The term fundamental station is used for special observatories which combine several space positioning techniques like VLBI, satellite laser ranging, GPS, Glonass, etc.
They are the basis of plate tectonic analysis, allowing the monitoring of continental drift rates with millimetre accuracies.
Important fundamental stations include: Worldwide about 30 fundamental stations are in existence: about 5 in the United States and in Commonwealth of Independent States countries, and 2–3 in South America, Africa, Eastern Asia, Australia and Antarctica.
[citation needed] The basic coordinate system is the ITRF reference frame, which is related to the ICRS celestial inertial system by means of very precise Earth Orientation Parameters (EOPs), containing polar coordinates, Earth rotation and nutation parameters.
The ITRF data set is revised every 3–5 years, the actual accuracy is in the millimeter area.