Satellite-based augmentation systems (SBAS) aim to improve the accuracy and reliability of satellite-based positioning in support of safety-critical navigation applications, such as aircraft operations near airfields.
Typically, SBAS make use of an extra, now geostationary, satellite that has a large service area, for example a continent, and which sends differential data about standard positioning satellites that are currently in view in its service area. If multiple ground reference stations are used, the quality of the differential data can be quite good and reliable. Usually this satellite will use radio signals of the same frequency as those in use by the positioning satellites, so that receivers can receive the differential code without problem.
Not all advantages of satellite augmentation will be useful for all receivers. For consumer market receivers, the biggest advantage, as compared to standard relative positioning, is that SBAS provides an ionospheric correction grid for its service area, from which a correction specific for the location of the receiver can be retrieved. This is not true in relative positioning, where the reference station determines the error it experiences and simply broadcasts this information for nearby target receivers to use. With SBAS, the receiver obtains information that is best viewed as a geostatistical interpolation of errors from multiple reference stations. More advanced receivers will be able to deploy also other differential data such as corrections on satellite position and satellite clock drift.
Currently, three systems are operational: for North America WAAS (Wide-Area Augmentation System) is in place; EGNOS (European Geostationary Navigation Overlay Service) for Europe; and MSAS (Multi-functional Satellite Augmentation System) for eastern Asia. The ground segment of WAAS consists of 24 control stations, spread over North America; that of EGNOS has 34 control stations. These three systems are compatible, guaranteeing international coverage. Usually signals from the geostationary SBAS satellites (under various names, such as AOR, Artemis, IOR, Inmarsat, MTSAT) can be received even outside their respective service areas. But the use of these signals there must be discouraged, as they will not help improve positional accuracy. Satellite identifiers, as shown by the receiver, have numbers above 30, setting them apart from standard positioning satellites. Though originally intended to improve the safety of aircraft landings, SBAS, with its horizontal accuracy to within 3 m, has many other uses. At this level of accuracy, vehicle position can be determined to a specific road lane, and “automatic pilots” become a possibility.