Ionospheric delay gradient threat model for GBAS based on GPS data in China

Ground-Based Augmentation Systems (GBAS) provides differential corrections and integrity information so that the airborne users correct global navigation satellite system (GNSS) measurements. Among all the errors, the influence of ionospheric delay is significant and uncertain. In general, GBAS ground stations determine and broadcast a conservative one-sigma parameter to bound the ionospheric uncertainty. However, the ionospheric abnormal conditions, just like storm and scintillation, may cause extreme gradients which is hardly to be bound. Therefore, the ionospheric threat model is defined to bound worst-case ionospheric delay gradients. However, current models can not be used in China directly because the characteristics of the ionosphere in China are complex and unique for the large latitude span. Therefore, it is urgent and necessary to establish a threat model for GBAS in China. This paper analyzes the ionospheric anomalies in China from 2011 to 2014, based on real GPS data from Crustal Movement Observation Network of China (CMONOC). The four years includes the most recent solar maximum period, hence the data from which are representative. We process the data by the improved “Simple Truth” method. Then, two typical ionospheric anomaly events are demonstrated and analyzed and we verify the max gradient event. Finally, we demonstrate the statistic result of ionospheric anomaly gradients larger than 100mm/km and the max gradient in this research is 196mm/km. There is no obvious relationship between the distribution of anomaly gradients and satellite elevation angle. Therefore, a uniform constant is recommended as an upper bound for the ionospheric threat model in China. More data is needed to determine the value of upper bound of extreme gradient and establish an integrated ionospheric threat model for GBAS in China. These results may be helpful for understanding the characteristic of ionosphere and provide reference for the ionospheric threat model for GBAS in China.