Assessment of the Proposed GPS 27-Satellite Constellation

P. Massatt, F. Fritzen, M. Perz

Abstract:	For several years, GPS has maintained between 26 and 29 on-orbit satellites to support a 24-satellite constellation. The Air Force requested Aerospace to assess the current constellation design to determine whether an increase in formal constellation orbital slots would be more effective in utilizing the currently available satellites. The Aerospace analysis showed that a 27-slot constellation would more effectively ensure continued high levels of global coverage in the event of one or more satellites being either failed or off line for maintenance. The constellation change represents a significant shift in the way that the Air Force is considering managing the GPS constellation. Rather than populating the constellation with spare satellites in several orbit planes, the Air Force is considering placing greater reliance upon the robustness of a larger constellation where all satellites contribute more effectively to overall GPS coverage. The new constellation both improves overall performance and avoids transition problems. By adjusting three of the existing 24 constellation slots and by adding three new slots, the constellation can be made more robust than simply managing to the original 24 slots and non-optimized or separately optimized spare placement. By keeping a similar constellation design, the constellation also retains the flexibility to reposition satellites in the event of a satellite production shortfall or a launch standdown. By making greater use of satellites previously designated as spares, the constellation significantly improves coverage for RAIM, urban canyon operations, and high-precision navigation. The proposed constellation can also support a transition to a larger constellation design should future requirements dictate that such a transition is warranted. The paper describes the objectives used for determining the new constellation design and the areas of performance that are likely to be improved through such a constellation change. GPS civil standards are reported in the Standard Position Service (SPS) Performance Standard. Current standards are built around the performance attributes associated with current maintenance of the GPS 24-satellite constellation. The standard will need to be altered to support a 27-slot constellation. Since the procured satellite resources for the immediate future cannot be changed, guaranteed minimum performance levels will also not be increased. However, as long as GPS continues without any shortage of satellites for replenishment purposes, users will see significant increases in performance. This paper will address some of the changes that are currently being considered. This paper summarizes the analysis conducted in the development of the 27-slot constellation and evaluates performance against current management criteria given no change in sustainment policies. The paper finishes with an assessment of implications for select user applications, such as general aviation and military theater operations.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	399 - 406
Cite this article:	Massatt, P., Fritzen, F., Perz, M., "Assessment of the Proposed GPS 27-Satellite Constellation," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 399-406.
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