The models that define the geocentric reference system along with the station coordinates and velocities used to realise the International Terrestrial Reference Frame currently give the best representation of the solid Earth and its time variations on a global scale. The coordinates of the reference frame are based on combinations of several geodetic space techniques including VLBI, SLR, DORIS and GPS. The ITRF reference system is global and as such is not fixed to any specific tectonic plate. Therefore, coordinates defined in ITRF will change over time due to the displacement of the plates and possibly to some intra-plate motions. It is for that reason that the ITRF frame defined in terms of both coordinates and velocities is referenced to a specific epoch of time.

The specific ITRF frame used by PPP is the one realised by IGS at the epoch for which the precise GPS orbits were computed. With the production of orbit estimates on a daily basis, the ITRF realisation epoch will always be within a day of the submitted GPS data.

International Terrestrial Reference Frame (ITRF) Definition

ITRF2000 Definition