116 research outputs found
Physically adequate proper reference system of a test observer and relativistic description of the GAIA attitude
A relativistic definition of the physically adequate proper reference system
of a test observer is suggested within the framework of the PPN formalism.
According to the nomenclature accepted within the GAIA project this reference
system is called Center-of-Mass Reference System (CoMRS). The interrelation
between the suggested definition of the CoMRS and the Resolutions 2000 on
relativity of the International Astronomical Union (IAU) are elucidated. The
tetrad representation of the CoMRS at its origin is also explicated. It is
demonstrated how to use that tetrad representation to calculate the relation
between the observed direction of a light ray and the corresponding coordinate
direction in the Barycentric Celestial Reference System of the IAU. It is
argued that the kinematically non-rotating CoMRS is the natural choice of the
reference system where the attitude of the observer (e.g. of the GAIA
satellite) should be modeled. The relativistic equations of rotational motion
of a satellite relative to its CoMRS are briefly discussed. A simple algorithm
for the attitude description of the satellite is proposed.Comment: 16 page
Relativistic scaling of astronomical quantities and the system of astronomical units
For relativistic modelling of high-accuracy astronomical data several time
scales are used: barycentric and geocentric coordinate times, TCB and TCG, as
well as two additional time scales, TDB and TT, that are defined as linear
functions of TCB and TCG, respectively.
The paper is devoted to a concise but still detailed explanation of the
reasons and the implications of the relativistic scalings of astronomical
quantities induced by the time scales TDB and TT.
We consequently distinguish between quantities and their numerical values
expressed in some units.
It is argued that the scaled time scales, the scaled spatial coordinates and
the scaled masses should be considered as distinct quantities which themselves
can be expressed in any units, and not as numerical values of the same
quantities expressed in some different, non-SI units (``TDB units'' and ``TT
units'').
Along the same lines of argumentation the system of astronomical units is
discussed in the relativistic framework. The whole freedom in the definitions
of the systems of astronomical units for TCB and TDB is demonstrated. A number
of possible ways to freeze the freedom are shown and discussed. It is argued
that in the future one should think about converting AU into a defined quantity
by fixing its value in SI meters
Analysis of astrometric catalogues with vector spherical harmonics
Comparison of stellar catalogues with position and proper motion components
using a decomposition on a set of orthogonal vector spherical harmonics. We
show the theoretical and practical advantages of this technique as a result of
invariance properties and the independence of the decomposition from a prior
model. We describe the mathematical principles used to perform the spectral
decomposition, evaluate the level of significance of the multipolar components
and examine the transformation properties under space rotation. The principles
are illustrated with a characterisation of the systematic effects in the FK5
catalogue compared to Hipparcos and with an application to the extraction of
the rotation and dipole acceleration in the astrometric solution of QSOs
expected from Gaia.Comment: accepted for publication in Astronomy & Astrophysic
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