Fluctuating amounts of water vapor over an antenna in an interferometer add phase shifts to the instrument, shifting the fringe pattern on the sky to decrease signal correlation. Optically thin line radiation from the same water vapor that causes the decorrelation can be accurately measured, however, allowing an estimate of the column of water above individual antennas. Knowledge of the water column allows an equivalent but opposite phase to be inserted in the data processing; this is the radio equivalent of adaptive optics. This memo explores the instrumental requirements for a multi-channel radiometer capable of measuring pathlength differences to 35 m, a goal for interferometry at 1 mm wavelength. An examination of linear and nonlinear error sources shows that residual amplifier or detector nonlinearity is likely to be the most significant instrumental limit for atmospheric phase correction.
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Last modified: May 04, 2000
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