Towards ephemerides of ICRF quasars

ICRF quasars are not stationary radio sources on the sky. Their positions change in time along complex jittery trajectories that are quite obvious in precision geodetic VLBI measurements separated by day of observation. The contribution of this intrinsic instability is best seen for the radio-bright sources, which tend to be defining and most frequently observed, where it becomes up to a factor of 3 greater than the formal error of astrometric measurement. These epoch-dependent variations, which we call "cosmic error" in analogy with astrometric binaries, have adverse effect on the main applications of ICRF, such as UT1-UTC monitoring and Earth orientation parameters determination. We investigate the properties of this effect in the statistical sense and quantify the impact on the Maximum Likelihood mean positions and excursions for the general ICRF3 sample.