| dc.description | Quasars and galactic nuclei are compact object with angular diameters at the level of the miliarcsecond. They can only be observed with high-resolution interferometers. Most of the extragalactic radio sources are at distances of several thousands of MPc and their redshifts are cosmological. The proper motions are beyond the precision of the modern observational techniques, and therefore they can be considered as inexistent. The extragalactic radio sources are the best adapted objects to materialize a celestial reference system kinematically stable for astrometry. The technique of Very Long Baseline Interferometry (VLBI) permits to determine positions of extragalactic radio sources precise at the level of 0.001" (Hinteregger, 1972; Whitney, 1974). The equatorial coordinates of the extragalactic radio sources implicitly define the directions of the axes of the extragalactic celestial reference frame. During the last ten years the different VLBI groups have elaborated catalogues of positions of the milliarcsecond (Ma et al. 1981, Fanswloe et al. 1981, Fanselow et al. 1984, Robertson et al. 1986). However, there still exist some inconsistencies between the independent realizations of the extragalactic frame which can reach soem 0.001". These inconsistencies arise from the adopted models, the distribution of stations in the networks, the distribution of radio sources in the sky, the data acquisition systems and the methods of reduction of observations. We have compared catalogues elaborated at three VLBI laboratories in the USA: the Goddard Space Flight Centre (GSFC) (Ryan and Ma, 1985, Ma, 1988), the Jet Propulsion Laboratory (JPL) (Sovers, 1986, 1988) and the U.S. National Geodetic Survey (NGS) (Robertson, Fallon y Carter, 1986). We have analysed three different aspects: a) the relative orientation between frames, b) the calibration of formal errors, c) the regional deformations between frames. In the frames, the arbitrary origin on the equator has been fixed by the adopted right ascension of the quasar 3C273B (1226 + 023) (Hazard et al., 1971). The equatorial coordinates of the radio sources are in J2000.0 by means of the conventional models IAU 1976 of the precession and IAU 1980 of the nutation. The astrometric comparison of these frames indicates that the direction of axes of the different realizations are consistent at a level better than 0.0015" (Arias et al. 1988), and detects relative regional deformations in some zones of the sky which can reach, in mean, 0.002"( Arias and Lestrade, 1990). Concerning the calibration of errors, the analysis show that formal errors in the individual catalogues have been sub-estimated, and that this effect is larger in declination than in right ascension (Arias et al. 1988). The relative orientation between individual frames could be diminished by adopting a) a better strategy to fix the right ascension origin (it must be noted that the rotation around the polar axis, which implies a misalignment of the catalogue equinoxes, in the most significative of the three angles); b) more precise models, particularly to determine the celestial pole position (the IAU conventional models have inaccuracies which can amount a few miliarcseconds in some terms of the nutation series, as has been shown by Herring, 1986); and c) a more homogeneous distribution of radio sources in the sky. The regional deformations are most probably originated in the geometry of the networks, limiting the spatial distribution of objects and the precision of their positions. Most of the VLBI network operates stations in the Northern hemisphere, and as a consequence, the Southern sky has a few objects with precise positions. On the other hand, the lack of baselines with long components in the North-South direction (except for the baseline Australia-California of the Deep Space Network of the JPL) give a low precision determination of declinations of radio sources near the equator. | |
