arXiv subjects

G. Bourda

Publications and source records attributed to G. Bourda.

18 recordsLinked to original sources

arXiv2020

The third realization of the International Celestial Reference Frame by very long baseline interferometry

A new realization of the International Celestial Reference Frame (ICRF) is presented based on the work achieved by a working group of the International Astronomical Union (IAU) mandated for this purpose. This new realization, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry. The ICRF3 includes positions at 8.4 GHz for 4536 sources, supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources, for a total of 4588 sources. A subset of 303 sources among these, uniformly distributed on the sky, are identified as "defining sources" and as such serve to define the axes of the frame. Source positions are reported for epoch 2015.0 and must be propagated for observations at other epochs for the most accurate needs, accounting for the acceleration toward the Galactic center, which results in a dipolar proper motion field of amplitude 0.0058 milliarcsecond/yr (mas/yr). The frame shows a median positional uncertainty of about 0.1 mas in right ascension and 0.2 mas in declination, with a noise floor of 0.03 mas in the individual source coordinates. A subset of 500 sources is found to have extremely accurate positions at 8.4 GHz, in the range of 0.03 to 0.06 mas. Comparing ICRF3 with the Gaia Celestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames. Significant positional offsets between the three ICRF3 frequencies are detected for about 5% of the sources. Moreover, a notable fraction (22%) of the sources shows optical and radio positions that are significantly offset. There are indications that these positional offsets may be the manifestation of extended source structures. This third realization of the ICRF was adopted by the IAU at its 30th General Assembly in August 2018 and replaced the previous realization, ICRF2, on January 1, 2019.

astro-ph.GA↗

arXiv2018

Gaia Data Release 2: The astrometric solution

Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these results performed within the astrometry task. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G<14 mag) sources, 0.1 mas at G=17 mag, and 0.7 mas at G=20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas/yr, respectively. The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas/yr. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas/yr in proper motion are seen on small (<1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.

astro-ph.IM↗

arXiv2016

Gaia Data Release 1: The reference frame and the optical properties of ICRF sources

As part of the data processing for Gaia Data Release~1 (Gaia DR1) a special astrometric solution was computed, the so-called auxiliary quasar solution. This gives positions for selected extragalactic objects, including radio sources in the second realisation of the International Celestial Reference Frame (ICRF2) that have optical counterparts bright enough to be observed with Gaia. A subset of these positions was used to align the positional reference frame of Gaia DR1 with the ICRF2. We describe the properties of the Gaia auxiliary quasar solution for a subset of sources matched to ICRF2, and compare their optical and radio positions at the sub-mas level. Their formal standard errors are better than 0.76~milliarcsec (mas) for 50% of the sources and better than 3.35~mas for 90%. Optical magnitudes are obtained in Gaia's unfiltered photometric G band. The comparison with the radio positions of the defining sources shows no systematic differences larger than a few tenths of a mas. The fraction of questionable solutions, not readily accounted for by the statistics, is less than 6%. Normalised differences have extended tails requiring case-by-case investigations for around 100 sources, but we have not seen any difference indisputably linked to an optical-radio offset in the sources.

astro-ph.IM↗

arXiv2016

Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes

Gaia Data Release 1 (Gaia DR1) contains astrometric results for more than 1 billion stars brighter than magnitude 20.7 based on observations collected by the Gaia satellite during the first 14 months of its operational phase. We give a brief overview of the astrometric content of the data release and of the model assumptions, data processing, and validation of the results. For stars in common with the Hipparcos and Tycho-2 catalogues, complete astrometric single-star solutions are obtained by incorporating positional information from the earlier catalogues. For other stars only their positions are obtained by neglecting their proper motions and parallaxes. The results are validated by an analysis of the residuals, through special validation runs, and by comparison with external data. Results. For about two million of the brighter stars (down to magnitude ~11.5) we obtain positions, parallaxes, and proper motions to Hipparcos-type precision or better. For these stars, systematic errors depending e.g. on position and colour are at a level of 0.3 milliarcsecond (mas). For the remaining stars we obtain positions at epoch J2015.0 accurate to ~10 mas. Positions and proper motions are given in a reference frame that is aligned with the International Celestial Reference Frame (ICRF) to better than 0.1 mas at epoch J2015.0, and non-rotating with respect to ICRF to within 0.03 mas/yr. The Hipparcos reference frame is found to rotate with respect to the Gaia DR1 frame at a rate of 0.24 mas/yr. Based on less than a quarter of the nominal mission length and on very provisional and incomplete calibrations, the quality and completeness of the astrometric data in Gaia DR1 are far from what is expected for the final mission products. The results nevertheless represent a huge improvement in the available fundamental stellar data and practical definition of the optical reference frame.

