The direction of the axes of ICRF are consistent with those of the FK5 system within the uncertainties of the latter. On the basis of Fricke's (1982) and Schwan (1988) considerations, the uncertainty of the pole position relative to the mean pole at J2000.0 is ±50 mas. Studies by Morrison et al. (1990) and Lindegren et al. (1995) have shown that the FK5 origin of right ascensions has an uncertainty of the order of ±100 mas.
The accuracy of the tie of the Hipparcos stellar frame to ICRS is ±0.6 mas at the Hipparcos mean epoch of observation (1991.25) and ±0.25 mas/year for the time evolution.
The tie between the dynamical planetary frame and ICRS is known within ±3 mas (Folkner et al. 1994).