The International Terrestrial Reference System (ITRS) constitutes a set of prescriptions and conventions together with the modelling required to define origin, scale, orientation and time evolution of a Conventional Terrestrial Reference System (CTRS). The ITRS is an ideal reference system, as defined by the IUGG resolution No. 2 adopted in Vienna, 1991 ( IUGG1991 Resolutions (PDF, 105KB, Not barrier-free file.)). The system is realised by the International Terrestrial Reference Frame (ITRF) based upon estimated coordinates and velocities of a set of stations observed by VLBI, LLR, GPS, SLR, and DORIS. The ITRS can be connected to the International Celestial Reference System (ICRS) by use of the IERS Earth Orientation Parameters (EOP).

Contents	Definitions of origin, scale, orientation, and time evolution for the realization of a CTRS:
	Origin:	The center of mass being defined for the whole earth, including oceans and atmosphere.
	Scale:	The unit of length is the metre (SI).
	Orientation:	Initially given by the BIH orientation at 1984.0
	Time evolution:	The time evolution of the orientation is ensured by using a no-net-rotation condition with regard to horizontal tectonic motions over the whole earth.
Accuracy	/
Updates	/
Public access	https://itrf.ign.fr/en/homepage
Format	/
Documentation	IERS Technical Note No. 36 ( IERS Conventions (2010) ).
Product Centre	IERS ITRS Centre