This is interesting … Wiki gives a
nice concise list of what results the VLBLI network has achieved (somewhat more concise than my previous post, too
)
Amongst these is the maintenance of the definition of the 'celestial reference frame'. This is done by measuring the positions of 212 extragalactic sources, (mostly quasars), which then gives the effective centre of mass of the Solar System.
This is then taken as the closest we can get to a 'true' inertial reference frame for defining the positions of all of the planets and other astronomical objects.
Wiki calls this a 'quasi inertial reference frame'. They say that Relativity implies there is no 'true' inertial reference frame although, I'm not sure this is accurate. I would have thought that this had more to do with the expansion of the universe and the Cosmological Principle, than with Relativity?
Interesting … this would seem to be the fundamental origin of all navigational and astronomical measurements/co-ordinates.
Cheers