Comparison and Combination of CRF Catalogues
  In 2007, a joint IERS/IVS Working Group has been established to consider
practical issues of creating the next ICRF generation, ICRF-2. The goal of the
WG is to seek after ways to improve the existing ICRF. In this study we
investigate a possibility of ICRF improvement by means of using combined ICRF
catalogue instead of a catalogue computed in a single analysis centre, even
though using most advanced models and software. In this work, we present a new
version of Pulkovo combined catalogue of radio source positions computed using
the method proposed in \cite{SokMal07}. Radio source catalogues that were
submitted in 2007 in the framework of the WG activity were used as input for
mutual comparison and combination. Four combined catalogues have been
calculated: Two first of them provide a stochastic improvement of the ICRF, and
last two of them allow us to account also for systematic errors in the current
ICRF version.
