RAIM Performance in the Post SA Era

Tim Murphy, Matt Harris

Abstract:	In May of 2000, Selective Availability (SA) was set to zero resulting in a dramatic improvement in the nominal performance of GPS for all users. This welcome change had some interesting ramifications for aviation related equipment which had already been fielded. All aviation receivers are required to validate GPS integrity by either using an augmentation signal (such as SBAS or GBAS) or by using autonomous Fault Detection (FD) capabilities. The most common form of autonomous FD capability is provided through Receiver Autonomous Integrity Monitoring (RAIM) algorithms. RAIM algorithms have been designed and fielded since the early 1990’s. Prior to the removal of SA, RAIM algorithms were designed to perform Fault Detection or Fault Detection and Exclusion (FDE) in an environment where the observation noise was dominated by SA. In fact, the majority of the RAIM algorithms in the field now on air transport airplanes have been designed in accordance with the assumption that SA is still active. The result of this is that those RAIM algorithms, when operating in the post SA environment are quite conservative in many respects. This paper presents analysis showing how such an algorithm performs given the current GPS performance (post SA). The paper also discusses and compares the performance of newer algorithms designed and implemented after SA was removed. Finally trade-offs related to upgrading older receivers are discussed in light of emerging requirements and applications such as ADS-B.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	139 - 150
Cite this article:	Murphy, Tim, Harris, Matt, "RAIM Performance in the Post SA Era," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 139-150.
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