VLBI (Very Long Baseline Interferometry), the process of using extragalactic
radio sources, such as quasars, to pinpoint changes in the Earth's
orientation to inertial space is a tremendously helpful tool.
However, as precise as VLBI is in recording and documenting the Earths orientation
to points far outside the solor system, VLBI does not account for motion of the solar system frame nor does it determine the "causes" for the change
in orientation. Rather the official NASA VLBI website recognizes two
historically accepted categories of causes for such a change and then
relies on the geoscientists to model the data.
Here is a quote from the NASA VLBI website:
in the Earth's orientation in inertial space have two causes: the
gravitational forces of the Sun and Moon and the redistribution of
total angular momentum among the solid Earth, ocean, and atmosphere.
VLBI makes a direct measurement of the Earth's orientation in space
from which geoscientists then model such phenomena as atmospheric
angular momentum, ocean tides and currents, and the elastic response
of the solid Earth."
Essentially, VLBI makes the measurements and geoscientists build the model. The problem with this methodology is that all the focus is on local dynamical causes (the work of the geoscientists) and none on reference frame or measurement issues (nobody's job). It is assumed that the earth's changing orientation to points "inside" the solar system (local measurements) must be the same as changes in the earth's orientation to points "outside" the moving solar system - but there is no evidence of this. In a circular argument: the static SS model constrains geoscientists to explain all changes in earth orientation through local dynamics, and because all changes in earth orientation are explained through local dynamics, the SS is constrained to zero motion (for EOP purposes).
The IAU recently adopted a new reference frame that uses a set of 295 quasars to define celestial positions (effective 1-1-2010, IAU News), meaning astronomers can now even more accurately determine the positions and motions of objects in the sky. However no amount of interferometry can tell us how much the SS curves through space unless measurements from "inside" the moving frame (where our radio telescopes are located) to points "outside" the moving frame (where the reference points are located) account for the motion of the SS. Present VLBI methodology does not account for the differing frame.