VLBI is an acronym standing for Very Long Baseline Interferometry. VLBI is a form of radio astronomy that combines the signals from two widely separated radio receivers to obtain very high resolution images. The technique is used in geodesy to compute distances between receiver antennas to very good accuracy, typically 2 cm or less over distances of thousands of km.
VLBI is one of the primary measurement techniques that is used to establish, or "realize", geodetic datums. The NAD83 datum is tied to the international terrestrial reference frame (ITRF) using the location of about a dozen North American VLBI antennas.