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

Abstract

Abstract. 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 investigated over the time span [1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][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. Chapter 7 concerning the observation of the core and inner core free nutations is paralleled by a twin paper (Dehant et al. 2005(Dehant et al. , A&A, 438, 1149) that proposes a theoretical development for their atmospheric and oceanic excitation.

Comparison of VLBI radio source catalogs

“…At the level of accuracy reached nowadays, these variants can have noticeable consequences on the derived VLBI products (Feissel-Vernier 2003;Feissel-Vernier et al 2005, 2006Lambert & Dehant 2007;Heinkelmann & Tesmer 2013). These options include the choice of the sessions to be analyzed, the exclusion of a part of the data (e.g., because of bad quality), and various aspects relevant to the parameterization (split between global, sessionwise, and arc parameters) and to the constraints (especially those applied globally to the station and radio source coordinates).…”

Time stability of the ICRF2 axes

“…Some IVS analysis centers have processed the geodetic VLBI observational database to obtain Table 1 is available in electronic form at http://www.aanda.org radio source coordinate time series. These time series represent the motion of the centroid of the radio source as seen by the network, and were intensively used in the past to build up a more stable reference frame (see, e.g., Feissel-Vernier 2003;Feissel-Vernier et al 2005Lambert & Gontier 2009). The selection of the ICRF2 defining sources was partly based on these series (Fey et al 2010): the authors defined a stability criterion and ranked the sources from the most to the less stable.…”

Atmospheric and oceanic excitation of the rotation of a three-layer Earth

“…When the modelled nutation (MHB2000 without the free modes) is subtracted from the observation, the signal associated with the free core nutation is the major source of residuals. From the analysis of those residuals, Feissel-Vernier (2003) and Feissel-Vernier et al (2005) have shown the presence of some power in the FICN frequency band. This motivated us to compute theoretically the nutational response of a three-layer Earth for the normal nutation modes excited by the noise of the atmosphere and/or the ocean at those frequencies.…”