At present a new realization of the International Celestial Reference Frame is being prepared. One of the aspects of improving the forthcoming realization is the compilation of combined catalogues of radio sources. We proposed a new method for calculation of external uncertainties of catalogues of radio sources and correlation coefficients. A comparison of the combined catalogues created at MAO NASU with ones compiled at CAO RAS and with the current realization of the International Celestial Reference Frame is performed.