Atmospheric Pressure Loading Service for VLBI and SLR

Time series of 3D site displacements caused by atmospheric pressure loading are computed for all VLBI and SLR sites from May 1976 using 6-hourly pressure field with a spatial resolution of 2.5x2.5 degrees from NCEP Reanalysis. Atmospheric pressure tides are removed from the NCEP Reanalysis data. Loading due to atmospheric tides is computed separately using Ponte-Ray (2002) model. These series are automatically updated on a daily basis. They are available on the Web at http://gemini.gsfc.nasa.gov/aplo . We have validated our model of atmospheric pressure loading by estimating the admittance factors of the pressure loading time series using the data set of 3.5 millions of VLBI observations. These admittance factors can be interpreted as correlation coefficients between the true (unknown) site displacements and our model. The average admittance factors are 0.95 -+ 0.02 for vertical displacement and 1.00 -+ 0.07 for the horizontal displacements. Closeness of these admittance factors to unity allows us to conclude that our model is adequate at the level of measurements noise.