Using a Wide Area Receiver Network to Support GBAS Ionospheric Monitoring

Abstract

Ionospheric anomalies, like ionospheric gradients, might produce a difference between the ionospheric error experienced by the Ground Based Augmentation System (GBAS) reference station and the aircraft on approach. This ionospheric delay difference could lead to position errors if undetected. For that reason, the GBAS Approach Service Types (GAST) C and D provide solutions against this threat, but the methods employed still face challenges. In the case of GAST C, the GBAS integrity parameters are inflated in order to exclude potentially usable satellite geometries that could produce unacceptably large position errors if affected by a worst case gradient. The GAST D concept contains a significant amount of monitors to cover different regions of the ionospheric threat space, which includes a combination of gradient sizes, directions and speeds. Both methodologies have a negative impact on the availability of the system, especially when trying to implement GAST C and D in regions with severe ionospheric behavior, i.e. equatorial regions, because of the exclusion of too many satellite geometries in the case of GAST C and a high rate of false alarms in GAST D. This paper proposes a real-time ionospheric monitoring approach that supports the already existing GAST C and D solutions without changing the overall integrity concept. The proposed concept is based on a multi-frequency multi-constellation receiver network external to the GBAS installation that provides real-time ionospheric information to the GBAS stations. A GAST C reference station could use this information to reduce the maximum gradient to consider in the inflation of the integrity parameters while a GAST D station could benefit from prior probabilities of appearance relaxing the monitoring thresholds and thus provoking less false alarms. The monitoring approach is first explained and then evaluated with simulated data. This data is constructed based on real measurements of noise and multipath taken with both Choke-ring and Multipath Limiting Antennas (MLAs), and models for the nominal ionospheric decorrelation. Simulation results show that a considerable reduction of the threat space that the remaining monitors in GAST C and D have to consider seems feasible. Furthermore, the main effects of the presence of high levels of noise and multipath in the measurements used are discussed.