Comparison of Wet Troposphere Calibration Techniques and Their Effects on VLB I Geodetic Measurements

Abstract

The water vapor in the troposphere is currently the largest source of error in VLBI geodetic measurements. Scientists have been working on this problem for over 10 years. Major improvements have been made. This report presents a direct comparison of three techniques for measuring the water vapor content of the troposphere, and then a look at some of their effects on the final VLBI baseline solutions. The three tropospheric calibration techniques consist of one which uses surface meteorology data as input and two which use water vapor radiometer data as input -- one of which incorporates a time dependent instrumental gain adjustment as well as other modifications. It was found that the water vapor radiometer model with the time dependent gain adjustment yields the most physically reasonable interpretation of troposphereic water vapor. On the final VLBI baseline solutions, water vapor radiometers present a substantial improvement over surface meteorological methods in accuracy and repeatability, but surface meteorological models can be improved by incorporating more estimation in the final fitting process.