Quantifying GDOP Degradations Caused by Removing Satellites from a GPS Constellation

Martin J. Smith and Timothy I. Wegner

Abstract:	In the interest of flight safety and understanding all navigational influences on the space shuttle, United Space Alliance conducted this study to determine the impact to available Geometric Dilution of Precision (GDOP) caused by effectively removing satellites from the visible sky. Primary emphasis is to predict the depth of satellite geometry integrity that the constellation can provide to a shuttle on or near the launch pad. The space shuttle will not rely on GPS for ascent, but with the planned phase-out of Tactical Air Navigation (TACAN), orbiter safety will become highly dependent upon GPS during landing, which includes landings made as part of the Return To Launch Site (RTLS) abort procedure. This study uses GPS constellation and vehicle position inputs to run three simulations with slightly different strategies. Each simulation employs an algorithm that, each second, removes all possible sets of one or two satellites from the visible sky of the shuttle and evaluates the best remaining GDOP. • The first simulation evaluates the GDOP response to removing two satellites at a time in a range of related 27-satellite constellations. The constellations differ from each other only by the absence of a different single satellite from the base 28-satellite almanac. • The second simulation evaluates the response to two-satellite removal in 15 past shuttle missions and the constellations that were actually in effect at those times. Only the prelaunch and ascent phases of the mission are evaluated. • The third simulation is similar to the first two simulations, but with only one-satellite removed instead of two at a time. The results indicate that there are many times when poor GDOPs caused by the mechanisms described above are possible. The first simulation shows that high GDOP or 3-satellite visibility conditions are theoretically possible for up to 41 minutes continuously. The second simulation shows that in 6 past shuttle missions, conditions existed such that if the right two satellites had been removed, GDOPs over 40 would have occurred. The singular chance of these conditions existing is about 1%, based on relative time comparisons, but for a 3 hour launch window, there would be a 36% chance of experiencing a period of vulnerability. During rare instances of 5- satellite visibility at the launch pad, it is also possible for one-satellite removal to result in excessive GDOP. The GDOP events described herein can result from receiver satellite removal, as can be experienced by a shuttle on the launch pad, or any mechanism that renders one or more visible satellites unusable for navigation, such as satellite failure or structural blockage.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	2594 - 2599
Cite this article:	Smith, Martin J., Wegner, Timothy I., "Quantifying GDOP Degradations Caused by Removing Satellites from a GPS Constellation," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 2594-2599.
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