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The space astrometry mission GAIA will construct a dense optical QSO-basedcelestial reference frame. For consistency between optical and radio positions,it will be important to align the GAIA frame and the International CelestialReference Frame (ICRF) with the highest accuracy. Currently, it is found thatonly 10% of the ICRF sources are suitable to establish this link, eitherbecause they are not bright enough at optical wavelengths or because they havesignificant extended radio emission which precludes reaching the highestastrometric accuracy. In order to improve the situation, we have initiated aVLBI survey dedicated to finding additional suitable radio sources for aligningthe two frames. The sample consists of about 450 sources, typically 20 timesweaker than the current ICRF sources (down to the 20 mJy flux level), whichhave been selected by cross-correlating optical and radio catalogues. Thispaper presents the observing strategy to detect, image, and measure accuratepositions for these sources. It will also provide results about the VLBIdetectability of the sources, as derived from initial observations with theEuropean VLBI Network in June and October 2007. Based on these observations, anexcellent detection rate of 89% is found, which is very promising for thecontinuation of this project.

Multi-step VLBI observations of weak extragalactic radio sources to align the ICRF and the future GAIA frame