Impact of Dual-Frequency Multi-Constellation GBAS Data Link Constraints on Integrity Monitoring

Abstract

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, L1/E1 and L5/E5a.

However, unintentional Radio Frequency Interference (RFI) could affect one of the frequencies, typically L1/E1, potentially leaving GBAS with only a single operating frequency. In such cases, GAST E cannot function as intended, making it necessary to define single-frequency fallback modes, GAST D1 and GAST D2, to maintain CAT II/III service. A key challenge for these new modes is the need to transmit additional measurements through the limited GBAS data link bandwidth, which makes data compression methods at the ground station and data reconstruction methods at the airborne side essential.

This paper evaluates the impact of the Message Type (MT) 23 compression and reconstruction method being proposed at the International Civil Aviation Organization (ICAO) level, together with the lower ground data update rate (0.33 Hz instead of 2 Hz), on one of the key integrity monitors for these fallback modes: the Dual Solution Ionospheric Gradient Monitoring Algorithm (DSIGMA). We first derive DSIGMA thresholds for L1/E1 and L5/E5a using nominal flight test data and evaluate their consistency with existing GAST D standards. We then assess monitor performance with simulated ionospheric gradients to evaluate the impact of these new constraints under controlled anomalous ionospheric conditions, followed by an evaluation with real flight data collected under strong scintillation.

The results with both simulated and real data demonstrate that the MT23 compression and reconstruction method, as well as the ground data lower update rate, can be used without visible effects on DSIGMA monitoring.