GBAS ionosphere monitoring and assessment based on GPS data in the Beijing area

Ionospheric delay is one of the largest and most variable sources of error for users of Ground-Based Augmentation System (GBAS). During extreme ionospheric storms, GBAS users and the GBAS ground facility may experience different ionospheric delays, leading to a big differential error and threatening the safety of users. Therefore, ionospheric monitoring and assessment is an important part of GBAS integrity monitoring. To understand and mitigate ionospheric threats to GBAS, the United States and South Korea established the ionospheric anomaly threat model based on extreme ionospheric gradients observed during the last solar maximum (2000-2004) and proposed a conservative value for the one-sigma vertical ionospheric gradient of σ_vig =4 mm/km for the conterminous United States. However, this model cannot be applied directly in China because ionospheric behavior varies significantly between locations with different solar radiation and geomagnetic environments. Thus it is necessary to monitor and evaluate the ionosphere and to characterize ionospheric behavior in China. This paper presents results obtained from processing GPS data for the Beijing region. We describe an improved ionospheric delay algorithm based on the simple truth algorithm. The accuracy and usability of the improved ionospheric delay algorithm are verified through comparison with the Long-term Ionospheric Anomaly Monitoring (LTIAM) software developed by the US Federal Aviation Administration. We then compute ionospheric threat gradients for the Beijing area by processing GPS data from 16 reference stations in the area. The maximum slant spatial gradient within the Beijing threat space is 31.09 mm/km, the maximum standard deviation of the vertical ionospheric gradient is 2.40 mm/km, and the overbound is 3.59 mm/km. We conclude that ionospheric activity was stable in the Beijing area during the study period. These results may be helpful for understanding ionospheric spatial decorrelation and could provide a theoretical reference for the use of GBAS in the Beijing area and throughout China.