This paper defines the GPS 21 Primary Satellite Constellation, along with its maintenance strategy, to be implemented in the mid-1990s. This constellation will consist of 24 satellite positions (including 3 on-orbit spares) to reduce significantly its sensitivity to satellite failure. Prior to its implementation, the Optimal 21 Satellite Constellation will be maintained. This constellation consists of 21 satellite positions providing complete full coverage, but it is also more sensitive to satellite failure. For both constellations, a slight (0.2 percent) altitude change is being considered for the fall of 1989. This would remove the need for all on-orbit station-keeping maneuvers. This paper presents satellite locations for both constellations, as well as positions for the first four Block II launches.