Constructing Ionospheric Irregularity Threat Model for Korean SBAS

Single-frequency based Satellite-Based Augmentation Systems (SBAS), the augmentation of the Global Navigation Satellite System (GNSS), broadcast estimates of vertical ionospheric delays and confidence bounds on the delay errors at Ionospheric Grid Points (IGPs). Using an ionospheric irregularity undersampled threat model, the integrity bounds, called Grid Ionospheric Vertical Errors (GIVEs), must be augmented to bound ionospheric irregularity threats which may exist between or beyond Ionospheric Pierce Points (IPPs) under ionospheric storm conditions. Since the ionospheric disturbed conditions can vary significantly from one region to another region, threat models need to be built for regions where SBAS will be operational. This paper presents a new method for constructing an undersampled threat model for SBAS in the Korean region, examines the influence of threat model to system availability, and demonstrates the performance of a newly developed threat model. The existing method tabulates undersampled threats in the threat model as a function of two metrics which measure the density and uniformity of IPP distribution in a region. Thus, the threat model metrics, which characterize threatening undersampled geometries including the density of IPP distribution accurately, play a critical role in improving system performance. The first threat metric, fit radius, is defined by an IPP search method used for a planar fit algorithm. This paper first determines a range of the fit radius optimized for the Korean region by considering the ionospheric observability and quality of the planar fit. Next this paper proposes a new second metric, the Relative Bin Number (RBN) metric, alternative to the Relative Centroid Metric (RCM) currently used in WAAS. RBN is more effective than the existing threat metric in capturing the sparseness of the IPP distribution by measuring the ratio of the number of partitions in which IPPs are absent to the total number of partitions. In addition, other essential parameters for the Korean SBAS threat model construction, including GEO MT28 (Message Type 28), IGP formations, and the number of reference stations, are determined. In a preliminary assessment, the undersampled ionospheric threat model based on the new methodology increased the coverage of 99.9% availability for APV-I service from 18.48% to 91.10%.