Proving SBAS Integrity Through Fault Tree-Based Systems Engineering

G.T. Watt, C.L. De La Cruz, H.L. Habereder, D.R. Heine

Abstract:	Proving system integrity of a Satellite-Based Augmentation System (SBAS) requires a comprehensive systems engineering approach with integrated safety assurance objectives to ensure all significant integrity threats are identified and mitigations for them are implemented to effectively reduce any residual safety risk to an acceptable level. Integrity is the assurance that the system will not only behave as intended (i.e., as described in the system requirements), but it also means that the SBAS will take the appropriate actions necessary to ensure overall system integrity when an unexpected error or failure condition is detected. Proving integrity involves significant challenges and is high dependant on accurately characterizing system integrity threats and rigorously verifying the effectiveness of threat mitigations in order to provide adequate assurance that the risk of errors and failure conditions contributing to an integrity failure (i.e., a hazard occurrence) is acceptably small. One of the most widely used techniques for identifying and assessing hazards in general and determining the effectiveness of proposed and implemented mitigations is Fault Tree Analysis (FTA). This technique uses Boolean logic to describe the combination of individual faults that can constitute a hazardous event, which in the case of an SBAS is failure of the system to provide a timely warning to users when individual corrections or satellites should not be used for navigation. Although the FTA cannot by itself prove system integrity, it is an excellent means of aggregating the causes of hazards with their mitigations to verify that the integrity threats have been identified, that their effect on the system is accurately characterized, and that the implemented mitigation effectively mitigates the safety risk to an acceptable level. This paper describes the critical role of the FTA in a proactive System/Safety Engineering environment in proving system integrity of an SBAS.
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:	2730 - 2738
Cite this article:	Watt, G.T., La Cruz, C.L. De, Habereder, H.L., Heine, D.R., "Proving SBAS Integrity Through Fault Tree-Based Systems Engineering," 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. 2730-2738.
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