Recent developments in GNSS data processing have allowed the estimation of ZTD with a high degree of accuracy using continuously operating GNSS networks. From this delay, integrated water vapour can be derived by means of additional meteorological information, in particular observed pressure or numerical weather prediction model pressure.

E-GVAP (The EUMETNET GPS Water Vapour Programme) is a inter-European project tasked with taking GPS meteorology in Europe to an operational status and to deliver high quality GPS ZTD observations to the meteorological community in Europe. Within the project, started in April 2005, European national met offices are in collaboration with geodetic institutes and GNSS network providers. Today UK Met Office and Meteo France uses Near-Real Time (NRT) GPS ZTD in their operational forecasts, many other European met offices plan to start utilizing them in their operations.

Currently 12 E-GVAP Analysis Centres are processing raw GNSS data and sending NRT ZTD products hourly to a common ftp-server at the UK Met Office: ASI (Italy), BKG (Germany), GFZ (Germany), GOP (Czech Republic), IGE(Spain), KNMI (Netherlands), LTP (Switzerland), METO (UK), NGAA (Sweden), ROB (Belgium), and SGN (France).

ASI/CGS is an E-GVAP Analysis Center but it is also working as an E-GVAP Combination Center. The number of operational GNSS sites used in E-GVAP has steadily increased to more than 1000 sites. Of these about 200 are analyzed by 3 Analysis Centres. A combined solution is a ZTD estimate made by a statistical combination of a number (at least 3) of individual ZTD estimates. For details refer to: Combination Methods of Tropospheric Time Series, R.Pacione, B. Pace, S. de Haan; H.Vedel, F. Vespe, Adv. Space Res., 47(2), 323-335, 2011, Doi: 10.1016/j.asr.2010.07.021

Combining different solutions of the atmospheric signal is a good way to detect problems in a single ACs and to provide a quality indicator for each solution. In addition comparisons with other independent techniques, as radiosonde observations or numerical weather prediction model, must be performed to evaluate the quality of atmospheric parameters directly estimated or retrieved from the GPS system.

1) ASI Combined Network Status

2) NRT GPS Zenith Total Delay Time Series

3) NRT GPS Quality Control Evaluation

4) Integrated Water Vapour Field