Integrity monitoring requirements for FAA´s GPS Wide-Area Augmentation System (WAAS)

Three years ago, FAA started on an ambitious program to perform research, development and implementation of satellite navigation based on the approval and use of the DoD´s Global Positioning System (GPS). One of the research and development (R&D) projects was to determine the capabilities and requirements for a GPS Wide-Area Augmentation System (WAAS), a ground-based GPS augmentation. The FAA´s WAAS will consist of up to about 30 ground reference stations that will combine data to estimate separate corrections for clock and ephemeris for each monitored satellite, and also generate a correction term for ionospheric delays at lattice points on a grid formed by the intersection of lines of constant latitude and longitude. The corrections are relayed through geostationary satellites in order to reach all users in a wide geographic area. Since WAAS is expected to support Category I (CAT I) precision approaches, stringent integrity monitoring requirements will have to be satisfied. Existing single location navigation sources, such as ILSs, are monitored at a single location, whereas WAAS must be monitored at many locations and the information combined to produce a single integrity broadcast message suitable for a wide geographic area. In addition, the WAAS message includes separate differential correction components that can be individually monitored, but the integrity information must be correlated with an aircraft position error. The authors develop integrity monitoring requirements, starting with the instrument landing system (ILS), and extrapolating them for WAAS