At present, point positions are measured at least twice annually for 40 globally distributed VLBI antennas. The historical data base contains positions for 123 sites, with the smallest errors approximately 1 mm in horizontal, and 1-2 mm in vertical.
Vertical rates have uncertainties typically two to three times larger.
The lengthening of the baseline between Westford, MA, and Wettzell, Germany, is evidence of the gradual opening of the Atlantic ocean floor. Extragalactic objects form a true inertial reference frame because they are at such great distances that their motions across the sky are undetectable. Positions of stars in our galaxy are now tied to this reference frame, and this is the same reference frame used for measuring Earth orientation. VLBI is unique in its ability to make rapid, accurate measurements of the orientation of the terrestrial reference frame with respect to the celestial reference frame. Variations in the Earth's orientation are due to the integrated effect of all changes in the Earth's angular momentum.