C. Ma, M. Eubanks
Extension and Maintenance of the ICRF
 
                   ABSTRACT
 
 
VLBI positions of 212 extragalactic radio sources were accepted by the
IAU at the XXIII General Assembly as defining the International Celestial
Reference Frame (ICRF), replacing the stellar FK5.  Geodetic and
astrometric data through July 1995 were used to estimate the positions
of the 212 defining objects as well as 294 candidate objects and 102
"other" objects with excessive position variation.  The position accuracy
of the ICRF is ~0.25 mas and the orientation of the ICRF is determined
to ~0.020 mas.  Since 1995 relevant VLBI observations have continued,
providing information on new and old sources.  The conceptual basis,
analysis configuration, and dissemination procedures for using the new
data are described.  The process will result in regular updating of
the catalog consistent with the International Celestial Reference System
(ICRS) and will be one of the primary responsibilities of the IVS
(International VLBI Service for Geodesy and Astrometry).  In parallel is
a research effort to prepare for the next ICRF realization.
 
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In Proceedings of the 13th Working Meeting on European VLBI for Geodesy
and Astrometry, Viechtach, 1999
 
Keywords: ICRF, celestial reference frame, astrometry