Satellite-based augmentation systems (SBAS), such as EGNOS, complement existing global navigation satellite systems (GNSS). SBAS compensate for certain disadvantages of GNSS in terms of accuracy, integrity, continuity and availability.
For example, neither the USA’s GPS nor Russia’s GLONASS meet the operational requirements set by the International Civil Aviation Organisation (ICAO) for use during the most critical phases of aircraft flight, in particular final approaches. To solve it, ICAO decided to standardise several GNSS augmentation systems including SBAS.
The SBAS concept is based on GNSS measurements by accurately-located reference stations deployed across an entire continent. The GNSS errors are then transferred to a computing centre, which calculate differential corrections and integrity messages which are then broadcasted over the continent using geostationary satellites as an augmentation or overlay of the original GNSS message. SBAS messages are broadcast via geostationary satellites able to cover vast areas.
Several countries have implemented their own satellite-based augmentation system. Europe has the European Geostationary Navigation Overlay Service (EGNOS) which covers the EU and possibly beyond. The USA has its Wide Area Augmentation System (WAAS). Japan is covered by its Multi-functional Satellite Augmentation System (MSAS). India has launched its own SBAS programme named GPS and GEO Augmented Navigation (GAGAN) to cover the Indian subcontinent. Both Korea (2013) and China (2014) have announced plans to start their own SBAS implementation.
All of the systems comply with a common global standard and are therefore all compatible (do not interfere with each other) and interoperable (a user with a standard receiver can benefit from the same level of service and performance whether located in the EGNOS or WAAS coverage area).
In addition to their use in the aviation sector, SBAS systems are essential for applications where accuracy and integrity are critical. In particular, they are indispensable for all applications where people’s lives are at stake or for which some form of legal or commercial guarantee is required.
For example, SBAS make it possible to improve and extend the scope of applications for GNSS in areas such as precision farming, the guidance of agricultural machinery, on-road vehicle fleet management, oil exploration for the positioning of platforms at sea or for scientific applications such as geodesy.