The US military's Navstar global positioning system (GPS) is operated by the US Air Force's 2nd Space Operations Squadron at Schriever AFB in Colorado.
Navstar is currently a constellation of 28 satellites, four of which, and potentially a spare, operate in each of six orbital planes at an altitude of 20,200km (12,550 miles). Only 24 satellites are needed to provide full operational capability.
Navstar's control system consists of four ground antennas and five monitoring stations that use high- fidelity GPS receivers to passively track the satellites' military P(Y) and civilian L2C signals on the L1 and L2 frequencies. The P stands for precision and the Y for encrypted status.
Tracking information is used by the master control station at Schriever to calculate and send updated navigation information to the satellites using S-band ground antennas. Those antennas also transmit commands to the satellites and receive the telemetry data.
Today the constellation consists of 15 older-generation Navstar GPS-II and GPS-IIA satellites, 12 of the block GPS-IIRs and one GPS-IIR-Modernised (M). Launched from 1997, the GPS-IIR has reprogrammable electronics enabling in-flight problem solving and upgrades.
The first of seven planned new GPS-IIR-Ms was launched in 2005. GPS-IIR-M satellites transmit the improved military signal M on the L1 frequency and on the L2 frequency alongside the civil signal L2C.
The GPS-IIF, to be launched from 2008, will have the IIR-M's improvements, plus a 12-year lifespan, faster electronics, more computer memory and a second civil signal.
From 2012 GPS-IIIA could be deployed. It is expected to have a fourth civilian signal, L1C, designed to be compatible with Galileo.
All of the world's satellite navigation augmentation systems, the US WAAS, the Japanese MSAS, the Indian GAGAN, and Europe's EGNOS are using or will use Navstar GPS.
Source: Flight International
