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Along with polar
motion, Earth rotation, precession, and nutation, which are used to
connect terrestrial and inertial frames, modeling geocenter motion is
needed to complete the realization of the ITRF. The temporal variations
of the external gravitational field represent a dynamic aspect of the
mass redistribution within the solid-Earth/ocean/atmosphere
system. Variations in the position of the center-of-mass of this system
with respect to crust-fixed observers is a geometric aspect of the
same phenomenon. These motions are time dependent offsets of the
crust-fixed tracking network with respect to the unique center of mass
of the orbital reference system and contribute to the variance seen in
the SLR observation residuals. A theoretical prediction of the
tidally driven motion of the geocenter with respect to a crust fixed
system can be found in Brosche and Wunsch [1993]. Direct observation
of these millimeter-level effects as observed using SLR data was
discussed by Watkins and Eanes, [1993] for multiple years of Lageos
data. Vigue et al., [1992] have assessed geocenter motion
monitoring capabilities utilizing GPS.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union