Tropospheric corrections in GNSS orbit determination without the mapping step

Neutral gas atmosphere bends and delays propagation of microwave signals in satellite-based navigation. Weather prediction models can be used to estimate these effects by providing 3-dimensional refraction fields for signal delay computation. In this study, a global numerical weather prediction model (Open Integrated Forecasting System (OpenIFS) licensed for Academic use by the European Centre for Medium-Range Weather Forecast) is used to generate the refraction fields. The slant delays are produced using a Least Travel Time (LTT) ray-tracer. Finally, the GNSS satellite orbits are solved using the GROOPS (Gravity Recovery Object Oriented Programming System) software toolkit of the Technical University of Graz which applies the raw observation method. Specifically, our implementation supplies the slant delays directly to the orbit solver without an intermediate mapping step, i.e., mapping of zenith delay to a prescribed functional form of azimuth and elevation angles. Essentially, this removes the assumption that signal delays follow some functional form, and allows hence to take full advantage of local refraction field asymmetries in GNSS signal processing that are partially lost in the mapping procedure. Our results indicate that this has clear benefits, both in terms of accuracy of the tropospheric correction and stream-lining the information flow in GNSS processing. Our view is that this new framework exposes the synergies in space geodesy and meteorology better than the earlier approaches.