The Global Positioning System

Abstract

The Navstar Global Positioning System, proposed for deployment in 1985, will have eight satellites equally spaced in each of three orbit planes at an inclination of 63°. Since the satellites will be in circular, 20000 km (12 h period), orbits and the nodes of the three orbit planes will be equally spaced, at least four satellites will be in view at any location. Range to three of the satellites, computed from the travel time of signal from the satellite to the ground, would give the position of the ground receiver. The measurement to the fourth satellite is required to synchronize the ground station clock with the satellite to provide a sufficiently accurate travel time. In order that the system may be demonstrated in early 1979, six satellites are now being launched into orbits that will provide the operational configuration over the southwestern part of the United States for a few hours each day. The accuracy of the instantaneous absolute position is expected to be 10 m. The relative position of two stationary receivers could be determined to 1 m accuracy in a few minutes even if the receivers are separated by 1000 km. Relative positions could be determined to better than 10 cm accuracy within a day.