Sidereal orientation of the Earth and stability of the VLBI celestial reference frame

The consideration of time stability of extragalactic radio sources observed by VLBI is shown to allow the realisation of more consistent celestial reference frames. The impact on the estimation of precession and nutation components is inves- tigated over the time span 1984-2002. The precession correction to the IAU 2000 value that is obtained when excluding the unstable sources reaches 49 ± 5 µas/year, to be compared to 12 ± 5 µas/year using the current conventional celestial frame. The determination of the obliquity rate is unaffected and remains at the level of 27 ± 2 µas/year. The observed correction to the 18.6-year nutation amplitude using the current conventional celestial frame ie sizeably corrupted by the unstable sources. After accounting for this effect, the estimations relative to both sets of reference radio sources confirm a discrepancy with the IAU 2000 nutation model with a total amplitude of 320 ± 100 µas for the observed nutation in longitude, to be compared to the 80 µas discrepancy found by Mathews et al. (2002, JGRB, 107, 1029). The discrepancy in obliquity amounts to 50 ± 16 µas. The effect of source instability is shown to have an impact on the determination of universal time at the one microsecond level. The high and medium frequency nutation terms (up to periods of a few years) are impacted only in the early years of the program.