The Gaia mission simulator
X. Luri, E. Masana, P. Altamirano, Y. Isasi, and C. Fabricius
Dept. Astronomia i Meteorologia, Universitat de Barcelona, Martı́ Franquès 1,
08028 Barcelona
xluri@am.ub.es
Summary. The treatment and reduction of the Gaia mission data is a huge computational challenge. The observations of more that a billion objects will generate
about 100TB of raw data and its reduction process will require about 1020 flops.
The design and validation of the viability of this reduction process has required
the implementation of a mission simulator to generate simulated telemetry allowing
to test the algorithms used for the reduction. The complexity of these algorithms and
the amount of data to handle will increase progressively in the coming years until a
full mission simulation is created before launch for an end-to-end final verification
of the system.
These simulated data are used, among other things, to validate the concept
of Global Iterative Solution (GIS) that provides the astrometric and photometric
parameters of the observed objects, as well as the instrument calibrations.
The Gaia simulator is developed in Java and, given its complexity and the
amount of data generated, it is executed in the Mare Nostrum supercomputer of
the Barcelona Supercomputer Centre (BSC). Each simulation uses some hundreds
of processors, each one simulating a mission interval. This contribution provides an
overview of the simulator design, of the team developing it and of the current results.
1 Introduction
Gaia is an astrometric mission of the European Space Agency (ESA), approved
by ESA’s Science Programme Committee in 2000, and redesigned in 2002 as a
consequence of a cost-reduction exercise. Gaia is now completing its B2 phase
and thus almost ready to start its construction. The launch data has been
fixed by end 2011. More details about the mission can be found in [4]
The mission
The Gaia payload consists of three distinct instruments mounted on a single
optical bench. Unlike HST and SIM, which are pointing instruments observing
a pre-selected list of objects, Gaia is a scanning satellite that will repeatedly
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X. Luri, E. Masana, P. Altamirano, Y. Isasi, and C. Fabricius
survey in a systematic way the whole sky, tying together without regional
errors widely separated sources. A brief description of the Gaia main performances can be summarized with a few numbers: a survey to V = 20 of all
point sources amounting to more than one billion objects, with an astrometric
accuracy of 25 mas at V = 15 and 7 mas for the few million stars brighter
than V = 13; radial velocities down to V = 17, with an accuracy ranging from
1 to 10 kms−1 ; multi-epoch spectrophotometry sampling from the visible to
the near IR.
Beyond its sheer measurement accuracy, the major strength of Gaia follows
from (i) its capability to perform an all-sky and sensitivity limited absolute
astrometry survey, (ii) the unique combination into a single spacecraft of the
three major electronic detectors carrying out nearly contemporaneous observations, (iii) the huge number of objects and observations allowing to amplify
the accuracy on single objects to large samples with deep statistical significance, a feature immensely valuable for astrophysics and not shared by a
mission like SIM, a pointed mission able to concentrate on few sources with
high astrophysical potential.
The science case
The immediate objectives of space astrometry are, in principle, the same as
those of ground based astrometry: to determine the apparent positions of celestial bodies and derive from them astrophysically important parameters such
as distances, proper motions, movements within double and multiple star systems. However, the current generation of CCD-based astrometric instruments
can also be used as stable photometers to acquire photometric measurements.
Such an approach has appeared in every proposal of a scanning mission having astrometry as a first goal, like the German DIVA mission or the different
versions of the US led project FAME. In addition Gaia carries a dedicated
spectrometer allowing to determine radial velocities for at least 100 million
stars.
