Improving the performance of ground based augmentation system (GBAS) in low latitude areas

Abstract

A Ground Based Augmentation System (GBAS) is a type of Differential GPS (DGPS) for safety-critical applications, and many such systems have been developed all over the world. Several challenges need to be addressed when it is used for aircraft approach and landing, particularly for low-latitude areas. When deployed in low latitude areas, GBAS is degraded by ionospheric effects. This has become a major challenge for GBAS developments at low latitudes. The verification of integrity risk is another challenge, and the conservative assumptions have been made to ensure the extremely small integrity risk. These issues limit the applications of GBAS in demanding operations, such as Category (CAT) II and CAT III operations. In addition, Receiver Autonomous Integrity Monitoring (RAIM) is required in any augmentation system because it checks all error sources. Nevertheless, current RAIM techniques try to distinguish the statistical distribution of measurement errors, and they could not meet integrity requirements specified in the position domain. This study investigates the accuracy and integrity improvements of GBAS in low latitude areas. Firstly, a modified stochastic model is developed to improve the accuracy of GBAS during ionospheric disturbances, which happen frequently in low latitude regions. Another factor that degrades the GBAS accuracy is the large spatial decorrelation of the ionosphere. To reduce the effects of the ionospheric spatial decorrelation, the ionospheric gradient model is proposed. The performances of the modified stochastic model and the ionospheric gradient model are investigated based on real GPS observations in Hong Kong, and results show that these two models can improve the accuracy of GBAS accuracy significantly. In addition to the accuracy improvement, this work investigates the integrity improvement. For areas where both the GBAS service and the Satellite Based Augmentation System (SBAS) service are available, these two types of services are combined in the user equipment to improve the integrity performance. For areas where only the SBAS service is available, the monitor station is used to detect the anomalies in SBAS data, and more importantly, to reduce tails in the statistical distribution of SBAS errors. The performances of the combination method and the monitor station method are evaluated by the simulation as well as real data in North America. Results show significant improvements in the integrity performance made by using the combination method and the monitor station method. In addition, a new Measurement-based RAIM (MRAIM) technique is developed to improve the performance of RAIM. MRAIM not only meets the integrity requirements in the position domain, but also deals with the problem of multiple faults. Its performance is evaluated by using real GPS data. Results show that the all the large position errors are bounded successfully with multiple faults assumed.