SIMBAD references

In preparation for the evolution of the definition of the VLBI-based International Celestial Reference Frame (ICRF), various elements of the analysis strategy are investigated, such as reference source selection and the impact of the status of the terrestrial reference frame in the data analysis model. We conclude that including the determination of both the celestial and the terrestrial reference frames in the analysis does not affect the quality of the celestial reference frame. The determination of precession and nutation components is not affected by the status of the terrestrial reference frame but it is affected by the selection of reference radio sources. Extending an earlier study, three lists of reference sources based on progressively larger tolerances are proposed. They include 181, 225 and 247 objects, respectively.