GPS Ground Segment Evolution

NTS-1 and NTS-2 employed ground segments customized for these experimental satellites. Six tracking stations, dispersed across Earth, were used for NTS-1, while four tracking stations were used for NTS-2, providing almost continuous tracking coverage. Control operations were manually intensive.

The first generation GPS ground segment supporting Block I operations was known as the Initial Control Segment (ICS). The ICS, consisting of a control station, ground antenna, and monitor station, was installed at Vandenberg AFB, California, in the late 1970s. Other monitor stations were located at Elmendorf AFB, Alaska; Andersen AFB, Guam; and the Naval Communication Station, Hawaii.

Development of the Operational Control System (OCS) began in 1980. The OCS began operations at Vandenberg AFB in 1985. In 1988, command and control of the satellites was transitioned to the 2nd Satellite Control Squadron (2SCS) located at Falcon Air Force Station (renamed Schriever Air Force Base in 1998) in Colorado Springs, Colorado.

By the early 1990s, system testing had been completed, demonstrating interoperability between the ground segment and the satellites, and operations commenced using an architecture based on an IBM mainframe computer. Five monitor stations were employed in Hawaii, Colorado Springs, Ascension Island, Diego Garcia, and Kwajalein, communicating with the Master Control Station (MCS) at Falcon Air Force Station.

Ground antennas for TT&C of the satellites were located at most of the monitor stations, as well as at Cape Canaveral, Florida. The ground antennas uplinked data to the satellites via SGLS. This data includes ephemerides and clock correction information transmitted within the navigation message, as well as command telemetry from the MCS. While data could be uplinked to each satellite three times per day, it was usually updated just once a day.

Modernization of the GPS ground segment began with the Legacy Accuracy Improvement Initiative (L-AII). New monitor stations, operated by the National Geospatial Agency, were incorporated into the GPS ground segment. These additional monitor stations provided greater visibility of the constellation, and hence greater accuracy: Cape Canaveral was incorporated in 2001, and in 2005 six new stations were added at Adelaide (Australia), Buenos Aires, Hermitage (United Kingdom), Manama (Bahrain), Quito (Ecuador), and Washington, DC. In 2006, five more stations were added at Fairbanks (United States), Osan (South Korea), Papeete (Tahiti), Pretoria, and Wellington (New Zealand). L-AII was completed in 2008, yielding a 10% to 15% improvement in the accuracy of clock and ephemeris information provided by the broadcast GPS signals. At any time, each satellite could be monitored by at least three monitor stations.

In conjunction with L-AII, the Air Force implemented the Architecture Evolution Plan (AEP). AEP replaced the IBM-mainframe-based architecture in the MCS with the then-modern distributed architecture of Sun workstations. AEP improved the flexibility and responsiveness of GPS operations, and paved the way for operation of Block IIF satellites. AEP also introduced an alternate MCS (AMCS), a fully operational backup for the MCS located at Vandenberg AFB, CA. The final version of AEP was declared fully operational in 2011. The current GPS MCS can command and control an operational constellation of up to 32 satellites.

In 2007, 2SOPS fielded the Launch, Anomaly Resolution, and Disposal Operations (LADO) system to control GPS satellites outside the operational constellation: 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 only the Air Force Satellite Control Network (AFSCN) remote tracking stations, 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.

In 2010, the Air Force awarded a contract to Raytheon for development of the Next Generation Operational Control System, called OCX. OCX would involve new hardware and software architectures, enhanced cybersecurity, and the ability to fully control the modernized signals: M, L1C, L2C, and L5. It would also perform launch, checkout, and control for GPS III satellites, and replace AEP for operating the remaining Block II satellites. Eventually, it would also perform monitoring of civil signals, while controlling more than 32 satellites.

In 2016, after years of delays and cost overruns in the OCX program, the Air Force declared a Nunn-McCurdy breach, indicating that costs would increase by more than 25% over the original plan. Later that year, the Office of the Secretary of Defense certified that continuing the OCX program was essential and preferred over alternatives. OCX capabilities will now be delivered in stages after 2020.