WAAS Algorithm Contribution to Hazardously Misleading Information (HMI)

Timothy R. Schempp, Stephen R. Peck and Robert M. Fries

Abstract:	The Wide Area Augmentation System (WAAS) is a safety-critical, software-intensive system, augmenting the satellite-based Global Positioning System (GPS). The system provides airborne users with positions of adequate accuracy, availability, continuity, and integrity to support different phases of flight. Under the "Free Flight" concept of the National Airspace System (NAS) adopted by the Federal Aviation Administration (FAA) at the turn of the century, the GPS/WAAS infrastructure is assuming a critical role in ensuring the safe and efficient flight operating capability of the NAS. The far-reaching impact on flight operations has made WAAS a safety-critical system. A system failure hardware or software has the potential of impacting a significant volume of airspace and aircraft in the course of navigation or landing. Each component of the WAAS corrections is either generated on a trusted platform or checked by one of the nine integrity monitoring mechanisms located on trusted platforms. A "trusted" platform is a platform with software developed and certified to an appropriate level relative to the hazard severity that the software is designed to mitigate. The development process is designed in accordance with RTCA/DO-178B. The WAAS system has a top level safety requirement to protect users at every point in space and time with seven 9’s of integrity (10-7 probability of HMI). This document provides a top level description of the approach used to prove the algorithms meet their integrity requirements. Each monitor in the safety processor is assigned a specific set of threats which they are designed to mitigate. The a-priori probability that a threat occurs is considered when mitigating a hazard, however, most threats come from platforms where the software was not developed to a high enough DO-178B level. These threats have a failure probability of 1. The system level integrity requirement is allocated to each monitor and hardware component in a manner that guarantees that the probability a user experiences HMI at any point in time during a 150 second approach is less than 10-7. This paper will describe the generic approach used to show an algorithm in the WAAS software meets its integrity allocation. The User Differential Range Error Monitor (UDRE Monitor) will be used as an example to walk the reader through the process.
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:	1831 - 1837
Cite this article:	Schempp, Timothy R., Peck, Stephen R., Fries, Robert M., "WAAS Algorithm Contribution to Hazardously Misleading Information (HMI)," 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. 1831-1837.
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