RADIOMETRIC MONITORING OF ATMOSPHERIC WATER-VAPOR AS IT PERTAINS TO PHASE CORRECTION IN MILLIMETER INTERFEROMETRY

Water vapor in the Earth's troposphere produces fluctuations in the ph ase of millimeter-wavelength radiation from astronomical sources. Such fluctuations seriously limit the spatial resolution achievable with c urrent millimeter interferometers. Since water vapor is also a source of atmospheric opacity at these wavelengths, radiometric measurements of sky brightness may be used to monitor the fluctuating water vapor c ontent of the atmosphere and thereby the fluctuations in the interfero metric phase. The atmospheric opacity depends on the frequency and on the physical conditions of those atmospheric regions in which the wate r vapor is located. Atmospheric temperature influences the strengths o f the various absorption lines, and pressure influences the degree of line broadening. The magnitude of the phase fluctuations relative to t he brightness fluctuations is therefore also dependent on frequency, t emperature, and pressure. The frequency of a radiometric monitoring sy stem may be chosen to minimize the dependence of this ratio on the atm ospheric parameters.