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	Gazette IERS Gazette IERS Gazette IERS Gazette IERS Gazette 
                                   ________________________________________
	No 47, 18 November 1999   /
_________________________________/        Contact: iers@obspm.fr
                                     ftp: hpiers.obspm.fr (145.238.100.28)
                                     WWW: http://hpiers.obspm.fr
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  Authors: C. Ma, A.-M. Gontier
  Subject: ICRF-Ext.1, the first maintenance and extension of ICRF

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Since 1 January 1998 the conventional celestial reference system has been 
the  International  Celestial Reference System (ICRS).  The International 
Celestial Reference Frame (ICRF) is the first realization of this system. 
The IERS, jointly with the  IAU Working Group on the ICRS, is responsible 
for the  maintenance of the conventional system and frame and of its ties 
to other frames.

The realization of the ICRF (Ma et al., A.J. 116:516-546, 1998),  although
officially adopted in 1998, was made by the IAU Working Group on Reference
Frames (WGRF) during 1995.  A first extension of ICRF, ICRF-Ext.1,  is now
available,  and the  data  analysis is described in the 1998  IERS  Annual 
Report (1999).

The primary objectives of this first maintenance and extension of the ICRF 
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 maintenance/extension catalog ICRF-Ext.1 contains all the 667 observed
extragalactic  sources in the  VLBI astrometric/geodetic  data set through 
April 1999. It is available on ftp and web at the following address:

 anonymous ftp: hpiers.obspm.fr (145.238.100.28)
                cd iers/icrf/iau/icrf-Ext.1  

           web: http://hpiers.obspm.fr
      under "results: The International Celestial Reference Frame (ICRF)"
           (web file: webiers/results/icrf/Icrf.html) 

In ICRF-Ext.1  radio sources are  divided in four categories : defining,
candidate, other (as in ICRF) and new sources. For the defining sources, 
the positions and errors are unchanged from the ICRF catalog 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.
 
The maintenance/extension of the ICRF is the joint effort of subgroup T-1,
Maintenance and Extension of the ICRS, of the  IAU  Working  Group on the 
ICRS, the ICRS section of the IERS, and the International VLBI Service for
Geodesy and Astrometry.
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