Radiometric Correction of Anomalous Refraction

We propose a scheme to measure and correct anomalous refraction effects for millimeter wave antennas. A radiometer tuned to the 183 GHz atmospheric water vapor line scans the atmosphere above the aperture of an antenna to determine the spatial derivative of the refractive index. In an array of antennas, such as the MMA, it would be used simultaneously to measure the phase delay correction for the antenna. The radiometer feed illuminates an area half the aperture diameter and is scanned by a rotating prism. As well as being cheaper than using three or four radiometers, a single radiometer avoids many of the problems due to systematic differences that would arise with separate receivers. On the Chajnantor site, where anomalous refraction conditions are not correlated with high opacity significant amounts of time could benefit from such correction.