SIMBAD references

The celestial reference system of the International Earth Rotation Service (IERS), ICRS, was recommended to the International Astronomical Union (IAU) for adoption as the conventional system, under the name of International Celestial Reference System, by the IAU working group on reference frames. This system agrees with the decisions taken by IAU in 1991 and it is consistent with the FK5 system at J2000.0. It is maintained on the basis of high-accuracy observations of extragalactic radio sources by very long baseline interferometry (VLBI). The maintenance algorithm ensures stable directions of its axes within ±20 microarcseconds. The typical accuracy of the source coordinates is ±300 microarcseconds. ICRS is accessible directly by the observation of extragalactic objects and indirectly through the major reference frames attached to the Galaxy, the Solar System and the Earth.