16,468 research outputs found
Empirical Earth rotation model: a consistent way to evaluate Earth orientation parameters
It is customary to perform analysis of the Earth's rotation in two steps:
first, to present results of estimation of the Earth orientation parameters in
the form of time series based on a simplified model of variations of the
Earth's rotation for a short period of time, and then to process this time
series of adjustments by applying smoothing, re-sampling and other numerical
algorithms. Although this approach saves computational time, it suffers from
self-inconsistency: total Earth orientation parameters depend on a subjective
choice of the apriori Earth orientation model, cross-correlations between
points of time series are lost, and results of an operational analysis per se
have a limited use for end users. An alternative approach of direct estimation
of the coefficients of expansion of Euler angle perturbations into basis
functions is developed. These coefficients describe the Earth's rotation over
entire period of observations and are evaluated simultaneously with station
positions, source coordinates and other parameters in a single LSQ solution. In
the framework of this approach considerably larger errors in apriori EOP model
are tolerated. This approach gives a significant conceptual simplification of
representation of the Earth's rotation.Comment: To be published in the Proceedings of the Geodetic Reference Frame
symposium held in Muenchen in October 2006. 6 pages, 2 table
The KCAL VERA 22 GHz calibrator survey
We observed at 22 GHz with the VLBI array VERA a sample of 1536 sources with
correlated flux densities brighter than 200 mJy at 8 GHz. One half of target
sources has been detected. The detection limit was around 200 mJy. We derived
the correlated flux densities of 877 detected sources in three ranges of
projected baseline lengths. The objective of these observations was to
determine the suitability of given sources as phase calibrators for dual-beam
and phase-referencing observations at high frequencies. Preliminary results
indicate that the number of compact extragalactic sources at 22 GHz brighter
than a given correlated flux density level is twice less than at 8 GHz.Comment: Accepted for publication by the Astronomical Journal. 6 pages, 3
figures, 3 table. The machine readable catalogue file, kcal_cat.txt can be
extracted from the source of this submissio
The catalogue of positions of optically bright extragalactic radio sources OBRS-1
It is expected that the European Space Agency mission Gaia will make possible
to determine coordinates in the optical domain of more than 500000 quasars. In
2006, a radio astrometry project was launched with the overall goal to make
comparison of coordinate systems derived from future space-born astrometry
instruments with the coordinate system constructed from analysis of the global
very long baseline interferometry (VLBI) more robust. Investigation of their
rotation, zonal errors, and the non-alignment of the radio and optical
positions caused by both radio and optical structures are important for
validation of both techniques. In order to support these studies, the
densification of the list of compact extragalactic objects that are bright in
both radio and optical ranges is desirable. A set of 105 objects from the list
of 398 compact extragalactic radio sources with declination > -10 deg was
observed with the VLBA+EVN with the primary goal of producing their images with
milliarcsecond resolution. These sources are brighter than 18 magnitude at V
band, and they were previously detected at the European VLBI network. In this
paper coordinates of observed sources have been derived with milliarcsecond
accuracies from analysis of these VLBI observations following the method of
absolute astrometry. The catalogue of positions of 105 target sources is
presented. The accuracies of sources coordinates are in the range of 0.3 to 7
mas, with the median 1.1 mas.Comment: 6 pages, 1 figure, accepted by the Astronomical Journal, ID:
AJ-10606. Electronic table 2 with the catalogue is available in the source
code of this submissio
On significance of VLBI/Gaia position offsets
We have cross matched the Gaia Data Release 1 secondary dataset that contains
positions of 1.14 billion objects against the most complete to date catalogue
of VLBI positions of 11.4 thousand sources, almost exclusively active galactic
nuclei. We found 6,064 matches, i.e. 53% radio objects. The median uncertainty
of VLBI positions is a factor of 4 smaller than the median uncertainties of
their optical counterparts. Our analysis shows that the distribution of
normalized arc lengths significantly deviates from Rayleigh shape with an
excess of objects with small normalized arc lengths and with a number of
outliers. We found that 6% matches have radio optical offsets significant at
99% confidence level. Therefore, we conclude there exists a population of
objects with genuine offsets between centroids of radio and optical emission.Comment: 6 pages, 6 figures, 3 tables; accepted by MNRAS Letters; full
electronic versions of 2 tables are available from the preprint source; text
and tables are updated, a figure adde
Precise absolute astrometry from the VLBA imaging and polarimetry survey at 5 GHz
We present in this paper accurate positions of 857 sources derived from the
astrometric analysis of 16 eleven-hour experiments from the Very Long Baseline
Array imaging and polarimetry survey at 5 GHz (VIPS). Among observed sources,
positions of 430 objects were not determined before at a milliarcsecond level
of accuracy. For 95% of the sources the uncertainty of their positions range
from 0.3 to 0.9 mas, with the median value of 0.5 mas. This estimate of
accuracy is substantiated by the comparison of positions of 386 sources that
were previously observed in astrometric programs simultaneously at 2.3/8.6 GHz.
Surprisingly, the ionosphere contribution to group delay was adequately modeled
with the use of the total electron contents maps derived from GPS observations
and only marginally affected estimates of source coordinates.Comment: Accepted for publication by the Astronomical Journal. 7 pages, 2
tables, 4 figures. Submission contains an ascii file with the catalogue. You
can get the catalogue by downloading the source of this paper and extracting
file table2.tx
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