GROUND CONTROL

NOTE GPS will become the primary reference navigation system for surface ships, submarines, and aircraft. Refer to the DOD policy statement under the LORAN and OMEGA section of this chapter for specific details on this important transition. GROUND CONTROL The ground control segment tracks the satellites, monitors and controls satellite orbits, and updates the satellite navigation data message. The ground control system consists of unmanned monitor stations and a manned control center. Monitor stations, located throughout the world, use GPS receivers to track each satellite. Tracking information gathered by the monitor stations is sent to the control center, where a precise position and a clock error for each satellite are calculated. The control center also calculates satellite positioning for the group of satellites. Positioning data for a single satellite is called ephemeris data; data for a group of satellites is called almanac data. Once each 24 hours, the control center transmits the ephemeris and almanac data to each satellite to update the navigation data message. SATELLITES There are 21 active operational and 3 active spare satellites in circular orbits, with a 55-degree inclination to the earth. These satellites provide navigation data to the navigation sets. The satellites are arranged in six concentric rings that allow them to orbit the earth twice a day and provide world-wide continuous coverage. Each satellite broadcasts two Figure 1-11.—NAVSTAR GPS major elements. 1-14