Integrity Concepts for a GPS Wide-Area Augmentation System (WAAS)

Robert Loh and James P. Fernow

Abstract:	The Federal Aviation Administration (FAA) is in the process of developing and implementing a GPS Wide-Area Augmentation System (WAAS). The first phase of WAAS will improve the integrity and availability of GPS-based satellite navigation for en route, terminal, and nonprecision approach phases of flight. The second phase, if implemented, would improve the accuracy sufficiently to support Category I precision approaches. (Implementation of the second phase will require a policy decision by the U.S. Departments of Defense and Transportation.) WAAS will consist of a network of around 15-30 ground stations, 2 master control stations, and 6 geostationary satellite transponders (and associated ground earth stations) that simultaneously provide three capabilities: an integrity broadcast; additional ranging signals; and wide-area differential GPS corrections. In contrast, civil aviation navigation is currently based primarily on FAA ground systems that provide single point sources of navigation for users within line-of-sight (about 30 to 130 nmi) of each individual ground station. Consequently, all current civil aviation requirements for integrity, accuracy, and availability are bati on tho.se systems. WAAS will provide the three capabilities listed above for all users within the entire CONUS. Therefore, the requirements must be modified for users navigating with WAAS. The objective of this paper is to describe the rationale for and the development of integrity concepts for a WAAS. Of all the requirements for civil aviation, integrity is the most important and stringent because it has a direct impact on safety. Integrity concepts for a WAAS are difficult to develop for the following reamns: l WAAS signals cannot be monitored at all locations where the capabilities are being provided. IJsers at different locations have the potential for using a large number of different sets of satellites. l An integrity decision made by a ground station will affect many or all users simultaneously. l The ground network of ground stations and communications links that is part of WAAS is a potential source of errors and must be monitored. l IJsers conducting different types of flight operations have vastly diffcrcnt error tolerances (protection limits). In order to maintain a low alarm rate, users avionics must be responsible for m‘aking certain integrity decisions. This paper describes current concepts for what is monitored by the ground stations and how the information is used by avionics.
Published in:	Proceedings of the 1994 National Technical Meeting of The Institute of Navigation January 24 - 26, 1994 Catamaran Resort Hotel San Diego, CA
Pages:	127 - 135
Cite this article:	Loh, Robert, Fernow, James P., "Integrity Concepts for a GPS Wide-Area Augmentation System (WAAS)," Proceedings of the 1994 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 1994, pp. 127-135.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In