Uncertainty in the velocity between the mass center and surface of Earth

Using spectral analysis and data decimation, we estimate the uncertainty in the velocity between the cumulative mass center of Earth (CM) and geodetic sites on Earth's surface. Knowing this velocity is crucial for evaluating space geodetic observations of continental uplift and subsidence in terms of postglacial rebound and sea level rise. We find SLR observations of satellite LAGEOS to constrain the X and Y components of the velocity of CM to +/- 0.4 mm/yr and the Z component to +/- 0.9 mm/yr. (95% confidence limits, X is in the direction of 0 degrees N 0 degrees E, Y of 0 degrees N 90 degrees E, and Z of 90 degrees N.) The uncertainty in Z is high, so that the estimate includes the independent inference made jointly using site velocities, the rigid plate hypothesis, and models of postglacial rebound that the true velocity of CM has a Z component of 0.5-1.0 mm/yr relative to that in ITRF2008. Uncertainty in scale rate, an intermediate parameter in the determination of an ITRF, is +/- 0.36 mm/yr for VLBI, +/- 0.52 mm/yr for SLR, and +/- 0.20 mm/yr for GPS. The scale of GPS depends on that of VLBI and SLR, but the low GPS uncertainty indicates that GPS results are, for the first time, unbiased by changing satellite Block types, evidently due to newly incorporated satellite phase center variations. GPS constrains the velocity of CM nearly as well as SLR, representing a technical advance given that a GPS satellite is not a sphere and responds strongly to solar radiation pressure.