Vertical Protection Level Equations for Dual Frequency SBAS

T. Walter, J. Blanch, P. Enge

Abstract: The L1-only Satellite-Based Augmentation System (SBAS) Minimum Operational Performance Standards (MOPS) were developed long before any SBASs were certified for operation. During the development and certification of the Wide Area Augmentation System (WAAS), it was discovered that the zero-mean Gaussian basis of the Vertical Protection Level (VPL) equation was not strictly true for some error sources, and very difficult to sufficiently demonstrate for others. The actual data collected in support of system performance demonstrated non-Gaussian behavior. Further, sources of small uncorrectable biases were discovered after the original MOPS development. These biases can arise from consistent, minor differences in the signal structure from one satellite to another. Antenna biases at the satellite, at the reference stations, and at the user are other possible sources of these biases. Because the MOPS VPL equation is based upon zero-mean Gaussian error combination, much additional work was required to demonstrate that the actual errors could be sufficiently protected safely. Some performance is lost because the system has to implement conservative approaches to account for these discrepancies. The advent of dual frequency SBAS affords the opportunity to revisit the MOPS and use different approaches for this new class of user. Lessons learned from the L1-only system certification and operation can be leveraged to both ease development of the future dual frequency system and improve user performance. This paper examines the VPL equations and proposes changes to directly address these lessons. The handling of non- Gaussian behavior and small biases directly address both goals. The VPL can be further changed to directly address the threats that most limit availability. Without the corrupting influence of the ionosphere, satellite faults become the dominant source of significant error. New VPL equations are proposed to specifically account for individual satellite fault modes. This paper will demonstrate that by avoiding overly conservative steps required to handle all possible cases, the users will see reduced protection levels and higher availability.
Published in: Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010)
September 21 - 24, 2010
Oregon Convention Center, Portland, Oregon
Portland, OR
Pages: 2031 - 2041
Cite this article: Walter, T., Blanch, J., Enge, P., "Vertical Protection Level Equations for Dual Frequency SBAS," Proceedings of the 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp. 2031-2041.
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