astro-ph.GA↗

arXiv2015

The ICRF-3: Status, plans, and progress on the next generation International Celestial Reference Frame

The goal of this presentation is to report the latest progress in creation of the next generation of VLBI-based International Celestial Reference Frame, ICRF3. Two main directions of ICRF3 development are improvement of the S/X-band frame and extension of the ICRF to higher frequencies. Another important task of this work is the preparation for comparison of ICRF3 with the new generation optical frame GCRF expected by the end of the decade as a result of the Gaia mission.

astro-ph.IM↗

arXiv2014

Future mmVLBI Research with ALMA: A European vision

Very long baseline interferometry at millimetre/submillimetre wavelengths (mmVLBI) offers the highest achievable spatial resolution at any wavelength in astronomy. The anticipated inclusion of ALMA as a phased array into a global VLBI network will bring unprecedented sensitivity and a transformational leap in capabilities for mmVLBI. Building on years of pioneering efforts in the US and Europe the ongoing ALMA Phasing Project (APP), a US-led international collaboration with MPIfR-led European contributions, is expected to deliver a beamformer and VLBI capability to ALMA by the end of 2014 (APP: Fish et al. 2013, arXiv:1309.3519). This report focuses on the future use of mmVLBI by the international users community from a European viewpoint. Firstly, it highlights the intense science interest in Europe in future mmVLBI observations as compiled from the responses to a general call to the European community for future research projects. A wide range of research is presented that includes, amongst others: - Imaging the event horizon of the black hole at the centre of the Galaxy - Testing the theory of General Relativity an/or searching for alternative theories - Studying the origin of AGN jets and jet formation - Cosmological evolution of galaxies and BHs, AGN feedback - Masers in the Milky Way (in stars and star-forming regions) - Extragalactic emission lines and astro-chemistry - Redshifted absorption lines in distant galaxies and study of the ISM and circumnuclear gas - Pulsars, neutron stars, X-ray binaries - Testing cosmology - Testing fundamental physical constants

astro-ph.IM↗

arXiv2008

Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame

The ICRF, currently based on the position of 717 extragalactic radio sources observed by VLBI, is the fundamental celestial reference frame adopted by the IAU in 1997. Within the next 10 years, the European space astrometry mission Gaia, to be launched by 2011, will permit determination of the extragalactic reference frame directly in the visible for the first time. Aligning these two frames with the highest accuracy will therefore be very important in the future for ensuring consistency between the measured radio and optical positions. This paper is aimed at evaluating the current astrometric suitability of the individual ICRF radio sources which are considered appropriate for the alignment with the future Gaia frame. To this purpose, we cross-identified the ICRF and the optical catalog Véron-Cetty and Véron (2006), to identify the optically-bright ICRF sources that will be positioned with the highest accuracy with Gaia. Then we investigated the astrometric suitability of these sources by examining their VLBI brightness distribution. We identified 243 candidate ICRF sources for the alignment with the Gaia frame (with an optical counterpart brighter than the apparent magnitude 18), but only 70 of these (10% of the ICRF sources) are found to have the necessary high astrometric quality (i.e. a brightness distribution that is compact enough) for this link. Additionally, it was found that the QSOs that will have the most accurate positions in the Gaia frame tend to have less-accurate VLBI positions, most probably because of their physical structures. Altogether, this indicates that identifying other high-quality VLBI radio sources suitable for the alignment with the future Gaia frame is mandatory.

astro-ph↗

arXiv2008

Multi-step VLBI observations of weak extragalactic radio sources to align the ICRF and the future GAIA frame

