| dc.description | The celestial system maintained by the International Earth Rotation Service is described in terms of physical properties of the fiducial objects, international consistency of the farm and agreement with the FK5 and dynamical systems. The celestial reference frame of IERS is based on compact extra-galactic objects observed by Very Long Baseline Interferometry (VLBI). It is maintained on the basis of several independent Earth orientation programs analysed by various analysis centres. We present hereafter its latest realization, which is a combination of individual frames obtained by four groups: the Goddard Space Flight Centre (GSFC, Ma et al., 1990), the Jet Propulsion Laboratory (JPL, Steppe et al., 1990) the National Geodetic Survey (NGC; Carter and Robertson, 1990) and the U.S. Naval Observatory (USNO, Eubanks et al., 1990). The combination model is based on a three rotation angle model applied to a selection of radio sources common to the individual frames. The initial definition of the system and the maintenance process are described by Arias and Feissel (1990); the connenction to the conventional terrestrial system of IERS at the level of 0.001" is studied by Feissel (1990). The latest realization (IERS; 1990) of the IERS celestial reference system is materialized by the J2000.0 positions of 228 extra-galactic radio sources between +85 deg and -80 deg in declination. It is well known that the use of the conventional IAU 1976 Precession and IAU 1980 Theory of Nutation in the analysis of VLBI observations would give rise to systematic errors in the source positions, and to misorientation of the axes of the frames, both at the level of a few miliarcseconds. Therefore the common practice in catalogue work is to estimate additional parameters which describe the motion of the celestial pole relative to its conventional position. In the combination performed by the Bureau Central of the IERS, only individual frames obtained by this procedure are used. The realization of the celestial reference system published in the Annual Report of IERS for 1989 contains 228 sources with different status: primary, secondary and complementary. The 51 primary sources were chosen on the basis of consistency of their estimated coordinates in the four individual frames, after removing the relative rotations: only sources which showed position differences under 0.0015" in all comparisons two by two were retained as primary. Their position uncertainties in the IERS frame, derived from this consistency, are smaller than 0.0007". The other sources common to at least two frames but with larger position discrepancies, are considered secondary; there are 40 of them in the realization described here. Finally, 137 complementary sources in the IERS frame were available from only one individual catalogue. Altogether 113 sources have a position uncertainty smaller than 0.001", 104 between 0.001" and 0.003" and 11 over 0.003". The IERS celestial reference system is barycentryc through the appropriate modelling of observations by the analysis centres which contribute individual catalogs. The Ox axis was implicitly defined in the initial realization (Arias et al, 1988), and is in agreement with the FK5 origin of right ascensions (Feissel, 1990). In addition, it is in agreement with the equinox of the JPL planetary frame DE200/LE200 within 0.02" (Dickey, 1989). The Oz axis points in the direction of the mean pole at J2000.0 as defined by the IAU conventional models for precession and nutation. As a result of the inaccuracy of the conventional models (Herring, 1990), the Oz axis of the IERS celestial system is shifted from the expected position of the mean pole by about 0.01" in longitude, sin "epsilon" and 0.001" in obliquity. New realizations of the IERS celestial reference system are produced whenever justified by the progress in the observations or in the modelling. The successive realizations produced up to now have maintained the initial definition of the axes within 0.0001". | |
