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In this paper we outline several problems related to the realization of the international celestial and terrestrial reference frames ICRF and ITRF at the millimeter level of accuracy, with emphasis on ICRF issues. The main topics considered are: analysis of the current status of the ICRF, mutual impact of ICRF and ITRF, and some considerations for future ICRF realizations.
The current state of the link problem between radio and optical celestial reference frames is considered. The main objectives of the investigations in this direction during the next few years are the preparation of a comparison and the mutual orienta
An analysis of the source position differences between VLBI-based ICRF and $Gaia$-CRF catalogues is a key step in assessing their systematic errors and determining their mutual orientation. One of the main factors that limits the accuracy of determin
The goal of this presentation is to report the latest progress in creation of the next generation of VLBI-based International Celestial Reference Frame, ICRF3. Two main directions of ICRF3 development are improvement of the S/X-band frame and extensi
The link problem between radio (VLBI/ICRF) and optical (Gaia/GCRF) celestial reference frames is analyzed. Both systems should be a realization of the ICRS (International Celestial Reference System) at microarcsecond level of accuracy. Therefore, the
Astrometry, the measurement of positions and motions of the stars, is one of the oldest disciplines in Astronomy, extending back at least as far as Hipparchus discovery of the precession of Earths axes in 190 BCE by comparing his catalog with those o