VLBI measures the time differences in the arrival of microwave signals from extragalactic radio sources received at two or more radio observatories. Generally, geodetic observing sessions run for 24 hours and observe a number of different radio sources distributed across the sky. The observatories can be widely separated; the sensitivity of the observations to variations in the orientation of the Earth increases with the size of the VLBI network.
To date, geocentric coordinates have been measured for about 120 sites, both fixed and "mobile", using the bandwidth synthesis Mark-III (or equivalent) technique. More than half of these have sufficiently long histories that reliable three-dimensional velocity estimates are also available.
Likewise, celestial coordinates have been determined for more than 600 radio sources. These observations are used to maintain the International Celestial Reference System (ICRS).
VLBI is the only technique capable of measuring all components of the Earth's orientation accurately and simultaneously.
Currently, VLBI determinations of earth-rotation variations, and of the coordinates of terrestrial sites and celestial objects are made routinely and regularly with estimated accuracies of about ±0.2 milliarcsecond or better.