166 research outputs found
High Precision Astrometric Millimeter VLBI Using a New Method for Atmospheric Calibration
We describe a new method which achieves high precision Very Long Baseline
Interferometry (VLBI) astrometry in observations at millimeter wavelengths. It
combines fast frequency-switching observations, to correct for the dominant
non-dispersive tropospheric fluctuations, with slow source-switching
observations, for the remaining ionospheric dispersive terms. We call this
method Source-Frequency Phase Referencing. Provided that the switching cycles
match the properties of the propagation media, one can recover the source
astrometry. We present an analytic description of the two-step calibration
strategy, along with an error analysis to characterize its performance. Also,
we provide observational demonstrations of a successful application with
observations using the Very Long Baseline Array at 86 GHz of the pairs of
sources 3C274 & 3C273 and 1308+326 & 1308+328, under various conditions. We
conclude that this method is widely applicable to millimeter VLBI observations
of many target sources, and unique in providing bona-fide astrometrically
registered images and high precision relative astrometric measurements in
mm-VLBI using existing and newly built instruments.Comment: Astronomical Journal, accepted for publicatio
Multi-step VLBI observations of weak extragalactic radio sources to align the ICRF and the future GAIA frame
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between optical and radio positions,
it will be important to align the GAIA frame and the International Celestial
Reference Frame (ICRF) with the highest accuracy. Currently, it is found that
only 10% of the ICRF sources are suitable to establish this link, either
because they are not bright enough at optical wavelengths or because they have
significant extended radio emission which precludes reaching the highest
astrometric accuracy. In order to improve the situation, we have initiated a
VLBI survey dedicated to finding additional suitable radio sources for aligning
the two frames. The sample consists of about 450 sources, typically 20 times
weaker than the current ICRF sources (down to the 20 mJy flux level), which
have been selected by cross-correlating optical and radio catalogues. This
paper presents the observing strategy to detect, image, and measure accurate
positions for these sources. It will also provide results about the VLBI
detectability of the sources, as derived from initial observations with the
European VLBI Network in June and October 2007. Based on these observations, an
excellent detection rate of 89% is found, which is very promising for the
continuation of this project
VLBI2010: The Astro-Geo Connection
VLBI2010 holds out promise for greatly increased precision in measuring geodetic and Earth rotation parameters. As a by-product there will be a wealth of interesting new astronomical data. At the same time, astronomical knowledge may be needed to disentangle the astronomical and geodetic contributions to the measured delays and phases. This presentation explores this astro-geo link
VLBI observations of weak extragalactic radio sources for the alignment of the future GAIA frame with the ICRF
The space astrometry mission GAIA will construct a dense optical QSO-based
celestial reference frame. For consistency between the optical and radio
positions, it will be important to align the GAIA frame and the International
Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is
found that only 10% of the ICRF sources are suitable to establish this link,
either because they are not bright enough at optical wavelengths or because
they have significant extended radio emission which precludes reaching the
highest astrometric accuracy. In order to improve the situation, we have
initiated a VLBI survey dedicated to finding additional high-quality radio
sources for aligning the two frames. The sample consists of about 450 sources,
typically 20 times weaker than the current ICRF sources, which have been
selected by cross-correlating optical and radio catalogues. This paper presents
the observing strategy and includes preliminary results of observation of 224
of these sources with the European VLBI Network in June 2007
Optimum estimate of delays and dispersive effects in low-frequency interferometric observations
Modern radio interferometers sensitive to low frequencies will make use of
wide-band detectors. For such wide bandwidths, dispersive atmospheric effects
introduce variations in the fringe delay which change through the band of the
receivers. These undesired dispersive effects must be estimated and calibrated
with the highest precision. We studied the achievable precision in the estimate
of the ionospheric dispersion and the dynamic range of the correlated fringes
for different distributions of sub-bands in low-frequency and wide-band
interferometric observations. Our study is focused on the case of sub-bands
with a bandwidth much narrower than that of the total covered spectrum (case of
LOFAR). We computed the uncertainty of the ionospheric delay, the delay
ambiguity, and the dynamic range of the fringes using four different kinds of
sub-band distributions: constant spacing between sub-bands, random spacings,
spacings based on a power-law distribution, and spacings based on Golomb rulers
(sets of integers whose sets of differences have non-repeated elements). For a
large number of sub-bands (, depending on the delay window) spacings
based on Golomb rulers give the most precise estimates of dispersive effects
and the highest fringe dynamic ranges. Spacings based on the power-law
distribution give similar results, although better than those with the Golomb
rulers for smaller number of sub-bands. Random distributions result in large
fringe dynamic ranges, but the estimate of dispersive effects is worse. A
constant spacing of sub-bands results in very bad fringe dynamic ranges, but
good estimates of ionospheric dispersion. Combining all the results, the
power-law distribution gives the best compromise between homogeneity in the
bandwidth sampling, precision in the estimate of ionospheric effects, dynamic
range of the correlated fringes, and group-delay ambiguity.Comment: 8 pages, 7 figures. Accepted for publication in A&
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