Atmospheric propagation effects on radio interferometry

Very-long-baseline interferometry (VLBI) offers estimates of distances on the Earth of several thousand kilometers with uncertainties of a few centimeters or less. Since the completion of the Mark III VLBI system, combining group-delay measurements at two widely separated frequency bands to estimate ionospheric refraction, the source of error that limits this accuracy has been thought to be refraction by the neutral atmosphere. The largest component of the radio refractive index of air is due to the dry atmosphere. The integrated effect of this component can be estimated accurately from the surface pressure for a signal arriving at the site from the zenith direction. For a signal arriving from other directions, a model for the atmosphere must be combined with surface meteorological measurements to estimate the propagation delay. The accuracy of this estimation is limited by the accuracy of the atmospheric model. The remaining component of the radio refractive index of air is due to water vapor, which in the lower troposphere is unmixed, and its effect on the group delay can vary from 0 to 20% of the effect of the dry atmosphere. The wet delay is very difficult to model using surface meteorological parameters.