Whereas the direct product of the Gaia mission will be a highly accurate
astrometric and photometric survey to V = 20 mag, the true science goals are
much broader and account for the support of a large scientific community. A
top-level summary is as follows:
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Mapping the Milky Way in three dimensions (parallaxes, positions, extinction)
Galactic kinematics (proper motions and radial velocities)
Formation and evolution of the Milky Way
Stellar physics (classification, M , L, log g, Teff , [Fe/H] )
Distance scale (geometric distances to Cepheids and RR Lyrae, HR diagram)
Age of the Universe (cluster diagrams, distances, luminosity)
Dark matter (stars as tracers of gravitational potential)
The Gaia mission simulator
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Reference frame (quasars, absolute astrometry)
Extrasolar planet detection (∼ MJ , astrometry and photometric transits)
Fundamental physics (relativistic parameters γ ∼ 5×10−7 , β ∼ 5×10−4 )
Solar system science (taxonomy, masses, orbits for 5 × 105 bodies)
Gaia will measure the positions, distances, space motions, and many physical characteristics of about one billion stars in our Galaxy and beyond. Gaia
will provide the detailed 3-D distributions and space motions of all these stars,
complete to 20-th magnitude. The measurement precision, reaching a few millionths of a second of arc, will be unprecedented. This will allow our Galaxy
to be mapped, for the first time, in three dimensions. Some millions of stars
will be measured with a distance accuracy of better than 1 per cent; some 100
million to better than 10 per cent. Compared to Hipparcos, Gaia will improve
parallax and proper-motion accuracy almost a 100 times and the number of
stars observed 10,000 times. It will add the measurement of radial velocities
and extended photometry for all the sources. At the same time Gaia will survey a vast population of solar system bodies (major planets, natural satellites,
comets, and asteroids, including several thousand near-Earth objects) and extragalactic objects (half a million quasars and thousands of supernovae). Gaia
will address all these fields, covering a significant part of modern astrophysics.
2 The data reduction
Gaia will produce an impressive (by current standards) volume of raw data
with about 50 GB of uncompressed science data per day yielding at mission
completion a telemetry volume of roughly 100 TB. The greater part of this
data volume consists of hard coded numbers representing photon counts registered by the video chains of the CDDs in the astrometric, photometric, and
RVS fields of Gaia, altogether amounting to a Giga pixels camera. Although
all the science content is there, it is deeply hidden in the data coding and
measurement process. Transforming the raw data into scientifically meaningful quantities such as parallaxes, magnitudes and radial velocities is the
goal of the Gaia data processing. It is a huge and challenging task in many
respects: mathematics, physical modelling, computer programming and computing, data management, storage and access, international funding and a bit
of sociology.
Data processing
On a global scale the Gaia data processing problem can be viewed as an iterative adjustment of a very large number of instrumental, astrometric and
photometric parameters to the entire mission data set. Part of the adjusted
parameters will form the final science data of the mission. The Gaia data
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X. Luri, E. Masana, P. Altamirano, Y. Isasi, and C. Fabricius
processing consists of several iterative processes dealing with astrometry, photometry and spectroscopy. These iterations reflect the fact that, as a survey
satellite reaching potential accuracy and completeness levels never obtained
before, it has to be self-calibrating in many aspects of its observations
The goal of the data processing is thus to transform the raw data described
above into the Final Science Data. In simplistic terms, the Final Science Data
will consist of an astrometric-spectrophotometric catalogue of approximately
109 sources, based on the ∼ 80 observations made of each source.
The minimum targeted content of the Final Science Data is:
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Identifier
Astrometry
Spectrophotometry
Spectroscopy
Classification
Astrophysical Parameters
The reduction consortia
To carry on the daunting task of reducing the Gaia data, ESA will issue an
Announcement of Opportunity (AO) that will lead to the formation of a panEuropean consortium of scientific teams in charge of the data reduction, the
Gaia Data Processing and Analysis Consortium (DPAC).
The authors are part of a collaboration to build such a consortium to
answer the AO and have been working in the Gaia mission since 1998. The
proposed concept is to build the Gaia DPAC around a series of ‘coordination
units’ (CU), themselves sub-divided into ‘development units’ (DU). The CUs
are small in number, with clearly-defined responsibilities and interfaces, and
their boundaries fit naturally with the main relationships between tasks and
the associated data flow.
The coordination units will have a reasonable amount of autonomy in their
internal organisation and in developing what they consider as the best solution for their task. However they are constrained by the fact that any such
solution has to meet the requirements and time schedules determined by the
Consortium Executive for the overall data processing. The coordination units
reflect the top level structure of the data processing, with well-defined responsibilities and commitments to the DPAC. For practical reasons they are,
in some cases, organized into more manageable components, called development units (DUs). This a more operational level with a lighter management
which will take the responsibility for the development of a specific part of the
software with well defined boundaries.
