Brief History of GPS Backward Compatibility (BC) Requirements for Psat & Pconst

Calvin Miles, Karen Van Dyke

Abstract:	This paper provides a brief ‘outline style’ history on how the “probability of a broadcast signal-in-space (SIS) integrity failure affecting an individual satellite” (Psat) and the “probability of broadcast SIS integrity failures due to a common cause concurrently affecting multiple satellites” (Pconst) came to become two of the most important backward compatibility (BC) requirements for the sustainment and modernization of the Global Positioning System (GPS). Psat is the most critical performance parameter for applications which rely on GPS integrity. It is foundational for Receiver Autonomous Integrity Monitoring (RAIM) as well for as augmentation services around the globe such as those provided by a regional Satellite Based Augmentation System (SBAS) or a local Ground Based Augmentation System (GBAS). It is said that RAIM would be useless for conducting safe aerial navigation if the very stringent GPS performance standards for Psat were not met. Second in criticality only to Psat, Pconst is also a vital performance parameter which determines the statistically best integrity that RAIM, SBAS, GBAS, etcetera can possibly deliver to GPS users. While a low Psat value limits the exposure to random SIS integrity failures affecting two or more satellites at the same time, a low Pconst value limits the exposure to common-mode SIS integrity failures affecting two or more satellites at the same time. RAIM is particularly prone to missed detections (loss of integrity protection) when presented with concurrent SIS integrity failures. Advanced RAIM (ARAIM) appears to be evolving to be even more sensitive to concurrent SIS integrity failures. Maintaining the existing GPS performance commitments for delivering very low Psat and Pconst values is necessary to preserve the utility of RAIM, SBAS, GBAS, and ARAIM. In the context of GPS Modernization – with new satellites, a new ground control system, and several new signals all being added –assuring the existing performance commitments are continually met is known as sustaining backward compatibility (BC). It is often said that sustaining BC is the most importance GPS modernization requirement of all. In support of on-going efforts to maintain the existing GPS performance commitments for very low Psat and Pconst values during GPS modernization, this paper provides an outline of the history of the GPS Psat and Pconst requirements to place them in their proper context. It begins with their origin from a U.S. President’s decision and then traces their journey into the BC baseline. It goes on to address how that BC baseline was initially captured by the GPS modernization program, how Psat and Pconst were specified as GPS III requirements, and how they are being addressed in the various GPS III production and fielding activities. This paper ends with a recap of current national policy as laid out by another U.S. President which reiterates the importance of sustaining BC and maintaining the GPS performance commitments for very low Psat and Pconst values.
Published in:	Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023) September 11 - 15, 2023 Hyatt Regency Denver Denver, Colorado
Pages:	1064 - 1082
Cite this article:	Miles, Calvin, Van Dyke, Karen, "Brief History of GPS Backward Compatibility (BC) Requirements for Psat & Pconst," Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), Denver, Colorado, September 2023, pp. 1064-1082. https://doi.org/10.33012/2023.19442
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