DETECTION OF ATMOSPHERIC-PRESSURE LOADING USING VERY LONG-BASE-LINE INTERFEROMETRY MEASUREMENTS

Loading of the Earth by the temporal redistribution of global atmosphe ric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates of the se displacements are determined by convolving National Meteorological Center (NMC) global values of atmospheric surface pressure with Farrel l's elastic Green's functions. An analysis of the distances between ra dio telescopes determined by very long baseline interferometry (VLBI) between 1984 and 1992 reveals that in many of the cases studied there is a significant contribution to baseline length change due to atmosph eric pressure loading. Our analysis covers intersite distances of betw een 1000 and 10,000 km and is restricted Lo those baselines measured m ore than 100 times. Accounting for the load effects (after first remov ing a best fit slope) reduces the weighted root-mean-square (WRMS) sca tter of the baseline length residuals on 11 of the 22 baselines invest igated. The slight degradation observed in the WRMS scatter on the rem aining baselines is largely consistent with the expected statistical f luctuations when a small correction is applied to a data set having a much larger random noise. The results from all baselines are consisten t with approximately 60% of the computed pressure contribution being p resent in the VLBI length determinations. Site dependent coefficients determined by fitting local pressure to the theoretical radial displac ement are found to reproduce the deformation caused by the regional pr essure to within 25% for most inland sites. The coefficients are less reliable at near coastal and island stations.