Evolution of Corrections Processing for the MC/MF Ground Based Augmentation System (GBAS)

The Ground Based Augmentation System (GBAS) is currently standardized at the International Civil Aviation organization (ICAO) level to provide precision approach navigation services up to Category I using the GPS or LONASS constellations. Current investigations into he use of GBAS for a Category II/III service type known as GAST D are ongoing. However, some gaps in performance have been identified and open issues remain Multi-frequency and multi-constellation solutions are being explored within the European SESAR program (WP 15.3.7) to address these issues. The addition of a secondary constellation provides many advantages such as better geometry, robustness against signal outages, relaxing of demanding constraints. Furthermore, new signals offer the potential to combine measurements on multiple frequencies to mitigate the effects of the ionosphere including during disturbances and helps the stringent continuity and availability requirements to be met. However, whilst the advantages of using many more signals is clear, there exists a major constraint with respect to the available space for message transmission from the GBAS VHF Data Broadcast (VDB) unit [5]. Currently, corrections and their integrity are provided in combined messages broadcast every half second (2Hz). However, extending this approach to multiple correction types, based on the different signals and observables for two or more constellations will not be possible. Furthermore, if the need arises to include future signals from the modernized constellations or expand further than two constellations then no additional transmission space would be available. It is for these reasons that the authors have investigated the possibility of providing corrections at a lower rate than the current 2Hz, with a separate message type dedicated to providing the integrity status of each correction in a manner akin to the Satellite Based Augmentation System (SBAS) [6]. In order to justify this approach and to select the ideal correction message rate, a number of items must be addressed.