Equatorial plasma bubble model and integrity risk evaluation for ground based augmentation system in Hong Kong

The spatial gradient induced by the Equatorial Plasma Bubble (EPB) in low latitude regions is a major challenge for the Ground Based Augmentation System (GBAS). To facilitate the implementation and operation of GBAS Approach Service Type (GAST)-D at Hong Kong International Airport, the impact of EPB induced spatial gradients needs to be analyzed. Previous simulations using a two-dimensional trapezoid model neglected the three-dimensional structure of the EPB, assuming ionospheric delay on a thin shell at a specific altitude. To address this limitation, this paper adopts a cube above the magnetic equator to characterize the EPB threat model. The ionospheric delay difference between satellite signals passing through the EPB model is limited by the upper bound spatial gradient derived with the data collected from Hong Kong Satellite Positioning Reference Station Network. The simulation results reveal that ionospheric monitors in GAST D can satisfy the Category II/III approach requirement, i.e., the probability of missed detection P_MD for the differential range error E_r larger than 2.75 m is lower than 1×10^-9. The potentially hazardous event with the largest E_r of 2.47 m and P_MD of 1.86×10^-9 occurs when the satellite signal moves parallel to the equator near the Equatorial Ionization Anomaly (EIA) region, and its significant portion traverses in the EPB depletion region.