The authors are integrated in the proposed CU2, whose main task is to
develop a software system capable of covering the simulation needs of the
DPAC through the development of Gaia Simulator, described in the next
section.
The Gaia mission simulator
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3 Overview of the Gaia Simulator
The design of the Gaia Simulator was made with the objective of breaking
down the project into smaller, well-differentiated modules or logical blocks.
In this section, you will find a quick description of the main logical blocks
defined in the design of the simulator.
With the object oriented perspective in mind, most of these blocks were
defined with the aim of including the definitions (in terms of components i and
functionality) of some specific objects, needed in the simulations (for example
an instrument on-board Gaia). At the top level, the Simulator is divided in two
blocks (Figure 1), one of these blocks, called gaiasimu, is devoted to implement
all the necessary element to describe the Gaia system and the universe that
Gaia has to observe in simulations. The second block is composed by data
generators, GASS, GIBIS and GOG.
In addition to these building blocks, the Gaia simulator uses two common libraries developed to cover general DPAC needs: the Gaia Parameter
Database (a compilation of parameters of general use throughout the project)
and GaiaTools (a common library of numerical and astronomical tools).
Fig. 1. Structure of the Gaia simulator
3.1 GaiaSimu
3.2 GASS
The Gaia System Simulator (GASS, see [2]) is the first of the data generators
that are part of the Gaia Simulator. The main objective of GASS is to simulate
the telemetry stream, the scientific and housekeeping data (for instance the
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X. Luri, E. Masana, P. Altamirano, Y. Isasi, and C. Fabricius
attitude) gathered by the spacecraft and sent to ground. Telemetry is the
raw basic data generated by Gaia and contains all the scientific information
of the mission. Due to the big amount of observed sources is not possible in
terms of CPU time (even using big supercomputers) to reproduce the on board
detection and observation process in a very detailed way. The adopted solution
by GASS was carried out the simulations using models of the Universe and
Instruments. The models are build in such a way that ensure those simulations
are statistically compatibles with the true observations.
GASS is used for several purposes as :
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Testing the overall data flow mechanism, before the mission starts.
Test the data handling algorithms (cross matching , telemetry compression,...)
Test the data reduction algorithms, in particular the Global Iterative Solution (GIS).
Test and tune the main database
Estimate the number of objects to be observed by Gaia and the amount
of data to be downlinked to ground.
The GAia System Simulator (GASS) is one of the tools developed to provide realistic simulations of the observation data. In particular, GASS will
generate telemetry data as it will be sent to ground during the actual mission. As part of a more general Gaia Simulator, which includes a pixel data and
an intermediate data generator, GASS implements a set of common packages
and libraries that ensure the use of the same parameters, instrument models
and universe models for all the data generators.
The generation of simulated data in general, and of GASS data in particular, is closely related to the needs of the other CUs. So, as explained in
the introduction of this section, a close interaction between the different CUs
(specifying simulations needs), the DPACE (accepting and assigning priorities
to the CU needs), and the GASS development team of CU2 will be needed.
Goals
The main goal of GASS is to simulate the telemetry stream of the satellite,
using models to generate the observable objects on the sky (Universe Model)
and instrument models. These, in particular the instrument models, assume
some simplifications in order to allow the generation of a large amounts of
simulation data with current computer resources. The goal of GASS is not
the generation of very detailed data at the pixel level (this will be made by
GIBIS), but rather the generation of realistic observation-level data streams
for the 5-year mission.
The data generated by GASS is used for several purposes as:
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to provide basic input data to test the reduction chains in all CUs
The Gaia mission simulator
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to test the data handling algorithms (e.g. cross-matching, telemetry compression, first look)
to estimate the amount of telemetry to be down linked
GASS is developed in Java and uses common tools such as the Gaia Parameter Database and the GaiaTools library in order to ensure coherence
with other simulation modules as GIBIS or GOG, and more generally, with
the data processing environment.
