Analysis of Fix Displacement Tolerances for the Development of Instrument Procedures Criteria for Global Positioning System

Young C. Lee, Ralph Sexton

Abstract:	The current airway structure is based on conventional ground-based navaids such as VOR and DME. In this structure, various fixes are defined along with their associated fix displacement tolerances (FDTs), from which obstacle clearance areas are designed. In the existing system, FDTs are formulated using 95 percent values and depend upon which navaids are used to establish the fix and upon the relative geometry of the aircraft, path, and facilities. The fix tolerance can be as small as 0.5 nmi along track for a DME fib. While an FDT is a 95 percent value, the error with conventional navaids is rather tightly bounded by a local ground monitor. On the other hand, while Global Positioning System (GPS) is generally much more accurate than ground-based navaids, GPS does not have a local monitor, and it is necessary to use analyses of geometry and probabilistic computations in order to define the limits of the GPS position error. Therefore, direct application of a 95 percent GPS accuracy value is not appropriate for an FDT. The paper analyzes the problem to answer three related questions: 1) Does the GPS user equipped with a TSO-C129 receiver have compatible accuracy and integrity to fly over existing airways? 2) How to derive the FDT for GPS users with a TSO-C129 receiver and 3) How to derive the FDT for GPS/WAAS users? Similar analysis has been performed in support of FAA development of fix tolerance areas and is part of the effort to develop area widths and other parameters for instrument procedures criteria.
Published in:	Proceedings of the 1998 National Technical Meeting of The Institute of Navigation January 21 - 23, 1998 Westin Long Beach Hotel Long Beach, CA
Pages:	281 - 294
Cite this article:	Lee, Young C., Sexton, Ralph, "Analysis of Fix Displacement Tolerances for the Development of Instrument Procedures Criteria for Global Positioning System," Proceedings of the 1998 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 1998, pp. 281-294.
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