VLBI for climate studies

Goal of the project is the determination of reliable and highly consistent time series of tropospheric parameters e.g., of the wet zenith delays, for globally distributed VLBI (Very Long Baseline Interferometry) stations with a long history of observations. For the accurate derivation of long time series from VLBI data in the first part of the project, special care has to be taken - of the mapping functions and the cutoff elevation angle, - of the time intervals for the estimation of the parameters and - of the treatment of the terrestrial reference frame. In particular the latter is important for the determination of accurate zenith delays due to the relatively high correlation with the station heights. The second part of the project covers the comparison and combination with time series from other IVS (International VLBI Service for Geodesy and Astrometry) Analysis Centers which submit their estimates to the Institute of Geodesy and Geophysics (IGG) in Vienna. The combined long time series are used to derive linear trends, possible non-linear changes as well as periodical variations by means of Fourier and wavelet analyses. Additionally, the tropospheric parameters determined from VLBI are compared and combined with those from the GPS (Global Positioning System), water vapour radiometer (WVR) and other techniques. The combination with estimates from the IGS (International GPS Service) is also performed rigorously i.e., based on the normal equations provided in SINEX files. Finally, the long time series are used for climate studies. The wet zenith delays which correspond to values of precipitable water, provide valuable information about the content of water vapour above the VLBI sites. Thus, the long time series imply information about the development of the most important greenhouse gas: water vapour.

Goal of the project is the determination of reliable and highly consistent time series of tropospheric parameters e.g., of the wet zenith delays, for globally distributed VLBI (Very Long Baseline Interferometry) stations with a long history of observations. For the accurate derivation of long time series from VLBI data in the first part of the project, special care has to be taken of the mapping functions and the cutoff elevation angle, of the time intervals for the estimation of the parameters and of the treatment of the terrestrial reference frame. In particular the latter is important for the determination of accurate zenith delays due to the relatively high correlation with the station heights. The second part of the project covers the comparison and combination with time series from other IVS (International VLBI Service for Geodesy and Astrometry) Analysis Centers which submit their estimates to the Institute of Geodesy and Geophysics (IGG) in Vienna. The combined long time series are used to derive linear trends, possible non-linear changes as well as periodical variations by means of Fourier and wavelet analyses. Additionally, the tropospheric parameters determined from VLBI are compared and combined with those from the GPS (Global Positioning System), water vapour radiometer (WVR) and other techniques. The combination with estimates from the IGS (International GPS Service) is also performed rigorously i.e., based on the normal equations provided in SINEX files. Finally, the long time series are used for climate studies. The wet zenith delays which correspond to values of precipitable water, provide valuable information about the content of water vapour above the VLBI sites. Thus, the long time series imply information about the development of the most important greenhouse gas: water vapour.