WAAS Messages

WAAS
Title	WAAS Messages
Author(s)	GMV.
Level	Basic
Year of Publication	2011

The Wide Area Augmentation System (WAAS) is the United States Satellite Based Augmentation System. The programme, started in 1992, is being carried out by the Federal Aviation Agency (FAA) and is specially developed for the civil aviation community. The system, which was declared operational in late 2003, currently supports thousands of aircraft instrument approaches in more than one thousand airports in USA and Canada. WAAS service area includes CONUS, Alaska, Canada and Mexico. The WAAS programme is continuously in evolution; two development phases have been already covered, a third is in progress, and there are plans to improve the capability of the system in parallel with the evolution of the SBAS standards towards a dual-frequency augmentation service.

WAAS Messages

The WAAS Messages are mainly divided in two types: fast and slow. In the WAAS receiver, the two types of WAAS correction messages received (fast and slow) are used in different ways. The GPS receiver can immediately apply the fast type of correction data, which includes the corrected satellite position and clock data, and determines its current location using normal GPS calculations. Once an approximate position fix is obtained the receiver begins to use the slow corrections to improve its accuracy. Among the slow correction data is the ionospheric delay. As the GPS signal travels from the satellite to the receiver, it passes through the ionosphere. The receiver calculates the location where the signal pierced the ionosphere and, if it has received an ionospheric delay value for that location, corrects for the error the ionosphere created. While the slow data can be updated every minute if necessary, ephemeris errors and ionosphere errors do not change this frequently, so they are only updated every two minutes and are considered valid for up to six minutes.

For a detailed description of SBAS Messages, see article EGNOS Messages. This articles follows the standard described in RTCA MOPS DO-229-C “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation System airborne equipment” (particularly in its Appendix A “Signal characteristics and format”), applicable for every Satellite Based Augmentation Systems (SBAS). In the article, SBAS Message specification is also compliant with the ICAO SARPs “Standards and Recommended Practices”, Appendix B “Detailed technical specifications for the Global Navigation Satellite System (GNSS)”. These two standards are nearly aligned in what regards to the SBAS SiS specification, but the augmentation of GLONASS or other GNSS constellations except for GPS is not specified in MOPS, being MOPS the standard that follows WAAS, as WAAS does not provides augmentation to GLONASS satellites.