command and control center that will checkout all GPS III satellites. Unlike today's LADO system, which operates separately from the master control station, the LCC will be fully integrated with OCX. This approach will allow the operation of a single OCX-centric system that can sustain the GPS constellation from launch to disposal.
The LCC component of OCX will be delivered prior to OCX Block 1 in order to support the launch and checkout of the first GPS III satellite, scheduled for 2015. The LCC will ensure a timely launch so constellation availability remains optimal and not impacted by the late discovery of problems.
The Air Force awarded the contract for the provision of the LCC to Lockheed Martin in January 2012. View contractor's press release
At the same time, the Air Force awarded Raytheon a contract for the development of the Launch and Checkout System (LCS), a component of the LCC. View contractor's press release