Definitions

IERS is the current structure in charge to define, realize and promote the International Terrestrial Reference System (ITRS) as defined by the IUGG resolution No 2 adopted in Vienna, 1991 (Geodesist's Handbook, 1992), providing the definition of a CTRS which is the basis of the ITRS. The resolution recommends the following definitions of the CTRS:

1. CTRS to be defined from a geocentric non-rotating system by a spatial rotation leading to a quasi-Cartesian system,
2. the geocentric non-rotating system to be identical to the Geocentric Reference System (GRS) as defined in the IAU resolutions,
3. the coordinate-time of the CTRS as well as the GRS to be the Geocentric Coordinate Time (TCG),
4. the origin of the system to be geocenter of the Earth's masses including oceans and atmosphere, and
5. the system to have no global residual rotation with respect to horizontal motions at the Earth's surface.

The ITRS definition fulfills the following conditions:

• it is geocentric, the center of mass being defined for the whole Earth, including oceans and atmosphere.
  
• the unit of length is the meter (SI). This scale is consistent with the TCG time coordinate for a geocentric local frame, in agreement with IAU and IUGG (1991) resolutions. This is obtained by appropriate relativistic modelling.
  
• its orientation was initially given by the Bureau International de l'Heure (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.