FUTURE RADIO REFERENCE FRAMES AND IMPLICATIONS FOR THE GAIA LINK.

Since January 1st, 2010, the IAU (International Astronomical Union) fundamental celestial reference frame has been the 2nd International Celestial Reference Frame (ICRF2), which is composed of Very Long Baseline Interferometry (VLBI) positions for more than 3000 extragalactic radio sources. This frame is constantly improving through joint efforts of the VLBI community. By surveying the whole sky up to magnitude 20, the European space astrometric mission Gaia will soon create its own celestial reference frame directly in the optical domain and with many more sources. By 2015-2020, the two frames will thus cohabit and it will be important to align these to the highest accuracy for consistency between optical and radio positions. In this paper, we present the various observational approaches that are undertaken to improve the VLBI frame in the future. These include extension to weaker sources for densification, extension to higher radio frequencies to take advantage of the more compact morphology of the sources at these frequencies, and further observations in the southern hemisphere for homogeneous sky coverage. We also elaborate on how such future radio frames should contribute to highly-precise alignment between the VLBI and Gaia frames within the next decade. [ABSTRACT FROM AUTHOR]