The ICRF1-Ext.1: first extension of the ICRF1
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The primary objectives of this first maintenance and extension of the ICRF1 are to provide positions for extragalactic radio sources observed since July 1995 and to refine the positions of candidate sources from additional observations. The secondary objectives include monitoring sources to ascertain whether they continue to be suitable for their use in the ICRF and improving the data analysis. The data added to the ICRF spanned December 1994 through April 1999 from both geodetic and astrometric observing programs. In the ~600,000 observations from 461 sessions there were 59 new sources.

This maintenance/extension catalog ICRF1-Ext.1 (IERS, 1999) contains all the observed extragalactic sources in the VLBI astrometric/geodetic data set. For the defining sources, the positions and errors are unchanged from the ICRF since these are the ICRF. For candidate and other sources the improved positions and errors reflect the changes in the data set and the analysis. As in the original ICRF1, the formal errors were inflated applying an additive variance and a scaling factor to provide more realistic errors. 59 new sources have ICRS coordinates and IAU names assigned by following the IAU convention for designations. Structure indices have been updated based on the work of Fey & Charlot (1999) and are now available for 392 sources. Yearly 1982-99 position plots and data files are available for 192 sources.