VLBI measurement of the secular aberration drift

While analyzing decades of very long baseline interferometry (VLBI) data, we detected the secular aberration drift of the extragalatic radio source proper motions caused by the rotation of the Solar System barycenter around the Galactic center. Our results agree with the predicted estimate to be 4-6 micro arcseconds per year ({\mu}as/yr) towards {\alpha} = 266\circ and {\delta} = -29\circ. In addition, we tried to detect the quadrupole systematics of the velocity field. The analysis method consisted of three steps. First, we analyzed geodetic and astrometric VLBI data to produce radio source coordinate time series. Second, we fitted proper motions of 555 sources with long observational histories over the period 1990-2010 to their respective coordinate time series. Finally, we fitted vector spherical harmonic components of degrees 1 and 2 to the proper motion field. Within the error bars, the magnitude and the direction of the dipole component agree with predictions. The dipole vector has an amplitude of 6.4 \pm 1.5 {\mu}as/yr and is directed towards equatorial coordinates {\alpha} = 263\circ and {\delta} = -20\circ. The quadrupole component has not been detected. The primordial gravitational wave density, integrated over a range of frequencies less than 10-9 Hz, has a limit of 0.0042 h-2 where h is the normalized Hubble constant is H0/(100 km s-1)

Selecting stable extragalactic compact radio sources from the permanent astrogeodetic VLBI program

A set of stable compact radio sources is proposed, based on the analysis of VLBI-derived time series of right ascensions and declinations from mid-1989 through May 2002. Five selection schemes are tested, that are based on the usual and Allan standard deviations and on apparent drifts. The efficiency of the selection schemes is characterized by the ability of the sources to support the maintenance of the direction of the axes of the celestial reference frame that they materialize. When compared with the current set of ICRF defining sources, the best performing selection scheme keeps 199 sources and lowers the medium-term frame instability from 28 to 6 microarcsec

Analysis strategy issues for the maintenance of the ICRF axes

In preparation for the evolution of the definition of the VLBI-based International Celestial Reference Frame (ICRF), various elements of the analysis strategy are investigated, such as reference source selection and the impact of the status of the terrestrial reference frame in the data analysis model. We conclude that including the determination of both the celestial and the terrestrial reference frames in the analysis does not affect the quality of the celestial reference frame. The determination of precession and nutation components is not affected by the status of the terrestrial reference frame but it is affected by the selection of reference radio sources. Extending an earlier study, three lists of reference sources based on progressively larger tolerances are proposed. They include 181, 225 and 247 objects, respectively.

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 investigated 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. 
Chapter 7 concerning the observation of the core and inner core free nutations is paralleled by a twin paper (Dehant et al. 2005, A&A, 438, 1149) that proposes a theoretical development for their atmospheric and oceanic excitation.


Stability of VLBI, SLR, DORIS, and GPS positioning

International audienceAbstract: The residual signal in VLBI, SLR, DORIS and GPS station motion, after a linear trend and seasonal components have been removed, is analysed to investigate site-specific and technique-specific error spectra. The study concentrates on 60 sites with dense observation history by two or more space geodetic techniques. Statistical methods include the Allan variance analysis and the three-cornered hat algorithm. The stability of time-series is defined by two parameters, namely the Allan deviation for a one-year sampling time (noise level) and the slope of the Allan variance graph with its spectral interpretation (noise type). The site-specific noise level is found to be in the range 0.5-3.5 mm in either horizontal direction and 1-4.5 mm in height for most sites. The distribution of site-specific noise type includes both white noise and flicker noise. White noise is predominant in the East direction. Both types of noise are found in the North direction, with no particular geographical clustering. In the Up direction, the Northern hemisphere sites seem to be split in two large geographical sectors characterised either by white noise or by flicker noise signatures. Technique-specific noise characteristics are estimated in several ways, leading to a white noise diagnostic for VLBI and SLR in all three local directions. DORIS has also white noise in the horizontal directions, whereas GPS has a flicker noise spectrum. The vertical noise spectrum is indecisive for both DORIS and GPS. The three-dimensional noise levels for the one-year sampling time are 1.7 mm for VLBI, 2.5 mm for SLR, 5.2 mm for DORIS, and 4.1 mm for GPS. For GPS, the long-term analysis homogeneity has a strong influence. In the case of a test solution reanalysed in a fully consistent way, the noise level drops to the VLBI level in horizontal and to the SLR level in vertical. The three-dimensional noise level for a one-year sampling time decreases to 1.8 mm. In addition, the percentage of stations with flicker noise drops to only about 20% of the network

Remaining error sources in the nutation at the submilliarcsecond level

International audienceEarths precession and nutations are mainly generated by the luni-solar tidal torque.Diurnal retrograde variations in the atmospheric and oceanic angular momenta in an Earth-fixed reference system induce some additional nutation motions. Observed precession and nutations are derived from very long baseline interferometry (VLBI) data,assuming that the direction of the observed quasars are fixed in space. In this study, we consider the effects of two possible causes for explaining discrepancies between the observed nutations and those modeled in MHB2000 (model adopted by the International Astronomical Union): (1) the time variations in the atmospheric (and potentially oceanic)forcing of the nutations, of the free core nutation (FCN), and of the free inner core nutation (FICN), and (2) the possible contamination of VLBI-derived nutation amplitudes by apparent changes in the directions of the extragalactic radio sources. The robustness ofMHB2000 is tested by perturbing some of the parameters and assessing the validity of the resulting nutation amplitudes against realistic estimations. We show that even allowing for large discrepancies related to atmospheric forcing, the ranges of the possible changes in the FCN and FICN periods and damping factors are small