BUILDUP AND REPLACEMENT OF NAVSTAR GLOBAL POSITIONING SYSTEM AND THE 18-SATELLITE CONSTELLATION

Abstract

Changes in the Navstar Global Positioning System (GPS) program in 1980 resulted in a reduction in the number of satellites from 24 to 18, and consequently led to a reconsideration of the original orbital configuration and a revision in the way buildup and replacement would be performed. During the buildup phase of the program, the Space Shuttle will be used, launching up to two Navstar satellites per mission on launches shared with other payloads and up to four satellites on dedicated launches. The original concept for the 24 satellite Navstar GPS constellation consisted of three orbit planes with eight uniformly distributed satellites per plane. In that configuration, all of the satellites on each Shuttle launch would be placed in only one plane. With the reduction from 24 to 18 satellites, it was determined that there were advantages to evaluating other constellations; rather than simply reducing the baseline three-plane, 24-satellite configuration to 18 satellites by removing two satellites from each plane, a larger number of planes may be preferred for the restructured program. Indeed, the current baseline configuration is a sixplane, 18-satellite constellation with three satellites per plane, and the three-plane constellation is an alternate configuration. Buildup and replacement studies for the new configurations that have been investigated have not only addressed the performance goals of Navstar GPS but have considered the economic use of the Shuttle as a launch platform. In addition, the launch constraints imposed by the Shuttle must be considered in the strategy used for buildup and replacement. This paper discusses the fundamentals of the buildup and replacement and the performance of the GPS with 18 satellites. The constellations discussed are the sixplane GPS baseline and the alternate three-plane configuration.