AircraftNews.Com

Probably not many of us lie awake nights pondering how we can be sure our GPS positions are firmly related to reality. It just is enormously reassuring to have that machine telling us where we are. Because it is presented via flashing lights and after much button pushing we are inclined to take its accuracy for granted. Well mostly it is very accurate and a marvel of technology it is too. But when one stops to consider there are some technical difficulties which have been very ingeniously addressed. When you are a photon of electromagnetic radiation or a molecule vibrating in space and time as you go about your business, the Earth, rather than being a solid stable reference point, is a shifting plate of vibrating Blancmange. Not even once does a position on or in the earth trace out the same path. The surface of the Earth is perpetually vibrating and quivering and the centre of rotation of the earth is never the same from one moment to the next when one gets close to serious accuracy. Tides, thermal expansion, Nutation, the influence of the moon, relativistic effects due to the alterations in the earths orbital speed all mean we have to look outside the solar system for a base line to refer orbital motions to in order that we have a base line to reference our measuments to. First it was a collection of fixed stars but they too move. Now there is a new and improved base line. Called the ICRF2, this new map uses observations of about 3,000 quasars with the Very Long Baseline Interferometer (VLBI). Sort of does put it all into perspective.

The VLBI is a special network of radio telescopes from all over the world that together have the resolving power of a telescope thousands of miles in diameter. (Though quasars burn intensely brightly, their enormous distance makes them too faint to locate accurately with a conventional optical telescope.)

The new quasar observations were able to reduce the uncertainties in the quasar positions to angles as small as 40 microarcseconds — about the thickness of a 0.7-millimeter mechanical pencil lead in Los Angeles as viewed from Washington, D.C. This uncertainty is five times better than the original ICRF.
http://www.livescience.com/space/091103-st-gps-quasars.html