International Celestial Reference Frame

The International Celestial Reference Frame (ICRF) is a quasi-inertial reference frame centered at the barycenter of the Solar System, defined by the measured positions of 212 extragalactic sources (mainly quasars). Although relativity implies that there is no true inertial frame, the extragalactic sources used to define the ICRF are so far away that any angular motion is essentially zero. The ICRF is now the standard reference frame used to define the positions of the planets (including the Earth) and other astronomical objects.

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 datum points. The ICRF is the realization of the International Celestial Reference System, and agrees with the orientation of the Fifth Fundamental Catalog (FK5) "J2000.0" frame to within the (lower) precision of the latter.

References

• US Naval Observatory ICRF page
• ICRF page from the International Earth Rotation Service
• Very good ICRS and ICRF overview
• IERS Conventions 2003 (defines ICRS and other related standards)