Precession Constant Correction and Proper Motion Systems of FK5 and Hipparcos

Results of many researches have shown that the relation between the proper motion systems of FK5 and Hipparcos is not consistent with the precession constant corrections determined by VLBI and LLR. We analysed proper motion data of PPM and ACRS based on the FK5 system for many different sub-samples and found that consistent values of the precession correction and equinox motion correction can not be given by either PPM or ACRS proper motion data, thereby indicating that the internal systematic error of the FK5 proper motion is the main underlying factor of the inconsistency.