GNSS-based Heighting in Australia: Current, Emerging and Future Issues

Ellipsoidal heights, i.e., w.r.t. a geometrical Earth figure, determined from Global Navigation Satellite Systems (GNSS) are inherently their least accurate coordinate, due mainly to satellite,geometry and atmospheric refraction. For most practical purposes, however, these GNSS-derived ellipsoidal heights have to be transformed to heights that relate to the Earth's gravity field, which generally adds further uncertainty The reduction in accuracy of the transformed height is due to errors in gravimetric quasi/geoid models, but this is compounded yet further in Australia and elsewhere because of the imperfect realisation of local vertical datums. This paper comments upon current, emerging and future issues with height determination on the Australian Height Datum (AHD) using GNSS. This comprises the reference frame used for GNSS ellipsoidal heights, theory- and data-driven inaccuracies in modelling the quasi/geoid, and deficiencies in the realisation of the AHD. While some of these issues will be redressed, in part, by the production of AUSGeoid2008 that is fitted to the AHD, there will always be the need to routinely apply checks on GNSS-derived heights in Australia, and elsewhere.