Memo 209 : 183 GHz radiometric phase correction for theMillimeter

Radiometry can be used to correct the wavefront distortions of millimeter and submillimeter signals introduced by the irregular distribution of water vapor in the Earth's troposphere. By measuring the emission due to water vapor along a line of sight it is possible to estimate the corresponding electrical path length. A correction system based on the strong water vapor transition at 183 GHz is considered. Simulations of the line proole show that this transition is at least partially saturated under even the driest conditions expected at the Chajnantor site in Chile. Potential errors in the radiometric correction process, including gain uctuations, uncertainty in the altitude of the water vapor uctuations, clouds, temperature uctuations and spillover are analyzed. It is shown that the line saturation is actually beneecial, as it allows the spectral signature of a water vapor uctuation to be discriminated from the various error components. Three or more channels are needed for this, but only moderate gain stability (1%) is required, greatly easing the level of calibration necessary. Correcting the path to 50 m is much easier to achieve when the column of water vapor is low (1 mm of Precipitable Water Vapor) than when it is high (PWV > 4 mm); the latter case requires a cooled radiometer to achieve the necessary sensitivity. Various practical implementations are also considered, including the relative beneets of cooled and un-cooled systems, and the use of dedicated radiometers versus the astronomical receivers for measuring the water line.