Analysis of Fix Displacement Tolerances for the Development of Instrument Procedures Criteria for GPS

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. While FDTs are formulated using 95 percent accuracy values in the existing system, the error with conventional navaids is rather tightly bounded by a local ground monitor. On the other hand, GPS does not have a local monitor, and therefore, direct application of a 95 percent GPS accuracy value is not appropriate for an FDT. This paper analyzes the problem and derives the FDTs for GPS users equipped with Technical Standard Order (TSO)-C129 or GPS/Wide Area Augmentation System (WAAS) receivers. This analysis was performed in support of the Federal Aviation Administration's (FAA) development of fix tolerance areas and is part of the effort to develop area widths and other parameters for instrument procedures criteria.