c © ESO 2007 Astronomy

Context. Radio source catalogues (RSCs) obtained from very long baseline interferometry (VLBI) observations are recommended by the International Astronomical Union (IAU) as realizations of the International Celestial Reference Frame (ICRF). As the accuracy of the VLBI observations is improving with time, development of more accurate methods of RSC construction is a topical problem. Aims. In this paper we search for an optimal method of representation of the RSC systematic errors and investigate a possibility for improving the ICRF by means of combining individual RSCs. Methods. Four methods of representing the RSC systematic differences have been examined by comparison of the differences in the radio source positions between the individual catalogues. Eight radio source catalogues and the latest ICRF realization, ICRF-Ext.2, were used for comparison. After selection of the best method of difference representation we computed the systematic differences between each individual catalogue and the ICRF. These differences were then used in the construction of the combined radio source catalogue. Results. Representation of the systematic differences between RSCs with the Legendre-Fourier functions proved to be the most accurate method. Using this method, two combined radio source catalogues have been constructed. The first of them provides a stochastic improvement of the current realization of ICRF, and the second, final one allows us to account also for possible systematic errors in the ICRF. Comparison of the celestial pole offsets obtained from the processing of VLBI observations using ICRF and th