Atmospheric Propagation Effects on Radio Interferometry.
Abstract
The technique of very-long-baseline interferometry (VLBI) offers geodesy the potential of estimating 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-which combines 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 quite 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. Water vapor in the lower troposphere is unmixed, and its effect on the group delay can vary from 0-20% of the effect of the dry atmosphere. The 'wet delay' is very difficult to model using measurements of surface meteorological parameters.
Document Details
- Document Type
- Technical Report
- Publication Date
- Apr 01, 1986
- Accession Number
- ADA178405
Entities
People
- J. L. Davis
Organizations
- Harvard College Observatory