From Ground to Air: The Paradigm Shift in GBAS Monitoring and Its Benefits

The Ground-Based Augmentation System (GBAS) is a differential navigation system that provides corrections and integrity parameters to support precision approaches of aircraft. The currently implemented GBAS Approach Service Types (GAST) C was developed to support CAT-I operations (and CAT-II operations in certain cases). The GAST D has been developed to support also CAT-III operations based on single-frequency GPS navigation. However, it is expected that availability of this service will not reach sufficiently high values in all parts of the world. The GAST E service type currently under development will include a significant shift in system architecture by incorporating dual-frequency, multi-constellation capabilities and not provide corrections anymore. Instead, under the GAST E concept, the ground station will transmit raw pseudorange and carrier phase measurements. This enables shifting the burden of integrity monitoring partially from the ground to the airborne receiver with potential benefits regarding integrity monitoring. This paper revisits the derivation of GBAS ground monitoring requirements in light of the proposed GAST E architecture and examines the potential for relaxation of these requirements. One monitor that could specifically benefit is the Code-Carrier Divergence (CCD) monitor, currently allocated to the ground subsystem in GAST D. By leveraging the airborne subsystem’s knowledge about relevant parameters used to derive the monitoring threshold, an airborne implementation of the CCD monitor can mitigate the need for conservative assumptions that are currently necessary to be applied by the ground system. Two monitoring concepts are considered: an adapted ground-based approach and an airborne-based monitoring approach. The results indicate that shifting CCD monitoring to the airborne subsystem enables a significant increase of the monitoring thresholds while maintaining compliance with the underlying safety requirements. The proposed adaptation reduces the risk of false alerts and has the potential to improve availability of the GBAS service.