83 research outputs found
The Ray Tracing Analytical Solution within the RAMOD framework. The case of a Gaia-like observer
This paper presents the analytical solution of the inverse ray tracing
problem for photons emitted by a star and collected by an observer located in
the gravitational field of the Solar System. This solution has been conceived
to suit the accuracy achievable by the ESA Gaia satellite (launched on December
19, 2013) consistently with the measurement protocol in General relativity
adopted within the RAMOD framework. Aim of this study is to provide a general
relativistic tool for the science exploitation of such a revolutionary mission,
whose main goal is to trace back star directions from within our local curved
space-time, therefore providing a three-dimensional map of our Galaxy. The
results are useful for a thorough comparison and cross-checking validation of
what already exists in the field of Relativistic Astrometry. Moreover, the
analytical solutions presented here can be extended to model other measurements
that require the same order of accuracy expected for Gaia.Comment: 29 pages, 1 figur
Time Transfer functions as a way to validate light propagation solutions for space astrometry
Given the extreme accuracy of modern space astrometry, a precise relativistic
modeling of observations is required. Concerning light propagation, the
standard procedure is the solution of the null-geodesic equations. However,
another approach based on the Time Transfer Functions (TTF) has demonstrated
its capability to give access to key quantities such as the time of flight of a
light signal between two point-events and the tangent vector to its
null-geodesic in a weak gravitational field using an integral-based method. The
availability of several models, formulated in different and independent ways,
must not be considered like an oversized relativistic toolbox. Quite the
contrary, they are needed as validation to put future experimental results on
solid ground. The objective of this work is then twofold. First, we build the
time of flight and tangent vectors in a closed form within the TTF formalism
giving the case of a time dependent metric. Second, we show how to use this new
approach to obtain a comparison of the TTF with two existing modelings, namely
GREM and RAMOD. In this way, we evidentiate the mutual consistency of the three
models, opening the basis for further links between all the approaches, which
is mandatory for the interpretation of future space missions data. This will be
illustrated through two recognized cases: a static gravitational field and a
system of monopoles in uniform motion.Comment: 16 pages, submitted to CQ
The Global sphere reconstruction (GSR) - Demonstrating an independent implementation of the astrometric core solution for Gaia
Context. The Gaia ESA mission will estimate the astrometric and physical data
of more than one billion objects, providing the largest and most precise
catalog of absolute astrometry in the history of Astronomy. The core of this
process, the so-called global sphere reconstruction, is represented by the
reduction of a subset of these objects which will be used to define the
celestial reference frame. As the Hipparcos mission showed, and as is inherent
to all kinds of absolute measurements, possible errors in the data reduction
can hardly be identified from the catalog, thus potentially introducing
systematic errors in all derived work. Aims. Following up on the lessons
learned from Hipparcos, our aim is thus to develop an independent sphere
reconstruction method that contributes to guarantee the quality of the
astrometric results without fully reproducing the main processing chain.
Methods. Indeed, given the unfeasibility of a complete replica of the data
reduction pipeline, an astrometric verification unit (AVU) was instituted by
the Gaia Data Processing and Analysis Consortium (DPAC). One of its jobs is to
implement and operate an independent global sphere reconstruction (GSR),
parallel to the baseline one (AGIS, namely Astrometric Global Iterative
Solution) but limited to the primary stars and for validation purposes, to
compare the two results, and to report on any significant differences. Results.
Tests performed on simulated data show that GSR is able to reproduce at the
sub-as level the results of the AGIS demonstration run presented in
Lindegren et al. (2012). Conclusions. Further development is ongoing to improve
on the treatment of real data and on the software modules that compare the AGIS
and GSR solutions to identify possible discrepancies above the tolerance level
set by the accuracy of the Gaia catalog.Comment: Accepted for publication on Astronomy & Astrophysic
POSSIBLE ASTRONOMICAL MEANINGS OF SOME EL MOLLE RELICS NEAR THE ESO OBSERVATORY AT LA SILLA
Abstact: This paper describes a peculiar, man-made circular stone structure, associated with the ancient rock engravings that are around the site of La Silla in Chile close to the European Southern Observatory, and are attributed to the El Molle Culture. Three stones of the circle, different from all the others, were likely to pinpoint the alignment of three bright stars close to the horizon, as seen from a specific vantage point inside the structure. The El Molle was the only period in which this alignment occurred significantly close to the horizon, moreover it was only in this epoch that it could also be associated with the transition from the warm to the cold season, a period of the year which was quite important for a society that supported itself by herding and farming
The MPI+CUDA Gaia AVU-GSR Parallel Solver in perspective of next-generation Exascale Infrastructures and new Green Computing milestones
We ported on the GPU with CUDA the Gaia Astrometric Verification Unit-Global Sphere Reconstruction (AVU-GSR) Parallel Solver. The code aims to find the astrometric parameters of ~10^8 stars in the Milky Way, the attitude and the instrumental settings of the Gaia satellite, and the global parameter of the PPN formalism, by solving a system of linear equations, × = , with the LSQR iterative algorithm. The coefficient matrix is large, having ~10^11 × 10^8 elements, and sparse. The CUDA code accelerates ≳ 14 times compared to the current version of the AVU-GSR code, parallelized on the CPU with MPI+OpenMP and in production since 2014. This acceleration factor is ~9.2 times larger than the one obtained with a preliminary GPU porting with OpenACC, equal to ~1.5. We obtained this result by running the codes on the CINECA SuperComputer Marconi100, that has 4 NVIDIA Volta V100 GPUs per node, where the MPI+CUDA application has been recently put in production. This analysis represents a first step to understand the exascale behaviour of a class of applications that follow the same structure of this code, employed in several contexts. In the next months, we plan to run this code on the pre-exascale platform Leonardo of CINECA, with 4 next-generation A100 GPUs per node, to better investigate this behaviour. Computing on highly parallel devices, such as GPUs, might imply a consistent power saving, which might go towards the achievement of a Green Computing milestone
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