Very Long Baseline Interferometry
Geodetic Very Long Baseline Interferometry (VLBI) observations provided the first direct confirmation of tectonic plate motion at the end of the 1980s. Now VLBI observations measure the motions of stations with accuracy better than 1mm/year together with other space geodetic techniques such as Global Positioning System (GPS) and Satellite Laser Ranging (SLR) analysis.
VLBI observations to stable distant quasars are an important component for the establishment of a reference system of coordinates in the sky - defining the Celestial Reference Frame.
The radio system (positions derived from radio observations to quasars) has replaced the traditional optical reference system based on star positions. The optical system which was used for the past 200 years had an average accuracy (of star positions) to about 10 milliarcseconds (equivalent to 0".01).
The current average accuracy of quasar positions observed by radiosystems is about 0.1-0.2 milliarcseconds (50-100 times better); therefore the International Astronomical Union has recommended using radiosystems for the definition of The International Celestial Reference Frame.