Evidence of systematic errors in FK5

A Strategy for Linking HIPPARCOS to an Extra-Galactic Reference Frame

“…In comparing it to FK5, it will be possible to smooth out small zonal errors known to exist in FK5 [1]. Then, linked to VLBI, there will be, for the first time, a homogeneous frame unified for optical and radio astrometry.…”

Link to an Inertial System

“…The FK5 frame contains global irregularities running up to 0".01 (Morrison et al (1991)), and while these zonal errors will not reappear in the Hipparcos Catalogue which will remain highly homogeneous and rigid, they will nevertheless affect the accuracy with which its J2000.0 equinox and equator can be realized. And the inertial properties of FK5 are not at present known to the accuracy needed for Hipparcos.…”