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The '''International Terrestrial Reference System''' ('''ITRS''') describes procedures for creating [[frame of reference|reference frame]]s suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a [[Standard (metrology)|physical standard]] might be described as a set of procedures for creating a ''realization'' of that standard. The ITRS defines a [[Geocentric coordinate system|geocentric]] system of coordinates using the [[SI]] system of measurement.
An '''International Terrestrial Reference Frame''' ('''ITRF''') is a realization of the ITRS. Its origin is at the center of mass of the whole earth including the oceans and atmosphere. New ITRF solutions are produced every few years, using the latest mathematical and surveying techniques to attempt to realize the ITRS as precisely as possible. Due to [[experimental error]], any given ITRF will differ very slightly from any other realization of the ITRF. The difference between the latest as of 2006 [[World Geodetic System|WGS 84]] and the latest ITRF
The ITRS and ITRF solutions are maintained by the [[International Earth Rotation and Reference Systems Service]] ([[IERS]]). Practical navigation systems are in general referenced to a specific ITRF solution, or to their own coordinate systems which are then referenced to an ITRF solution. For example, the ''Galileo Terrestrial Reference Frame'' (''GTRF'') is used for the [[Galileo_(satellite_navigation)|Galileo]] navigation system; currently defined as ITRF2005 by the [[European Space Agency]].<ref name=gtrf>{{cite web |title=Reference Frames in GNSS |url=https://gssc.esa.int/navipedia/index.php/Reference_Frames_in_GNSS |website=Navipedia |publisher=European Space Agency}}</ref>
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