International Celestial Reference Frame (ICRF-3)

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Astrometric radio source catalogs in general and the International Celestial Reference Frame (ICRF) in particular represent the most accurate and most stable realizations of a quasi-inertial celestial reference system. While previous ICRFs were composed of a single VLBI solution of all available global S/X-band observations, the new (third) frame, ICRF-3, to be compiled in the next few years on request of the International Astronomical Union (IAU), will be a combined multi-frequency catalog. The reason is that by now a competitive solution of X/Ka-band observations from NASA/JPL is available, which will be combined with multiple solutions of S/X-band observations. The project, which is applied for in the framework of the Research Unit 1503 "Space-Time Reference Systems for Monitoring Global Change and for Precise Navigation in Space", will develop the necessary methods and procedures for the combination of multiple catalogs. New is the approach of using intermediate results of the least squares solutions of the observations namely normal equation systems instead of the source positions themselves. This method permits for the first time the transfer of the full variance-covariance information to the final catalog. Furthermore, the parameterization of unstable sources may be refined introducing linear splines where deemed appropriate following critical investigations. For the combination to work properly, i.e., without introducing frame constraints and other distortions, the input normal equation systems have to be free of datum constraints or have to be made datum-free. In the project, the final ICRF-3 catalog will be compiled in close collaboration with the IAU Working Group on the Third Realization of the International Celestial Reference Frame.