The space astrometry mission GAIA will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the GAIA frame and the International Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is found that only 10% of the ICRF sources are suitable to establish this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional suitable radio sources for aligning the two frames. The sample consists of about 450 sources, typically 20 times weaker than the current ICRF sources (down to the 20 mJy flux level), which have been selected by cross-correlating optical and radio catalogues. This paper presents the observing strategy to detect, image, and measure accurate positions for these sources. It will also provide results about the VLBI detectability of the sources, as derived from initial observations with the European VLBI Network in June and October 2007. Based on these observations, an excellent detection rate of 89% is found, which is very promising for the continuation of this project.

astro-ph↗

arXiv2008

A Vlbi Survey of Weak Extragalactic Radio Sources for the Alignment of the Icrf and the Future Gaia Frame

The space astrometry mission GAIA will construct a dense optical QSO-based celestial reference frame. For consistency between the optical and radio positions, it will be important to align the GAIA frame and the International Celestial Reference Frame (ICRF) with the highest accuracy. Currently, it is found that only 10% of the ICRF sources are suitable to establish this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional high-quality radio sources for aligning the two frames. The sample consists of about 450 sources, typically 20 times weaker than the current ICRF sources, which have been selected by cross-correlating optical and radio catalogues. The paper presents the observing strategy and includes preliminary results of observation of 224 of these sources with the European VLBI Network in June 2007.

astro-ph↗

arXiv2008

Analysis and comparison of precise long-term nutation series, strictly determined with OCCAM 6.1 VLBI software

The IAU/IUGG Working Group on "Nutation for a non-rigid Earth", led by Véronique Dehant, won the European Descartes Prize in 2003, for its work developing a new model for the precession and the nutations of the Earth. This model (MHB2000, Mathews et al. 2002) was adopted by the IAU (International Astronomical Union) during the General Assembly in Manchester, in 2000. It is based (i) on some improvements for the precession model (with respect to the previous one of Lieske et al. 1977) owing to the VLBI technique, and (ii) on a very accurate nutation model, close to the observations. With this prize, the Descartes nutation project could offer for international scientists some grants, to be used for further improvements of the precession-nutation Earth model. At the IGG (Institute of Geodesy and Geophysics), with the OCCAM 6.1 VLBI analysis software and the best data and models available, we re-analyzed the whole VLBI sessions available (from 1985 till 2005) solving for the Earth Orientation Parameters (EOP). In this paper we present the results obtained for the EOP and more particularly for the nutation series. We compare them with the other IVS analysis centers results, as well as with the IVS combined EOP series from the analysis coordinator. The series are in good agreement, except for the polar motion coordinates that show a shift with respect to the other ones and that we discuss here. Finally, we analyse the nutation series in the framework of the free core nutation (FCN) study and modelisation.

astro-ph↗

arXiv2008

GINS: a new tool for VLBI Geodesy and Astrometry

In the framework of the "Groupe de Recherches de Géodésie Spatiale" (GRGS), a rigorous combination of the data from five space geodetic techniques (VLBI, GPS, SLR, LLR and DORIS) is routinely applied to simultaneously determine a Terrestrial Reference Frame (TRF) and Earth Orientation Parameters (EOP). This analysis is conducted with the software package GINS which has the capability to process data from all five techniques together. Such a combination at the observation level should ultimately facilitate fine geophysical studies of the global Earth system. In this project, Bordeaux Observatory is in charge of the VLBI data analysis, while satellite geodetic data are processed by other groups. In this paper, we present (i) details about the VLBI analysis undertaken with GINS, and (ii) the results obtained for the EOP during the period 2005-2006. We also compare this EOP solution with the IVS (International VLBI Service for geodesy and astrometry) analysis coordinator combined results. The agreement is at the 0.2 mas level, comparable to that of the other IVS analysis centers, which demonstrates the capability of the GINS software for VLBI analysis.

astro-ph↗

arXiv2008

VLBI analyses with the GINS software for multi-technique combination at the observation level

A rigorous approach to simultaneously determine a Terrestrial Reference Frame (TRF) and Earth Orientation Parameters (EOP) is now currently applied on a routine basis in a coordinated project within the Groupe de Recherches de Géodésie Spatiale (GRGS) in France. Observations of the various space geodetic techniques (VLBI, SLR, LLR, DORIS and GPS) are separately processed by different analysis centers with the software package GINS-DYNAMO, developed and maintained at the GRGS/CNES (Toulouse). This project is aimed at facilitating fine geophysical analyses of the global Earth system (GGOS project). In this framework, Bordeaux Observatory is in charge of the VLBI (Very Long Baseline Interferometry) analyses with GINS for combination with the data of the other space geodetic techniques at the observation level. In this paper, we present (i) the analyses undertaken with this new VLBI software, and (ii) the results obtained for the EOP from beginning 2005 until 2007. Finally, we compare this EOP solution with the IVS (International VLBI Service) Analysis Coordinator combined results. The agreement is at the 0.2 mas level, comparable to that of the other IVS Analysis Centers, which demonstrates the VLBI capability of the GINS software.

