IAU(1991) RECOMMENDATION VI
Selection of extragalactic objects on which the conventional celestial barycentric reference system will be based
The XXIst General Assembly of the International Astronomical Union,
the desirability of implementing a conventional celestial barycentric reference system based upon the observed positions of extragalactic objects, and,
the existence of tentative reference frames constructed by various institutions and combined by the International Earth Rotation Service (IERS) into a frame used for Earth rotation series,
1. that intercomparisons of these frames be extensively made in order to assess their systematic differences and accuracy,
2. that an IAU Working Group consisting of members of Commissions 4, 8, 19, 24, 31 and 40, the IERS, and other pertinent experts, in consultation with all the institutions producing catalogues of extragalactic radio sources, establish a list of candidates for primary sources defining the new conventional reference frame, together with a list of secondary sources that may later be added to or replace some of the primary sources, and,
1. that such a list be presented to the XXIInd General Assembly (1994) as a part of the definition of a new conventional reference system,
2. that the objects in this list be systematically observed by all VLBI and other appropriate astrometric programmes.
Note for Recommendation VI
This recommendation essentially describes the first part of the work that must be done to prepare the realization of the reference system defined by Recommendations I and II. The choice of objects must be made in the first place by considering their observability by VLBI, but special care should be taken to include a large proportion of extragalactic radio sources with well identified optical counterparts.