Improving Availability of SBAS Ionospheric Corrections

Abstract

The current WAAS/MSAS ionospheric correction procedure is based on the algorithm called as 'planar fit.' The availability of ionospheric corrections for MSAS is, so far, predicted as relatively low due to insufficient measurements for performing planar fit. The storm monitor often trips even for non-storm ionosphere conditions. The authors are introducing zeroth order fit to improve availability of ionospheric corrections. It will be used only when the standard planar fit cannot be applied due to trip of the storm monitor or insufficient number of observations. Additionally, quadratic fit is also introduced to improve the performance of estimation process. It needs good geometry of the distribution of observations being used for fit, so the authors have tested three candidate metrics to determine if quadratic fit can be applied or not. It will be shown that HDOP and condition number metrics are functional for this purpose. With performing zeroth fit, 5-6% of IGPs were saved from the unavailable states for a severe storm condition observed in October 2003. Quadratic fit reduced about 12% of residual error in sense of RMS for 70% of IGPs.