51 research outputs found
Secular Aberration Drift and IAU Definition of ICRS
The gravitational attraction of the Galactic centre leads to the centrifugal
acceleration of the Solar system barycentre. It results in secular aberration
drift which displaces the position of the distant radio sources. The effect
should be accounted for in high-precision astrometric reductions as well as by
the corresponding update of the ICRS definition.Comment: 6 page
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)
Effect of asymmetry of the radio source distribution on the apparent proper motion kinematic analysis
A new list of physical characteristics of 4261 astrometric radio sources,
including all 717 ICRF-Ext.2 sources has been compiled. Comparison of our data
of optical characteristics with the official International Earth Rotation and
Reference Systems Service (IERS) list showed significant discrepancies for
about half of 667 common sources. We also found that asymmetry in the radio
sources distribution between hemispheres could cause significant correlation
between the vector spherical harmonics, especially if the case of sparse
distribution of the sources with high redshift. We identified radio sources
having many-year observation history and lack redshift. This sources should be
urgently observed at large optical telescopes. The list of optical
characteristics created in this paper is recommended for use as a supplement
material for the next International Celestial Reference Frame (ICRF)
realization. It can be also effectively used for cosmological studies and
planning of observing programs both in radio and optics.Comment: 9 page
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
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
Analysis strategy issues for the maintenance of the ICRF axes
International audienceIn 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
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.
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