Wide Area Augmentation System

The GPS SPS service fails to meet the accuracy, availability and integrity requirements for high-risk navigation of, for example, aircraft. In order to meet these requirements the Wide Area Augmentation System or WAAS is being developed. WAAS is a safety-critical navigation system that augments GPS to provide a high quality of positioning information. This system will allow GPS to be used as a primary means of navigation for enroute travel and non-precision approaches in the U.S. The wide area of coverage includes the entire United States and some outlying areas such as Canada and Mexico. This is called the service area. Compatible systems are being developed to cover Canada, Europe and Japan.

WAAS is based on a network of a couple of dozen Wide-area ground Reference Stations (WRSs) spread around the service area. Each WRS is located at a known position. The WRS receives and collects data continuously from GPS satellites. The WRSs send the data to the Wide-Area Master Stations (WMSs) via a wide-area network. The WMSs calculate the error components of the GPS-received signals. The corrections are transmitted to a ground earth station (GES). The GES receives the GPS correction data from the WMS via the wide-area network and transmits the data to a geostationary satellite (GEO). The GEO receives the GPS correction data from the GES and retransmits the data to user receiver sets via the same frequency used by GPS (L1). The corrections are applied to the pseudoranges before the user calculates the GPS received position.

WAAS will improve GPS SPS accuracy to approximately 7.6 meters (95%) vertically and horizontally, improve system availability through the use of the GEOs for positioning, and provide important integrity information about the entire GPS constellation. It will give aviation users accurate positioning information to allow the use of Category I precision approaches.