Improved Adaptive Hatch Filter Method to Minimize Effects of Ionospheric Anomaly for GBAS

Abstract

Carrier-smoothed-Code (CSC) algorithm is widely used in ground based augmentation system (GBAS) receivers to suppress the high frequency noise of the pseudorange and the position accuracy will be improved accordingly. Hatch filter is a commonly used CSC algorithm. In general, the Hatch filter smoothing time is 100 seconds, which is suitable for most cases. However, the smoothed pseudorange error may be very large when the ionosphere is abnormal. In view of the phenomenon of ionospheric anomaly in the GBAS and the resulting reduction in the accuracy of the smoothed pseudorange, a new adaptive CSC algorithm is proposed. Firstly, the mathematical recurrence form of the Hatch filter is given and then the error propagation form of the Hatch filter is analyzed in detail. Secondly, in order to detect whether the ionosphere is abnormal, a new code-carrier divergence (CCD) monitor which is based on second-order linear time-invariant (LTI) low-pass filter is proposed to suppress the high frequency noise of the CCD. If the ionosphere is abnormal, the Hatch filter smoothing time will be adaptively calculated using the ionosphere delay changing rate from the CCD monitor. Finally, the experimental platform of GBAS developed in laboratory is used to verify the adaptive CSC algorithm proposed here, and the experimental results show that the smoothing time of the Hatch filter can be calculated adaptively, and also the positioning accuracy of the airborne can be improved when the ionosphere is abnormal.