A Satellite-based Augmentation System (SBAS) is a civil aviation safety-critical system that supports wide-area or regional augmentation – even continental scale - through the use of geostationary (GEO) satellites which broadcast the augmentation information. A SBAS augments primary GNSS constellation(s) by providing GEO ranging, integrity and correction information. While the main goal of SBAS is to provide integrity assurance, it also increases the accuracy with position errors below 1 meter (1 sigma).
Current satellite navigation systems (GPS, GLONASS) were not designed to meet the real-time integrity monitoring capability required by the civil aviation navigation safety needs. To overcome the above limitation, several Augmentation Systems have been put in place. A Satellite Based Augmentation System (SBAS) is a wide-area differential augmentation system. A network of ground stations at known positions is deployed over the SBAS service area in order to monitor the ranging signals of the satellite constellation. The SBAS collects and process all the input data provided by the station network in order to compute (see SBAS Fundamentals for further details) and provide corrections to the original navigation information of the primary constellation (satellite orbit and clock errors, ionospheric errors) and its integrity bounds information over a certain region.
The SBAS navigation message is disseminated over the service area by a GEO. Currently, several SBAS systems are in an operational capability, such as WAAS in United States, EGNOS in Europe, and MSAS in Japan, GAGAN in India.Every SBAS provides ranging signals transmitted by GEO satellites, differential corrections on the wide area and additional parameters aimed to guarantee the integrity of the GNSS user:
GEO Ranging: transmission of GPS-like L1 signals from GEO satellites to augment the number of navigation satellites available to the users.
Wide Area Differential (WAD): differential corrections to the existing GPS/GLONASS/GEO navigation services computed in a wide area to improve navigation services performance. This includes corrections to the satellite orbits and clocks, as well as information to estimate the delay suffered from the signal when it passes through the ionosphere.
GNSS/Ground Integrity Channel (GIC): integrity information to inform about the availability of GPS/GLONASS/GEO safe navigation service.