Retrieval of atmospheric water vapor by geodetic VLBI

The talk gives an overview about the retrieval of atmospheric water vapor by geodetic VLBI observations within the International VLBI Service for Geodesy and Astrometry (IVS). The models used to obtain zenith wet delays and finally integrated water vapor content are described and the role of in-situ meteorological data in this context is introduced. The VLBI derived zenith wet delay time series are compared with those obtained by other space geodetic techniques, such as the GNSS (Global Navigation Satellite Systems) for which we use results provided by the IGS (International GNSS Service), and also with zenith wet delays based on data of the European Centre for Medium-Range Weather Forecasts (ECMWF). The time series are discussed in terms of spatial and temporal availability, and the quality of the derived zenith wet delays is assessed by comparison between the techniques. Long and homogeneously processed time series of zenith wet delays provide important information for the determination of climate trends and for the validation of numerical weather models or radiosonde observations.