Preliminary Results from Ionospheric Threat Model Development to Support GBAS Operations in the Brazilian Region

Abstract

The Brazil ionospheric study project aims to develop a new Ground-based Augmentation System (GBAS) ionospheric threat model which reflect Brazilian low-latitude conditions. This study utilizes dual-frequency GNSS data collected on about 120 active ionosphere days to assess the Brazilian ionospheric behavior. Over 1000 threat points were generated from post-processed data, and most of these ionospheric gradients are caused by night-time Equatorial Plasma Bubbles (EPBs). A significant number of ionospheric gradients exceeded the upper bounds (375 – 425 mm/km) of the Conterminous U.S (CONUS) threat model. In particular, the largest gradient of about 850 mm/km is twice as large as the maximum gradient observed in CONUS. Since a larger bound would have a significant effect on system performance and availability, more-detailed study of the behavior of the most severe ionospheric gradients is desirable before finalizing the threat model. This paper also defines other threat model parameters to model the geometry of EPBs. The observations from Brazil were examined to quantify other existing parameters used to model ionospheric storms in mid-latitude regions and newly introduced parameters for EPBs. The maximum depletion of 35 meters and transition zone lengths of about 20 – 450 km have been estimated for the EPBs which produced the most extreme gradients. These EPBs appeared to be moving roughly eastward and parallel to the geomagnetic equator (within ±35 degrees of the magnetic north) with speeds of about 40 – 250 m/s. EPBs occurred mostly during post-subset hours between 1800 and 0500 local time with the peak in the late evening just before local midnight. Defining and modeling these characteristics of EPBs are key to the development of a GBAS ionospheric mitigation and safety case for operational approval in Brazil and the southern hemisphere.