The International Celestial Reference Frame (ICRF) is a quasi-inertial frame of reference centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources (mainly quasars). Although general relativity implies that there are no true inertial frames around gravitating bodies, the ICRF is important because it definitely does not exhibit any measurable angular motion since the extragalactic sources used to define the ICRF are so far away. The ICRF is now the standard reference frame used to define the positions of the planets (including the Earth) and other astronomical objects. It has been adopted by International Astronomical Union since 1 January 1998. ICRF had a noise floor of approximately 250 microarcseconds (µas) and an axis stability of approximately 20 µas; this was an order-of-magnitude improvement over the previous Fifth Fundamental Catalog (FK5).
The ICRF also contains positions of 396 additional non-defining sources for reference. The positions of these sources have been adjusted in later extensions to the catalogue.
Note that, in astrometry, a reference frame is the physical realization of a reference system, i.e. the reference frame is the reported coordinates of data points. The ICRF is the realization of the International Celestial Reference System (ICRS), and agrees with the orientation of the Fifth Fundamental Catalog (FK5) "J2000.0" frame to within the (lower) precision of the latter.
In 2009, an updated reference frame ICRF2 was created. ICRF2 is defined by the position of 295 compact radio sources (97 of which also define ICRF1). Including non-defining sources, it comprises 3414 sources measured using very-long-baseline interferometry. The ICRF2 has a noise floor of approximately 40 µas and an axis stability of approximately 10 µas. Maintenance of the ICRF2 will be accomplished by a set of 295 sources that have especially good positional stability and unambiguous spatial structure. The original ICRF is now referred to as ICRF1.