E-GVAP: The EUMETNET EIG GNSS water vapour programme

E-GVAP and ground based GNSS meteorology, basics

E-GVAP was set up, in April 2005, to provide its EUMETNET members with European GNSS delay and water vapour estimates for operational meteorology in near real-time.

The NRT GNSS delay data contain information about the amount of water vapour above the GNSS sites. Water vapour plays a key role in some of the most important weather phenomena: It is obviously related to precipitation, but also provides about half the energy to the atmosphere (via latent heat release), contributing to atmospheric dynamics, and it is the dominant greenhouse gas. There is a big lack of humidity observations in the meteorological observing system, usage of ground based GNSS data is one means by which to improve on this.

The vast majority of high quality permanent GNSS sites are installed for positioning purposes, by geodetic institutions and private firms. To them the atmospheric delay is a noise term. The core of E-GVAP is a close collaboration between geodesy and meteorology. Raw data from GNSS sites are collected by a number (more than 10) GNSS analysis centres, which process the data to estimate ZTDs (and other parameters). The ZTDs are then forwarded to a dataserver, for distribution to meteorological institutes. And for quality control and verification. E-GVAP contributes meteorological data, that can be used to validation GNSS delay estimation, and to improve GNSS positioning in the future.

For the moment the E-GVAP network consists of more than 1500 GNSS sites. Mainly in Europe, but recently processing and distribution of global GNSS data has started, since many E-GVAP members run global NWP models. For the same reason E-GVAP welcomes collaboration with both European and non European institutions, in order to densify the GNSS meteorological observing network.

Usage of ground based GNSS delay data in NWP has proven to improve forecast skill.