Impact of anomalous ionospheric gradients on GBAS in the low-latitude region of China

The ionospheric gradient caused by spatial decorrelation is a crucial factor affecting the integrity of a ground-based augmentation system (GBAS). We processed GPS data from the Crustal Movement Observation Network of China from 2008 to 2018 with the long-term ionospheric anomaly monitoring method and then analyzed and verified a typical anomalous event. Compared with the wedge, plasma bubbles in the low-latitude region can be better described by the trapezoid. The largest detected anomalous gradient in China was 196 mm/km, and the conservative upper bound of anomalous gradients was determined to be 300 mm/km. Wedge and trapezoid were applied to establish GBAS threat models for China. Based on these threat models, we analyzed the impact of anomalous ionospheric gradients on GBAS in the low-latitude region of China. The results show that the performances of ionospheric anomaly monitors satisfy the requirements of GBAS approach service type D when the wedge is applied. However, the performances do not satisfy the requirements under the trapezoid for the following scenarios: When the speed and direction of the aircraft are similar to the threat model with a narrower slope width, the aircraft is affected by anomalies before the time that the GBAS facilities are affected; moreover, the monitor performance decreases sharply, and large differential range errors may occur. Finally, the high-risk approach conditions were analyzed in detail, and we provided recommendations for conducting aircraft approaches at low latitudes.