Performance Evaluation of the Ionospheric Threat Mitigation Strategies in Dual-Frequency Multi-Constellation GBAS

In this paper, we evaluate the performance of the dual-frequency airborne ionospheric gradient monitor proposed for dual-frequency multi-constellation (DFMC) Ground Based Augmentation Systems (GBAS) at different distances from the airport. We use two types of thresholds for this assessment: (i) a constant threshold derived from operational requirements, and (ii) a dynamic threshold that increases with the distance from the airport. Increasing the threshold allows more ionospheric error within the position solution, but also enables the use of the primary single-frequency modes without the need to switch to the ionosphere-free (Ifree) solution for a longer period, which generally degrades the performance because it combines the noise and multipath of two frequencies. Furthermore, we compare the performance of the two potential architectures for DFMC GBAS: (i) the so-called GAST F architecture, which is based on single-frequency 100 seconds smoothing, and (ii) the GAST X architecture, which is based on divergence-free (Dfree) smoothing with variable and potentially longer smoothing time constants. Results with both simulated and real data show that the use of a variable threshold significantly reduces the probability of excluding satellites and switching to the Ifree mode for both GAST F and X, thereby increasing the availability of GBAS.