Nonlinear Filter for Ionosphere Divergence Error Reduction in LAAS

The Hatch filter, the industry-standard method for smoothing GPS code measurements with carrier data in the local area augmented system (LAAS), is susceptible to divergence errors during severe ionospheric storms. The work presented here develops an alternate single-frequency filter called a nonlinear divergence elimination (NLDE) filter that mitigates divergence errors without increasing measurement noise. Application of the filter to GPS data indicates that the nonlinear filter not only zeroes steady-state divergence but also reduces maximum transient errors by 0-24% relative to the Hatch filter.