GBAS Enhancement by using the ionospheric gradient correction

In Ground Based Augmentation System (GBAS), the temporal decorrelation is reduced by using the range rate correction. But the ionospheric spatial decorrelation is not compensated by the existing GBAS. Extensive studies have focused on the large ionospheric gradient and its effects on GBAS. The first purpose is to examine the characteristics of the ionospheric variability in low-latitude areas based on GPS data in the 11 -year cycle of the solar activity (2001-2012). An index is defined for describing the ionospheric variability. Both the standard deviation of the index and the maximum daily index are summarized and presented. The statistical results show that modeling the ionospheric spatially decorrelated error is necessary for achieving the meter level accuracy of GBAS. The second purpose of this paper is to propose a correction model for mitigating the ionospheric effects on GBAS. The model is represented by a vector including two components that describes the spatial change rates of the ionospheric delay along the east and the north directions for each satellite. In the modified GBAS messages, two more parameters for each satellite are transmitted to the user. By applying the correction, the user receiver is able to correct the difference of the ionospheric delay between the reference station and the user tracking the same satellite, and the improvement of the GBAS position performance can be achieved. GPS data collected from reference stations in low-latitude areas are used to test the performance of the proposed method. For a short baseline length of 16.9 km in Hong Kong, the horizontal positioning accuracy can be improved by 41 % and 61% during active solar years and geomagnetic storms respectively. The test results show that the ionospheric gradient correction can significantly improve the performance of the GBAS.