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Four types of augmentation
To meet aviation's strict operational requirements during various phases of flight, ICAO recommends the core constellations require augmentation:
1. Aircraft-Based Augmentation System (ABAS)
2. Satellite-Based Augmentation System (SBAS)
3. Ground-Based Augmentation System (GBAS)
4. Ground-based Regional Augmentation System (GRAS)
ABAS
ABAS is self contained on board the aircraft, and relies on processing techniques and relevant avionics. It is common on large modern aircraft for use in enroute and non-precision approaches.
SBAS
SBAS uses ground based monitoring equipment to validate and correct GNSS signals, then delivers this information to the aircraft via additional geostationary satellites. SBAS became operational in the United States in 2003. Other countries (Japan, India, Europe)
also plan to deploy similar systems. It has been adopted by many general aviation aircraft, where coverage is available.
SBAS has the advantage that the correction signals are broadcast widely within the footprint of a geostationary satellite, however the infrastructure costs of these satellites are high. Neither Boeing nor Airbus plans to offer compatible avionics on their aircraft. This is
because SBAS offers no operational advantages over their current ABAS technologies to justify the additional equipment.
GBAS
GBAS is designed to be installed at an airport and broadcasts the correction signals via Very High Frequency (VHF) radio to the aircraft. Unlike the other augmentation systems, GBAS is a precision approach and landing system, which replaces traditional Instrument Landing Systems (ILS). It provides greater accuracy and integrity of signals, compared with ABAS or SBAS. This allows the aircraft to reach lower decision heights during inclement weather. Both Boeing and Airbus actively support GBAS, and Qantas has just taken delivery of the worlds first GBAS certified aircraft.
GRAS
GRAS is a hybrid between SBAS and GBAS. It uses SBAS type ground equipment, but broadcasts to the aircraft in the same format as GBAS via VHF. With minor software changes, a GBAS avionics unit can accept the GRAS signal. Ground-based radio transmitters can be added as required, enabling a modular approach to regional augmentation, which also provides sovereign control. Together GBAS and GRAS provide a gate-to-gate navigation solution, the ideal for most aviation customers. GRAS procedures and recommended practices were approved by ICAO in 2006.