Session B4: GNSS Integrity and Augmentation

From Ground to Air: The Paradigm Shift in GBAS Monitoring and Its Benefits
Michael Felux, Michael Nietlispach, ZHAW - Zurich University of Appl. Sciences
Location: Beacon B

The Ground-Based Augmentation System (GBAS) can significantly enhance the performance of satellite-based navigation by providing corrections and integrity data. Increased accuracy and ensured performance thus make the use of Global Navigation Satellite Systems (GNSS) suitable for precision approach operations. While GBAS has originally been developed to replace currently used Instrument Landing Systems (ILS), its implementation has been relatively slow, with stations developed for Category I (CAT-I) operations (the so-called GBAS Approach Service Type (GAST) C) only gradually becoming operational at a few selected airports worldwide. With a first safety case made in Frankfurt, these stations can, in certain cases, now also support CAT-II operations, i.e. precision approaches with a decision height of just 100 ft. Although the standards for GAST-D, the service type developed for and intended to support Category II/III (CAT-II/III) operations, have been fully developed and published, there is currently no commercially available product to support these operations. Looking ahead, GAST-E, a new service type, also supporting CAT-II/III operations based on using navigation signals in two frequency bands and from different GNSS constellations GNSS, has been under development for several years. The concept has matured in general architectural considerations and continues to be developed within the Horizon Europe project EDGAR. This dual-frequency multi-constellation (DFMC) service type called GAST-E is expected to overcome several limitations of GAST-D by improving resilience against ionospheric disturbances, removing potential siting constraints that may be required for GAST-D station, and increasing overall system performance through the use of multiple constellations and frequencies to ensure the service is globally available with sufficiently high availability.
To support CAT-II/III precision approach operations, traditionally, a GBAS ground station must monitor for potential faults that could compromise navigation integrity and safety. These low-level monitoring requirements are derived from the standards AC 120-28D for the US and the CS-AWO (Europe), which mandate that aircraft touches down within the designated touchdown zone of the runway with a given, very high probability in the nominal case and with certain fault types. These requirement relate to the Total System Error (TSE) on aircraft level. The TSE mainly consists of the so-called Flight Technical Error (FTE), describing how well the autopilot can control the aircraft in varying conditions such as different wind scenarios and with different mass and center of gravity locations, as well as by the Navigation System Error (NSE) describing the performance of the navigation system. With a given FTE performance for a certain aircraft, the NSE budget can be calculated and further low-level monitoring requirements be derived.
The scope of this paper is to revisit the requirement derivation process for the navigation system in light of the current architecture considered for GAST-E. One of the main changes in philosophy compared to GAST-C/D is that the ground station will uplink measurements instead of corrections. A significant part of the processing and monitoring that was previously done in the ground station will now take part in the airborne receiver. Despite the increased required processing power, this paradigm shift brings along several potential benefits. While in service types C/D the requirement derivation had to make generalized and conservative assumptions regarding, for example, the FTE performance to accommodate all potential aircraft types that might use GBAS, in GAST-E the requirements can potentially be derived based on the performance of a specific aircraft type. Furthermore, one parameter that influences the remaining NSE budget in the trade-off is the glide path angle (GPA) of a given approach. Again, in the GAST-C/D case, the most conservative value (i.e. 2.5° in this case) had to be assumed. However, the GPA is a parameter that is transmitted within the GBAS message, so the actual GPA of the approach an aircraft is flying can be used to determine the monitoring thresholds for that approach.
In this paper we will analyze the impact of the above-mentioned considerations and show the potential benefits in terms of relaxed monitoring requirements.