Starting with ITRF2005 and continuing with ITRF2008, the International Terrestrial Reference
Frame (ITRF) construction used time series of station positions and Earth Orientation
Parameters (EOPs): weekly from satellite techniques (SLR, GNSS, DORIS) and sessionwise
from VLBI. Time series are essential for the ITRF to account not only for station nonlinear
motions and discontinuities but also to evaluate the stability over time of the physical
frame parameters, the origin and the scale, which are critical for Earth science applications.
The ITRF rigorous combination provides a self-consistent series of EOPs, including Polar
Motion from VLBI and satellite techniques, and Universal Time and Length of Day from VLBI
only.