65 research outputs found
Free core nutation: new large disturbance and connection evidence with geomagnetic jerks
Variations in free core nutation (FCN) are connected with various processes
in the Earth's fluid core and core-mantle coupling, which are also largely
responsible for the geomagnetic field variations, particularly the geomagnetic
jerks (GMJs). A previous study (Malkin, 2013) revealed that the epochs of the
observed extremes in the FCN amplitude and phase variations are close to the
GMJ epochs. In this paper, a new evidence of this connection was found. The
large FCN amplitude and phase disturbance occurred at the epoch close to the
newly revealed GMJ 2011. This event occurred to be the second large change in
the FCN amplitude and phase after the 1999 disturbance that is also associated
with the GMJ 1999. Moreover, the long-time FCN phase drift had changed suddenly
in 1998--1999, immediately before the GMJ 1999, and seemed to change again at
the epoch immediately preceding the GMJ 2011. The FCN amplitude showed a
general long-time decrease before GMJ 1999, and it subsequently grew until GMJ
2011, and then seemed to decrease again. A smaller FCN change can be observed
at the epoch around 2013, which is also suspected as the GMJ epoch. The latter
confirms the suggestion that a rapid change in the FCN amplitude and/or phase
can be used as an evidence of the GMJ that is not clearly detected from the
geomagnetic observations
A new version of the OCARS catalog of Optical Characteristics of Astrometric Radio Sources
A new version of the Optical Characteristics of Astrometric Radio Sources
(OCARS) catalog is presented. This compiled catalog includes radio sources
observed in different VLBI programs and experiments that result in source
position determination, their redshift, and photometry in the visible and
near-infrared bands. A cross-identification table between the OCARS and other
catalogs is also provided. The status of the catalog as of 2018 September 7 is
described in this paper. The OCARS catalog currently contains 6432 sources, of
which 3895 have redshift data and 5479 have photometric data. Compared with the
previous version, the current version has been enriched with extended redshift
and photometry information, and cross-identification with several catalogs in
radio, optical, infrared, ultraviolet, X-ray, and gamma-ray bands. The OCARS
catalog is updated every few weeks on average to incorporate new data that
appear in the NASA/IPAC Extragalactic Database (NED), SIMBAD database, and in
the literature
Application of the Allan Variance to Time Series Analysis in Astrometry and Geodesy: A Review
The Allan variance (AVAR) was introduced 50 years ago as a statistical tool
for assessing of the frequency standards deviations. For the past decades, AVAR
has increasingly being used in geodesy and astrometry to assess the noise
characteristics in geodetic and astrometric time series. A specific feature of
astrometric and geodetic measurements, as compared with the clock measurements,
is that they are generally associated with uncertainties; thus, an appropriate
weighting should be applied during data analysis. Besides, some physically
connected scalar time series naturally form series of multi-dimensional
vectors. For example, three station coordinates time series , , and
can be combined to analyze 3D station position variations. The classical AVAR
is not intended for processing unevenly weighted and/or multi-dimensional data.
Therefore, AVAR modifications, namely weighted AVAR (WAVAR), multi-dimensional
AVAR (MAVAR), and weighted multi-dimensional AVAR (WMAVAR), were introduced to
overcome these deficiencies. In this paper, a brief review is given of the
experience of using AVAR and its modifications in processing astro-geodetic
time series
Improving short-term EOP prediction using combination procedures
A well known problem with EOP prediction is that a prediction strategy proved
to be the best for some testing period and prediction length may not remain as
such for other period of time. In this paper we consider possible strategies to
combine EOP predictions made using different analysis technique to obtain a
final prediction with the best accuracy corresponding to the least prediction
error between input predictions. This approach can be used to improve the
short-term real-time EOP forecast.Comment: 5 pp., presented at the Journees 2008: Astrometry, Geodynamics and
Astronomical Reference Systems, Dresden, Germany, 22-24 Sep 200
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