Augmentation of a global navigation satellite system (GNSS) is a method of improving the navigation system's attributes, such as accuracy, reliability, and availability, through the integration of external information into the calculation process. There are many such systems in place and they are generally named or described based on how the GNSS sensor receives the external information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional vehicle information to be integrated in the calculation process.
Satellite-based augmentation system
Satellite-based augmentation systems (SBAS) support wide-area or regional augmentation through the use of additional satellite-broadcast messages. Using measurements from the ground stations, correction messages are created and sent to one or more satellites for broadcast to end users as differential signal. SBAS is sometimes synonymous with WADGPS, wide-area DGPS.
The GBAS and SBAS that have been implemented or proposed include:
- The Wide Area Augmentation System (WAAS), operated by the United States Federal Aviation Administration (FAA).
- The European Geostationary Navigation Overlay Service (EGNOS), operated by the ESSP (on behalf of EU's GSA).
- The Multi-functional Satellite Augmentation System (MSAS), operated by Japan's Ministry of Land, Infrastructure and Transport Japan Civil Aviation Bureau (JCAB).
- The Quasi-Zenith Satellite System (QZSS), being implemented by Japan, scheduled to commence in November 2018.
- The GPS Aided Geo Augmented Navigation (GAGAN) system being operated by India.
- The GLONASS (System for Differential Correction and Monitoring, SDCM), operated by Russia with global coverage.
- The Satellite Navigation Augmentation System (SNAS), proposed by China.
- The Wide Area GPS Enhancement (WAGE), operated by the United States Department of Defense for use by military and authorized receivers.
- The commercial StarFire navigation system, operated by John Deere and C-Nav Positioning Solutions (Oceaneering).
- The commercial Starfix DGPS System and OmniSTAR system, operated by Fugro
- The GPS·C, short for GPS Correction, was a Differential GPS data source for most of Canada maintained by the Canadian Active Control System, part of Natural Resources Canada - now decommissioned.
- Australia and New Zealand have recently (2018) commenced R&D into a second generation SBAS system (as yet unnamed) for their operational areas. Work is proceeding in using a multi frequency and multi constellation approach to reduce certain errors that first generation system like WAAS cannot handle. Also under research is using Precise Point Positioning as part of the design.  The resulting system is expected to reliability produce multi centimetre accuracy fixes.
Ground-based augmentation system
Each of the terms ground-based augmentation system (GBAS) and ground-based regional augmentation system (GRAS) describe a system that supports augmentation through the use of terrestrial radio messages. As with the satellite based augmentation systems detailed above, ground-based augmentation systems are commonly composed of one or more accurately surveyed ground stations, which take measurements concerning the GNSS, and one or more radio transmitters, which transmit the information directly to the end user from the ground up thus avoiding the constraints associated with GEO Satellites at high altitudes.
The shorter the distance between the ground station that calculates the differential corrections to the inbound plane, the higher the accuracy is likely to be. There are stricter Safety requirements on GBAS systems relative to SBAS systems since GBAS is intended mainly for the landing phase where real-time accuracy and signal integrity control is critical, especially when weather deteriorates to the extent that there is no visibility (CAT-I/II/III conditions) for which SBAS is not intended or suitable.
Various ground-based augmentation systems
- International Civil Aviation Organization Ground-Based Augmentation System (GBAS) applies to precision approach landing of civil aircraft. Originally this system was called the Local Area Augmentation System (LAAS)
- The US Nationwide Differential GPS System (NDGPS), An augmentation system for users on U.S. land and waterways.
- See also, the Differential GPS (DGPS) Wikipedia page
Aircraft-based augmentation system (ABAS)
The augmentation may also take the form of additional information from navigation sensors being blended into the position calculation, or internal algorithms that improve the navigation performance. Many times the additional avionics operate via separate principles than the GNSS and are not necessarily subject to the same sources of error or interference. A system such as this is referred to as an aircraft-based augmentation system (ABAS) by the ICAO. The most widely used form of ABAS is receiver autonomous integrity monitoring (RAIM), which uses redundant GPS signals to ensure the integrity of the position solution, and to detect faulty signals.
Additional sensors may include:
- eLORAN receivers
- Automated celestial navigation systems
- Inertial navigation systems
- Simple dead reckoning systems (composed of a gyro compass and a distance measurement)
- Kee, C., Parkinson, B. W., and Axelrad, P. (1991), "Wide area differential GPS", Navigation, Journal of the Institute of Navigation, 38, 2 (Summer, 1991), <http://ion.org/search/view_abstract.cfm?jp=j&idno=207[permanent dead link]>
- US Government page on GPS augmentation systems
- ICAO (2005). Global Navigation Satellite System (GNSS) Manual (PDF) (First ed.).