It reports on years of pinpointing quasars and radio galaxies hundreds of millions of light-years away and more, so far that they have no measurable motions across the line of sight. Narrow-band gigahertz signals received are taped at the same time with local atomic clocks, and the radio wave crests and troughs from each source at each station are centrally compared, even by the billion, to seek a best fit for relative delays and clock rate. Some 200 sources define the radio benchmarks for the new sky map. The Hipparcos starlight positions have been painstakingly fitted into this International Celestial Reference Frame , which also incorporates the necessary earth-motion data from the decades of work on the Katalogs. There is still nothing ultimate about this narrative.
Ambitious plans abound for space mapping to come.
Perhaps the limits will be realized at a few microarc seconds, because electromagnetic waves bend to the tiny repeated gravitational pulls of the lumpy distribution of dark and light mass as the waves thread their way to us.