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Reference systems


The ITRS (International Terrestrial Reference System) is the International Terrestrial Reference System recommended by the IERS. The ITRS' origin, orientation and scale are defined as follows:

  • Its origin is the center of mass of the Earth including oceans and atmosphere
  • Its orientation was initially given by the BIH orientation at 1984.0. The time evolution of the orientation is ensured by using a no-net-rotation condition with regards to horizontal tectonic motions over the whole Earth.
  • Its scale is the meter (SI) and is consistent with the TCG time scale

The ITRS realizations are called ITRF (International Terrestrial Reference Frame) and are regularly updated. ITRF88, ITRF89, ITRF90, ITRF91, ITRF92, ITRF93, ITRF94, ITRF96, ITRF97, ITRF2000, ITRF2005 and ITRF2008 solutions have already been published. These realizations are composed of coordinates of a group of stations distributed over the globe. These coordinates consist of a reference position and velocity, introduced from the ITRF91 (they were provided from geophysical models before this realization). Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging, DORIS orbit determination technique, and Global Satellite Navigation Systems are used to calculate them. Ground data on the relative positions of geodetic instruments, called local ties, are also included in the calculation of the ITRFs in order to determine the biases between different positions given by each measurement technique at co-location sites. A co-location site is a site where several space geodetic instruments are installed. Three-dimensional uncertainties were on the order of 2-3 mm for positions and 1 mm / year for velocities in the ITRF2005.

Site de co-localisation en 1997

Fig . Site de co-localisation en 1997

The ITRF product center is housed at LAREG. The latest realization of the International Terrestrial Reference System is the ITRF2008.
Data from ITRF94, ITRF96, ITRF97, ITRF2000, ITRF2005 and ITRF2008 are available at For additional information about ITRS and ITRF, please contact


A new on-line tool to visualize station coordinates time series of DORIS, GPS, SLR sites from the GRGS/IERS Analysis Centres is available here