Abstract: Satellite based augmentation system (SBAS) is built on the basis of traditional GNSS in order to further improve accuracy, integrity, continuity and availability. At present, many SBASs have been established around the world. Among them, the system which provides the longest service is the WAAS of the United States. WAAS has 38 reference stations, 3 master stations, 6 ground uplink stations and 3 GEO satellites. In order to guarantee system redundancy, each WRS has three independent receivers. The system structure of WAAS was introduced, and the similarities and differences of different types of message data transmitted by different GEO satellites in WAAS were compared. The consistency of message broadcasting and the impact on users due to different GEO broadcast messages were analysed. Also, the specifications and constraints of WAAS on user selection of SBAS signals in minimum operational performance standards were analysed. The essential difference between SBAS and traditional widearea difference system was revealed from the perspective of applying requirements, and it was pointed out that integrity realtime monitoring is based on establishing a sufficiently reliable ground monitoring network and satellite communication link.

Key words: satellite navigation, SBAS, WAAS, consistency, integrity, system architecture