Atmospheric VLBI: A method to validate long time series of water vapour content

G. Elgered, and R. Haas

Department of Radio and Space Science, Chalmers Univerity of Technology, 
Onsala Space Observatory, SE-439 92 Onsala, Sweden

Oral presentation

The possibility to infer the atmospheric water vapour content from space geodetic observations has been demonstrated, with increasing quality, during the last twenty years. First by using geodetic VLBI and thereafter based on GPS. Ground-based GPS receiver stations provide a relatively high spatial resolution and results in close to real time, at least compared to VLBI. Therefore, the water vapour estimates from GPS networks are nowadays used, and their impact assessed, in assimilation into numerical models for weather prediction. Another application, more and more attractive as the time series become longer, is to validate climate models as well as to monitor and search for trends in the water vapour content. The absolute accuracy, and especially the long term stability of the GPS results is then of fundamental importance. In order to assess the stability an independent, preferably more stable and accurate method is called for. We here investigate to what extent VLBI is an appropriate method for this task. For this purpose we use the water vapour time series at the Onsala Space Observatory estimated from geodetic VLBI, microwave radiometer, and radiosonde data since 1980. The time series of the water vapour content inferred from GPS observations starts in 1993.