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Real-Time GNSS for European Troposphere Delay Model

Periodic Reporting for period 1 - ReS4ToM (Real-Time GNSS for European Troposphere Delay Model)

Reporting period: 2019-08-16 to 2021-08-15

The project „Real-Time GNSS for European Troposphere Delay Model (ReS4ToM)” aims at developing “a novel real-time model of the troposphere by using Global Navigation Satellite Systems (GNSS) derived troposphere delays, gradient information and water vapor content”. Remote sensing of the troposphere with GNSS, so-called GNSS meteorology, provides observations of spatial and temporal resolution higher than any other technique and operates under all weather conditions. Therefore, hundreds of permanent GNSS stations in Europe are used by several analysis centers to operationally sense the troposphere under the E-GVAP project for monitoring water vapor by the help of GNSS. Troposphere products estimated from GNSS observations from the two oldest systems (GPS, GLONASS) are delivered with a latency reaching one hour. A real-time service, rather than a delayed provision of accurate troposphere products, from quad-constellation GNSS remains a goal. In addition to zenith total delay (ZTD), advanced troposphere products like horizontal gradients and slant delays gain more attention over recent years.
The main product of GNSS meteorology, the ZTD, can be assimilated into numerical weather prediction (NWP) models in order to improve weather forecasting. This is particularly important for severe weather events (heavy rainfalls, hailstorms) for which reliable prediction remains a challenge. The dynamics of troposphere gradients can reveal additional information on troposphere asymmetry, and slant delays can be used to reconstruct the three-dimensional distribution of water vapor. With low-cost GNSS receivers, the tracking network can be densified and thus the spatial density of sensing the troposphere can be increased from tens to single kilometers. This allows to observe local dynamics of water vapor and increases the accuracy of forecasts for urban areas.
Objectives of this Marie Skłodowska Curie Action (MSCA) were as follows: (a) to combine in a consistent and operational way multiple novel aspects of GNSS data processing; (b) to compute and validate a real-time GNSS-based troposphere delay model for users and to provide it online making use of open standards; (c) to check how such troposphere products support geodetic techniques and improve weather forecasting; (d) to investigate the potential and limitations of the low-cost GNSS receivers for GNSS meteorology. A parallel goal of the MSCA Individual Fellowship was to foster the development of the individual researcher, by increasing his visibility in the GNSS and meteorological communities, re-enforce his position of professional maturity in research and supervision, which will allow him to reach a promotion to achieve a fully independent academic position after this fellowship.
The project was structured in five work packages (WPs). In WP1 “Algorithms” multiple novel aspects of GNSS data processing in real-time were combined in a consistent and operational way, and implemented in the corresponding analysis software. The work carried out under this WP led to the transition to a new functional model of GNSS data processing and adjustments in the stochastic model, the exploitation of observations from all four GNSS and the estimation of troposphere gradients. In WP2 “Troposphere model” the fully scalable and highly automated real-time GNSS data processing engine (service) was established. Troposphere products for up to 161 European GNSS stations were delivered for the entire years 2019 and 2020. In WP3 “Validation” troposphere products were validated against independent data sources, i.e. a) Final ZTD products from the International GNSS Service (IGS) and analysis centers of the EUREF Permanent GNSS Network (EPN), b) analyses of global and European high-resolution numerical weather prediction models: GFS4, ICON-EU, COSMO-DE, WRF. In WP4 the applications of the real-time troposphere products for meteorology were investigated, e.g. dynamics of ZTD and gradients during a severe weather event were studied. Moreover, the feasibility of using low-cost receivers in real-time GNSS meteorology was demonstrated and the dynamics of water vapor content in the troposphere was observed at unprecedented high resolution on a city scale.
Results of this MSCA were reported under WP5: (1) four peer-reviewed scientific publications in high-impact journals, (2) eight international conference contributions from either the geodetic or meteorological communities, (3) social media reporting, (4) attendance of two workshops for early career researchers, (5) two publications in a Polish popular-science journal. Several datasets are freely available at the Zenodo repository and real-time troposphere products are submitted to the online visualization tool of the European Plate Observing System (EPOS) community. The fellow actively cooperated on the development and improvements of the service.
This MSCA has pushed the frontiers of real-time GNSS meteorology forward in numerous ways. The advanced GNSS data processing strategy has been developed, which exploits a quad-GNSS constellation, deals directly with all major signal propagation errors, provides advanced troposphere products, and is competitive with existing near real-time solutions in terms of accuracy, while also reducing the latency of products. The corresponding analysis software was developed and the service was established to process GNSS data from the European network of permanent GNSS stations. The accuracy of real-time ZTD in ReS4ToM is at the level of 5 to 8 mm, which corresponds to the accuracy of Integrated Water Vapor of 1.5 – 2.5 kg/m2.
It was also proven, that Galileo and supporting services are already mature enough to provide reliable information on the troposphere state in real-time. The combination of GPS and Galileo observations is superior to single-system processing, as it increases the accuracy and availability of troposphere products. Moreover, such a combination suppresses orbit-related artificial signals of high frequency in the ZTD time series.
Furthermore, it was demonstrated, that horizontal gradients form consistent signatures during the presence of a severe weather event. Thus, such parameters represent relevant information on troposphere asymmetry, which should be exploited further in weather forecasting.
Last but not least, the feasibility of using low-cost GNSS receivers for GNSS meteorology was also confirmed. Data from such devices leads to troposphere products, for which the accuracy is sufficient for assimilation into an NWP model. Therefore, densification of existing GNSS networks at a reduced cost is possible, thus allowing to observe local dynamics of water vapor content.
This action did not only combine a manifold of recent advancements in GNSS but also demonstrates that the transition from well-established near real-time processing to real-time processing will bring benefits for GNSS meteorology. It is anticipated, that E-GVAP analysis centers will update their processing strategies, while weather services will increase their efforts in assimilating real-time and advanced troposphere products. The demonstration of benefits for GNSS meteorology coming from low-cost GNSS receivers should convince meteorological offices and GNSS service providers to densify tracking networks. The Fellow’s PhD student is currently developing other engineering and monitoring applications of this technology.
The concept of GNSS meteorology (ReS4ToM in particular)