Reduction of ionosphere divergence error in GPS code measurement smoothing by use of a non-linear process

This paper develops a single-frequency filter for smoothing GPS code measurements using a nonlinear method. The method zeroes steady-state divergence errors caused by ionospheric storms. The filter has significant potential to improve the availability of safety-critical differential GPS systems such as Ground-Based Augmentation Systems (GBAS). These systems, of which the Local Area Augmented System (LAAS) is one example, typically rely on a Hatch Filter for code smoothing. Although the Hatch filter offers excellent noise reduction and ease of implement under nominal conditions, the filter amplifies ionosphere errors under rare, anomalous storm conditions. Our method generalizes the Hatch Filter by adding a correction term which is computed through a nonlinear process. The nonlinear process detects high ionosphere-delay gradients and estimates the appropriate correction term for the Hatch Filter. Experimental trials indicate that the nonlinear filter not only zeroes steady-state divergence but also reduces maximum transient errors by 0-25% relative to the Hatch Filter without impacting filter noise under nominal conditions.