The current version of GASS (GASS 3.0) follows the latest industrial design. However, some simplifications have been made in order to be able to
generate the volume of data corresponding to 5 years of observations with
reasonable computer resources. The simulated data volume can reach a few
hundreds of GB of telemetry data just for simulations limited to stars brighter
than G=12 mag.
3.3 GIBIS
To design and test the performances of the algorithms developed by the DPAC,
simulations as realistic as possible of the sky observed by Gaia and of the
instruments themselves are needed. The Gaia Instrument and Basic Image
Simulator (GIBIS, see [1]) has for task to generate simulated data down to
the maximum level of detail possible. To be able to obtain those data in
a reasonable amount of CPU time, GIBIS should allow to concentrate on
specific aspects of the mission. While GASS, described above, simulates a
huge amount of realistic raw telemetry stream, GIBIS has been created to
simulate a smaller amount of Gaia observations to a greater level of detail,
down to the pixel-level.
Pixel-level simulations will be needed in all studies of specific complexities that the data processing will have to handle. In particular GIBIS will be
needed to study the calibration of the instrument, develop imaging capabilities, reduce crowded fields, analyse the sky background, handle multiple star
contamination of the same observed data, reduce extended objects, etc.
Those detailed GIBIS simulations will be used to provide statistical results
to be applied by GASS for large scale telemetry simulations.
The user can specify the characteristics of the sky portion for which Gaia
observations are requested, the characteristics of the Gaia instruments, the
simulation methods to be used and the selection of the transits and fields to
simulate.
Then for each transit, GIBIS transforms the astrophysical characteristics
of the sources into observables in the fields requested, creates the CCD images
and runs the on-board algorithm prototypes to deliver to the user the resulting
raw Gaia observations per transit.
As most algorithm development will not need directly raw data but calibrated ones with a number of pre-processing already applied (cross-matching,
spectrum wavelength calibration, multiple transit combination, etc.), a calibration step is planned.
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X. Luri, E. Masana, P. Altamirano, Y. Isasi, and C. Fabricius
3.4 GOG
In the previous sections the pixel level data generator (GIBIS) and the telemetry generator (GASS) have been introduced. Although these two elements will
cover a broad range of DPAC needs, a third data generator will be necessary
to the development teams. This third data generator will be named the Gaia
Object Generator (GOG, see [3]).
GOG’s main purpose will be the generation of Main Data Base data (MDB
data). M DB data is the name given to those datasets that are stored in the
main ESAC database and generated in any of the several steps that compose the reduction process, including the Initial Data Treatment. Additionally,
GOG will also catalogue data directly generated by the Universe model.
One should note that MDB data could also be obtained from the telemetry
stream generated by GASS – and in some cases, processed through the main
ESAC database – or after processing GIBIS images. However, this method
could be very time consuming since it would generate a large amount of unneeded information. GOG will be a shorcut designed to avoid this overhead
in an efficient way.
GOG will thus be a tool to directly get catalogue and Main database (MDB)
data without the use of GASS telemetry, GIBIS or the main ESAC database.
Acknowledgements: the work on Gaia of the authors, including the simulator development described here, is funded by the Spanish Plan Nacional del
Espacio through the projects PNE2003-04352, ESP2005-24356-E, PNE200613855-C02-01 and ESP2006-26356-E.
References
1. Babusiaux, C., ”Gaia Instrument and Basic Image Simulator (GIBIS)
overview”, GAIA-C2-TN-OPM-CB-007 (2006)
2. Isasi, Y., Masana, E., Luri, X., ”Gaia System Simulator (GASS) overview”,
GAIA-C2-TN-UB-YI-001 (2006)
3. Masana, E. , Luri, X, Babusiaux, C., ”Proposal for the definition of the Gaia
Object Generator (GOG)”, GAIA-C2-SP-UB-EM-001(2006)
4. M.A.C. Perryman, K.S. de Boer, G. Gilmore, E. Hoeg, M.G. Lattanzi, L. Lindegren, X. Luri, F. mignard, O. Pace & P.T. de Zeeuw: Astron. Astrophys.,
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