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Astron. Astrophys. 359, 1195-1200 (2000)

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3. The International Celestial Reference Frame (ICRF)

The ICRS is materialized by the precise coordinates of extragalactic radio sources observed with the technique of Very Long Baseline Interferometry (VLBI). The previous realizations of the conventional celestial reference system assumed that the coordinates of the objects were conventional, i.e., their numerical values remained fixed for some time. This is the philosophy adopted in the series of FK catalogues; when a new catalogue was released, the reference system implied in it changed its orientation relative to the previous one. In the case of the ICRS, the axes of the system are considered conventionally fixed to those of the initial realization; source coordinates are susceptible to change if justified by their improvement. As the system should not change with the different realizations, a process of maintenance is applied. The ICRS maintenance implies a permanent monitoring of sources in ICRS to study their positional stability, the improvement of the models adopted and the control of the invariability of the directions of its axes.

The first realization of the ICRF (Ma et al. 1997, 1998) is based on radio positions obtained in a general solution for all dual frequency Mark III VLBI data available through the middle of 1995. The quasi-inertial reference system is defined by the mean J2000.0 coordinates of 212 high-astrometric-quality radio sources; their positional accuracy is better than about 1 mas both in right ascension and declination. Additional 396 objects that either need further observation to conclude about their quality, or being unsuitable to define an accurate reference frame, serve to link ICRF to other frames. The maintenance of the axes is assured by the 212 defining sources of the frame.

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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