3 MONTHS OF CONTINUOUS MONITORING OF ATMOSPHERIC WATER-VAPOR WITH A NETWORK OF GLOBAL POSITIONING SYSTEM RECEIVERS

Three months of continuous data from the Global Positioning System (GP S) using 20 sites in Sweden and 5 sites in Finland have been used to e stimate the integrated amount of atmospheric water vapor. The quality of the data has been assessed by comparisons with a microwave radiomet er (water vapor radiometer (WVR)) at the Onsala Space Observatory and with data from four different radiosonde stations. We found the agreem ent in integrated water vapor (IWV) between the GPS estimates and the radiometer data to be 1-2 kg/m(2) in terms of daily root-mean-square ( rms) differences. A major part of these rms differences were caused by a bias between the data sets. This bias (WVR-GPS) varied from day to day between -1.0 and +2.5 kg/m(2) with a mean value of +1.3 kg/m(2) Co mparisons with radiosonde data showed rms differences around or slight ly above 2 kg/m(2) for each station using the entire 3 month data set. Also here the GPS estimates were, on the average, below the radiosond e results. We show that the radomes used to protect the GPS antennas a re likely to cause a large part of the observed bias. Spatial structur e functions were calculated by using the GPS and the radiosonde data. An overall consistency between the GPS-based and the radiosonde-based structure functions indicates that the spatial correlations between th e GPS estimates are not affected by the estimation process used in the GPS data analysis.