L5/E5a-Based Fallback Mode for Dual-Frequency Multi-Constellation GBAS

The future architecture for Dual-Frequency Multi-Constellation Ground-Based Augmentation Systems (DFMC GBAS), known as GBAS Approach Service Type (GAST) E, is designed to rely on measurements from two frequency bands, specifically L1/E1 and L5/E5a. However, unintentional Radio Frequency Interference (RFI) could affect one of the frequencies, typically L1/E1, potentially leaving GBAS with just a single operating frequency. In such a scenario, GAST E cannot function as intended, necessitating the definition of single-frequency fallback modes, either on L1/E1 or L5/E5a, to maintain CAT III service. Therefore, the definition of a GAST D-like mode based on L5/E5a becomes important. However, the integrity monitors designed for GAST D in current standards have only been validated for L1 and GPS and are known to be sensitive and trigger excessive false alarms when the ionosphere is very active. Therefore, in this paper, we evaluate the performance of a GAST D equivalent mode implemented for L5/E5a. The purpose of this study is to assess if this fallback mode can support CAT III operations, or whether the integrity monitors trigger excessive false alarms and consequently regular switches to a CAT I fallback mode can be expected. For these studies, we implement and adapt the main existing GAST D monitors for anomalous ionospheric gradient mitigation to operate in the L5/E5a frequency band. First, we assess the performance of these monitors under nominal conditions using real data collected by the Multipath Limiting Antennas (MLAs) installed at Tenerife Norte Airport and a user ground receiver located within the airport’s protected area. Following the assessment under nominal conditions, we adapt the thresholds of these monitors and evaluate their performance using real data during scintillation events. Results show the need to adapt the monitors for L5/E5a and the suitability of the adapted thresholds to detect high ionospheric activity