|
Analysis of Fix Displacement Tolerances for the Development of Instrument Procedures Criteria for GPS
1998 Award Winner
Dr. Young C. Lee, The MITRE Corporation
Ralph Sexton, Innovative Solutions International, Inc.
ABSTRACT
The current airway structure is based on conventional ground-based navigational aids (navaids) such as very high frequency (VHF) Omnidirectional Range (VOR) and Distance Measuring Equipment (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 nautical miles (nmi) along track for a DME fix.
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. Therefore, direct application of a 95 percent GPS accuracy value is not appropriate for an FDT. For the case of GPS, it is necessary to use analyses of geometry and probabilistic computations in order to define the limits of the GPS position error. This paper analyzes the problem to answer three related questions. The first question is, "Does the GPS user equipped with a Technical Standard Order (TSO)-C129 receiver have compatible accuracy and integrity to fly over existing airways?" This question leads to a more fundamental, second question, "How to derive the FDT for the GPS users with a TSO-C129 receiver?" With the deployment of GPS/Wide Area Augmentation System (WAAS) in the National Airspace System (NAS) imminent, it is also necessary to answer the third question, "How to derive the FDT for the GPS/WAAS users?" This analysis has been performed in support of the Federal Aviation Administration (FAA) development of fix tolerance areas and is part of the effort to develop area widths and other parameters for instrument procedures criteria.
Publication
Published in Summer 1998. NAVIGATION: Journal of the Institute of Navigation, Vol.45, No. 2, pp. 103-115.
Additional Search Keywords
global positioning system, GPS, TSO-C129, fix displacement tolerance, FDT, GPS integrity, receiver autonomous integrity monitoring, RAIM, RNAV
|
