Formulation of a Time-Varying Maximum Allowable Error for Ground-Based Augmentation Systems

J. Rife and E. Phelts

Abstract: Ground-Based Augmentation Systems (GBAS), such as the Federal Aviation Administration's Local Area Augmentation System (LAAS), ensure integrity by enabling users to compute a conservative navigation error bound, called the Protection Level (PL). Although the PL covers both fault-free operations and certain faulted operations (reference-receiver faults or ephemeris faults), the PL does not explicitly account for all known fault modes. These remaining fault modes, including signal deformation, code-carrier divergence, and excessive acceleration, are covered implicitly through a rangedomain bound known as the Maximum-allowable Error in Range (MERR). This paper introduces a new formulation for MERR that accounts for several limitations of earlier methods. In contrast with earlier formulations, which defined MERR as a static value, the new method defines MERR as a function of time. This time-varying MERR bound can then be compared to the expected time-evolution of the navigation error associated with each fault mode. The GBAS can be proven to maintain integrity during a fault event so long as the error magnitude does not exceed the MERR. The time-varying MERR formulation addresses three limitations associated with earlier static formulations. Specifically, the new method (1) resolves ambiguity in the timing of bound application, (2) removes arbitrary restrictions on the performance of GBAS fault monitors, and (3) establishes a clear relationship between the MERR and the GBAS Time-to-Alert (TTA) requirement. In particular, TTA strongly impacts MERR magnitude, as illustrated by an application of the time-varying MERR to the signal-deformation fault mode.
Published in: Proceedings of the 2006 National Technical Meeting of The Institute of Navigation
January 18 - 20, 2006
Hyatt Regency Hotel
Monterey, CA
Pages: 441 - 453
Cite this article: Rife, J., Phelts, E., "Formulation of a Time-Varying Maximum Allowable Error for Ground-Based Augmentation Systems," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 441-453.
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