New Developments in Near Real-Time GNSS Zenith Total Delay Estimates at the University of Luxembourg

Recently, the University of Luxembourg (UL), in collaboration with the United Kingdom Met Office, has started providing accurate near real-time (NRT) Zenith Total Delays (ZTDs) from networks of GNSS ground stations. This initiative is in alignment with the operational meteorological products from various analysis centers available at the EUMETNET EIG GNSS Water Vapour Programme (E-GVAP) and the team in Luxembourg envisages to re-start its contributions in the near future. Active in Europe, E-GVAP coordinates NRT GNSS-based atmospheric content monitoring to support Numerical Weather Prediction (NWP) modelling with products that are crucial for mesoscale models throughout Europe, for example, for the Met Office. GNSS technology is essential for accurately measuring atmospheric parameters such as ZTD and Integrated Water Vapor (IWV) at high frequencies, regardless of weather conditions. In addition, GNSS data are low-cost when compared to conventional meteorological systems. Ensuring the NRT availability of these data for NWP assimilation systems requires numerous methods in GNSS data handling and processing, quality assurance, and distribution.  The study details the collaborative work between the UK Met Office and the University of Luxembourg in providing accurate and rapidly available meteorological data through GNSS technology. This collaboration has led to the development and update of various systems for the processing of GNSS observations to produce advanced NRT ZTD products at UL and the Met Office. These products are generated at 1-hour intervals on both global and regional scales, and at sub-hourly intervals regionally. This study primarily aims to provide a thorough review and accuracy assessment of NRT ZTD products from the UL, comparing their precision with benchmark data from both post-processed and NRT ZTD estimates from various EGVAP analysis centres. The NRT GNSS processing systems at UL use the Bernese GNSS Software (BSW) versions 5.2 and 5.4 with a double-differencing (DD) approach, and similarly, the post-processed benchmark ZTD estimates employs the DD positioning strategy using the same software packages.