ATMOSPHERIC GRADIENTS AND THE VLBI TERRESTRIAL AND CELESTIAL REFERENCE FRAMES

Gradients in the atmospheric refractive index can lead to errors in es timated vertical and horizontal station coordinates. These errors prod uce systematic errors in the terrestrial and celestial reference frame s determined from our very long baseline interferometry (VLBI) measure ments. Estimation of gradients for our global VLBI data set changes th e terrestrial reference frame length scale by -0.7 ppb and produces st ation position adjustments that vary approximately monotonically with latitude. Estimating gradients reduces the radio source declinations b y an amount that peaks at about 0.5 mas near the equator and decreases toward the poles. VLBI gradient estimates are consistent with gradien ts derived from a global three-dimensional model of assimilated meteor ological data. Both indicate that mean atmospheric delay gradients poi nt toward the equator in both the northern and southern hemispheres. T he correlation coefficient between VLBI and meteorological model gradi ents for VLBI sessions for the VLBI antenna at Westford, Massachusetts was 0.56.