VLBI plays an indispensable role in maintaining the International Terrestrial
Reference System, the world standard for defining three dimensional positions
(latitude, longitude, and height) on Earth.
Land surveying and navigation require a geodetic reference system, a combination of ground-based control points and a set of site coordinates of those points. With the introduction of space geodetic techniques such as VLBI and GPS that can measure the Earth from space, the world common geodetic reference system becomes reality for the first time.
In Japan, as of April 2002, the century old
Tokyo Datum was legally superseded by a
brand new one, the Japanese Geodetic
Datum 2000 (JGD2000), which is a local
realization of International Terrestrial
Reference System.
In this transition, triangulation points and GPS-based control points|the
base of all kinds of surveys in Japan|had new latitudes and longitudes
calculated from a VLBI station position that had been already determined
with long-term repeated international VLBI observations.
Currently GSI performs VLBI observations regularly at its four VLBI stations
in Japan including the Tsukuba station. The results are used to determine
the Japan's geographical position on a global basis and also for other
geodetic activities such as maintaining "JGD 2000", monitoring
the accuracy of GPS-based control points, detecting crustal deformations.
