Ionospheric threat mitigation by geometry screening in ground-based augmentation systems

Large spatial variations in ionospheric delay of Global Navigation Satellite System signals observed during severe ionospheric storms pose potential threats to the integrity of the Ground-Based Augmentation System, which supports aircraft precision approaches and landing. Range-domain monitoring within the Ground-Based Augmentation System ground facility cannot completely eliminate all possible ionospheric threats, because ionospheric gradients are not observable to the ground monitor if they impact the satellite-to-ground lines of sight with the worst-possible geometry and velocity. This paper proposes an algorithm called position-domain geometry screening to remove potentially hazardous satellite geometries under worst-case ionospheric conditions. This is done by inflating one or more integrity parameters broadcast by the ground facility. Hence, the integrity of the system can be guaranteed without any modification of existing avionics. This paper develops an algorithm that allows the ground station to conservatively estimate the worst-case ionospheric errors for Ground-Based Augmentation System users. The results of this algorithm determine which potential aircraft satellite geometries are safe and which are unsafe, and inflation of the broadcast σ_vig parameter is used to make all unsafe geometries unusable for the Ground-Based Augmentation System. Although the elimination of unsafe geometries reduces system availability, this paper shows that acceptable availability for category I precision approaches is attainable at Memphis International Airport and Newark Liberty International Airport while guaranteeing system integrity under anomalous ionospheric gradients.