Improved atmospheric mapping functions for VLBI and GPS

New mapping functions based on in situ meteorological parameters have been developed for calculating the radio path length through the atmosphere at elevations down to 3 degrees. The hydrostatic component of the mapping function is related to the geopotential height of the 200 mb isobaric pressure level above the site and provides a factor of two improvement in accuracy and precision over previous hydrostatic mapping functions at mid-latitudes. The wet component of the mapping function is calculated from the vertical profile of wet refractivity at the site but will provide an improvement of only about twenty-five percent. However, since the effect of known errors in the hydrostatic mapping function dominates that from the wet component, except near the equator, implementation of these mapping functions should reduce the contribution of the atmosphere to errors in estimates by VLBI and GPS of both the vertical component of site position and the radio propagation delay due to water vapor in the atmosphere.