The Navstar Global Positioning System six-plane 18-satellite constellation

In 1980, the Navstar Global Positioning System (GPS) program was restructured, and the constellation was reduced from 24 satellites to 18 at the 1987 initial operational capability (IOC). In connection with this change, a number of possible 18-satellite constellations were studied to determine the one with the best performance. The two candidate constellations selected include a three-plane, six-satellite-per-plane configuration and a six-plane, three-satellite-per-plane configuration. The three-plane configuration is a subset of the original 24-satellite constellation which had been identified in the GPS Program Management Directive. The present investigation is concerned with the reasons which led to the recommendation that the six-plane constellation be substituted as the primary constellation, with the three-plane constellation being carried as an alternate.