Mitigation of Ionospheric Mapping Function Error

Kurzfassung

Ionospheric delay is currently considered as a major error source for satellite based navigation systems. The operational standards set by the International Civil Aviation Organization (ICAO) for Wide Area Differential GPS (WADGPS) ensure Space Based Augmentation System (SBAS) users an ionospheric delay correction in addition to satellite clock and orbit corrections. In the current standard, the SBAS broadcasts vertical ionospheric delays in meters at the ionospheric grid points (IGP) located at every five degrees of latitude and longitude. The SBAS user receivers convert them to the slant delay using the so called obliquity factor derived under the assumption of a thin-shell ionosphere fixed at 350 km height. Although the ionospheric correction is available for all broadcast IGPs, the user does bilinear interpolation between IGPs those surround the ionospheric piercing point (IPP) at the thin-shell height. In this paper we present a new mapping function approach for vertical to slant delay conversion. Considering broadcast delays not only at a single IPP rather at different geographic locations along the ray path projected on the thin-shell at 350 km height, our algorithm incorporates the horizontal gradients in the slant delay computation. Instead of collapsing the vertical structure of the ionosphere into a thin-shell we consider the Chapman layer assumption for describing the vertical electron density distribution of the ionosphere. Our investigation shows that using the proposed algorithm instead of the thin-shell algorithm we can reduce the mapping function error by about 50% under high as well as low solar activity conditions.