Ionospheric monitoring in a dual frequency GBAS

This paper proposes a method for monitoring the difference in the ionosphere between a GBAS ground station and an airborne user from single frequency L1/E1 corrections and single frequency L5/E5a corrections. The absolute ionospheric delay at the ground station cannot be estimated from the corrections only. Therefore, the monitored test statistic is the difference of an ionospheric delay estimate at the airborne system and a similar pseudo-ionospheric delay estimation from the single frequency corrections from the ground station. In order to make the two estimates comparable the common part of the airborne ionospheric delay has to be removed. This test is affected by noise and multipath from the airborne measurements and from the ground corrections. Based on airworthiness performance requirements a monitoring threshold per satellite is derived. The decision then enables the airborne equipment to either exclude individual satellites from the single frequency processing or trigger a switch to an ionospheric free dual frequency processing.