We should also mention the star triples and greater multiplicity increase the density of reference frame for monitoring objects fainter than 20th magnitude in narrow fields of view of large telescopes, when the accuracy of the optical realization of ICRF using only objects of GAIA will be insufficient ensuring astrometric observations of variable objects of large amplitude of brightness, in particular the systematic search of optical transients. Two obvious areas were discussed in detail here: objects in the Solar system moving along complex trajectories and weak quasars. Billion objects brighter 20m will be observed of about 100 times, coordinate precision weakest of them will be worse than 100 microarcseconds. Please note that not all of them are yet mentioned during this discussion. Parameters of the experiment GAIA help clearly delineate the astrometric applications of ground observations. When you add in that many asteroids are fainter than 20th magnitude (the Gaia limit) much or all of the time, that small Near Earth asteroids will have perceptible Yarkovsky accelerations, and that Gaia will be puzzled about some objects (e.g., fast moving new asteroids or long-period binary stars), there will still be lots for ground based observers to do. If ground-based asteroid observations can be brought down to 10 mas, then Gaia is not beyond competition (as 100's of less accurate observations could be acquired in the time between Gaia scans). Gaia will also not be better than 1 mas for many asteroids, as it is most accurate for ~12th magnitude, and an asteroid that bright will be resolved. (In other words, asteroid data can fit to 10 mas in one night, but when many nights are added together, and especially when the asteroids move so far the reference stars change, the residual fit goes up by a factor of 10 or more.) Gaia will improve than considerably. Modern ground based astrometry for unresolved asteroids (ones smaller than the telescope's resolution) are good to ~ 10 milli arcseconds (mas) (yes), except that errors in the background stars and the timing push their errors up to > 100 mas. This will especially useful with asteroids. In fact, ground based astrometry will get considerably better, including retroactively, thanks to Gaia, as the background reference frame will become much better.
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