Guillermo Delgado (Onsala Space Observatory, European Southern Observatory), Angel Otarola , Victor Belitsky , Denis Urbain 
August 24, 1999
Keywords: Water Vapor, 183 GHz, Opacity
At millimetre wave lengths the radio seeing is severely limited by the inhomogeneous distribution of water vapour. This is a fundamental limit of the resolution of terrestrial based millimetre wave interferometers. One way of overcoming this limitation is by using radiometric measurements of the brightness temperature of the sky to estimate the water vapour content of the atmosphere thus correcting the phase fluctuation in the observed wave front.
Two radiometers to observe the 183 GHz water vapour line have been installed by the Onsala Space Observatory, Sweden, at Llano de Chajnantor, northern Chile. This as part of the site testing work carried out by the European Southern Observatory in collaboration with the National Radio Astronomy Observatory (NRAO) and other European organisations in the frame of the Atacama Large Millimeter Array project (ALMA).
Here we present a description of the radiometers and their stability. Also we show a method to estimate the amount of precipitable water vapour (PWV) from antenna brightness temperature measurements at three frequencies close to the water line transition at 183.31 GHz. The data is fitted to a simple atmospheric water line emission model, starting from ground-level weather information. Data has been collected and analysed continuously, starting September 1998 at Llano de Chajnantor.
The value of PWV obtained with the 183 GHz radiometer was correlated with radiosonde measurements and atmospheric opacity measured at 225 GHz.
The correlation with opacity measurements at 225 GHz is very good, giving confidence in the estimated value of PWV. In the case of radiosonde measurements, a systematic error in the determination of relative humidity (RH) by the radiosonde detectors was observed. More data is expected from the radiosonde manufacturer to further proceed with the analysis.
 European Southern Observatory
 Onsala Space Observatory
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