Launch Vehicle: Atlas E/F
First Launch: 22 February 1978
Total Weight: 1636 pounds at launch, 1016 pounds in orbit
Width: 210 inches (with solar panels extended)

The NAVSTAR (NAVigation System using Time And Ranging) satellites developed by Rockwell International (now part of Boeing) are the spacecraft element of the US Air Force/US Navy Global Positioning System (GPS). It is a successor to the US Navy Timation, Transit and Triad navigational satellite systems. The system is designed to provide pinpoint navigational accuracy to land, sea and air forces in the field. The GPS is designed to function with a galaxy of 18 NAVSTAR spacecraft, nine of which had been launched by July 1984. The first Block 2 spacecraft will be in orbit in 1986, and the complete system will be in place by 1988. The spacecraft are three-axis stabilized and placed in several overlapping planes circling the globe every 12 hours. NAVSTARs provide accurate three-dimensional position fixes within 52 feet with velocity to within 4 inches per second. The ground portions of the GPS include a master control station at US Air Force Space Command (SPACECOM) Headquarters in Colorado and monitoring stations in the Indian Ocean (Diego Garcia), the Atlantic Ocean (Ascension Island) and the Pacific (Hawaii, Guam, and the Philippines). Small user packs are also distributed to small units of all branches of the US armed services, including 7291 USAF aircraft, and as many as 4700 US Navy ships. Spacecraft, such as Landsat, also carry GPS receivers. NAVSTAR carries an atomic clock to provide timing data accurate to within one millionth of a second. An L-band transmitter provides positioning data, and has a pseudorandom noise code incorporated into the L-band transmissions that requires a match by the receiving station to prevent an unauthorized user from getting into the system.