Improved Least Travel Time ray-tracing operator for GNSS tropospheric delays

Modelling of the microwave signal delay in the neutral atmosphere (i.e., the tropospheric delay) is a crucial part of GNSS observations processing. The design of observation-modelling algorithms is based on signal ray tracing. Considering advancements in modern Numerical Weather Prediction (NWP) models and high standards of GNSS product quality, it is necessary to revise the existing ray-tracing algorithms. We developed an improved least-travel time (LTT) ray-tracer with robust physics assumptions, based on the original LTT algorithm. Both new and original LTT implementations, along with the state-of-the-art VieVS Ray-tracer (RADIATE), are supplied with numerical weather data by the Open Integrated Forecasting System model (OpenIFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). These three ray tracers are justly compared in the setup of modelling the skyview delays for 256 GNSS stations during one month (December 2016). The skill of these delay products is assessed as the quality of GNSS precise orbit determination (POD) products of the GPS constellation made by the orbit solver GROOPS (Gravity Recovery Object Oriented Programming System) software toolkit of the Graz University of Technology. The GNSS POD metrics which have been analysed are orbit midnight discontinuities (MD) and precise point positioning (PPP) error. In the context of these metrics, the usage of the new LTT algorithm leads to better orbit products, compared to the original LTT and the RADIATE ray tracers.