Flight Trials of the Wide Area Augmentation System (WAAS)

Abstract

The Wide-Area Augmentation System (WAAS) is being rapidly developed by the Federal Aviation Administration for supplemental operational use in 1997. In time, it will be a primary navigation aid for all phases of flight down to Category I precision approach. The WAAS will include a network of approximately 20 to 30 Wide-area Reference Stations (WRSs) distributed around the National Airspace System. These reference stations will observe all GPS satellites in view and send pseudorange and ionospheric observations back to one or more Widearea Master Stations (WMSs). The WMSs will use this data to form vector corrections for each GPS satellite. These vectors contain separate components for the satellite ephemeris, satellite clock and ionosphere. The corrections will be broadcast to WAAS users via a geostationary satellite, using a signal and data format, which has been designed by RTCA Special Committee 159. In the Summer of 1994, Stanford performed WAAS flight trials to provide WAAS operational experience as early as possible. Our flight trials used three WAAS Reference Stations (WRSs), which Stanford installed for the FAA in the Western United States. They also used an experimental WMS located at Stanford. The flights used Professor David Powell’s Piper Dakota to fly WAAS precision approaches to an uninstrumented airport (Palo Alto). The WAAS data dramatically improved the accuracy of the airborne GPS fix, and it drove an “ILS-like” display. This paper describes our experiment and reports the results of the flights.