astro-ph↗

arXiv2007

Gravitational potential, inertia and Earth rotation

Several satellite missions, devoted to the study of the Earth gravity field, have been launched (like CHAMP, recently). This year, GRACE (Gravity Recovery and Climate Experiment) will allow us to obtain a more precise geoid. But the most important is that they will supply the temporal variations of the geopotential coefficients (called Stokes coefficients). In the poster we show how the Earth gravitational potential is linked to the Earth rotation parameters. Indeed, through the Earth inertia coefficients, we can connect the variation of LOD and Polar Motion with the temporal variations of the Stokes coefficients. We also consider the nutations, that are related to the gravitational geopotential coefficients. We discuss the possibility of using the Stokes coefficients in order to improve our knowledge of the Earth rotation.

astro-ph↗

arXiv2007

Temporal variations of the gravity field and Earth precession-nutation

Due to the accuracy now reached by space geodetic techniques, and also considering some modelisations, the temporal variations of some Earth Gravity Field coefficients can be determined. They are due to Earth oceanic and solid tides, as well as geophysical reservoirs masses displacements. They can be related to the variations in the Earth's orientation parameters (through the inertia tensor). Then, we can try to improve our knowledge of the Earth Rotation with those space measurements of the Gravity variations. We have undertaken such a study, using data obtained with the combination of space geodetic techniques. In particular, we use CHAMP data that are more sensitive to such variations and that complete the ones already accumulated (for example with Starlette and LAGEOS I). In this first approach, we focus on the Earth precession nutation, trying to refine it by taking into account the temporal variations of the Earth dynamical flattening. The goal is mainly to understand how Geodesy can influence this field of science. Like this, we will be able to compare our computation with up to date determinations of precession nutation.

astro-ph↗

arXiv2007

Length of day and polar motion, with respect to temporal variations of the Earth gravity field

The masses distribution inside the Earth governs the behaviour of the rotation axis in the Earth (polar motion), as well as the Earth rotation rate (or equivalently, length of day). This masses distribution can be measured from space owing to artificial satellites, the orbitography of which provides the Earth gravity field determination. Then, the temporal variations of the Earth gravity field can be related to the variations of the Earth Orientation Parameters (EOP) (with the Inertia Tensor). Nowadays, owing to the satellite laser ranging (SLR) technique and to the new gravimetric satellite missions (such as CHAMP or GRACE), the temporal variations of the low degree coefficients of the Earth gravity field (i.e. Stokes coefficients) can be determined. This paper is one of the first study using gravity variations data in the equations already established (e.g. Lambeck 1988) and linking the variations of the length of day and of the C20 Stokes coefficient (or, linking the polar motion and the C21 and S21 coefficients). This paper combines the Earth rotation data (mainly obtained from VLBI and GPS measurements) and the Earth gravity field variations ones (e.g. Lageos I and II data, or GRACE data), in order to complete and constrain the models of the Earth rotation. The final goal is a better Earth global dynamics understanding, which possible application can be the constraint on the couplings into the Earth system (e.g. core-mantle couplings).

astro-ph↗

arXiv2007

Earth rotation and variations of the gravity field, in the framework of the "Descartes-nutation" project

The "Descartes-Nutation" Project is devoted to the "understanding of the next decimal of precession-nutation, from the theoretical point of view as well as from the observational point of view". In this framework, we made a proposal in order to contribute to the study of (i) the dynamical flattening of the Earth, (ii) the coupling effects of the lunisolar forcing, (iii) the effect of the geophysical fluids on the EOP and (iv) the Nutation observations. We investigate further the links between Earth Orientation and Gravity Field Variations. Indeed, the masses distributions inside the Earth govern the behaviour of the rotation axis in space (precession-nutation) and in the Earth (polar motion), as well as the Earth rotation rate (or equivalently, length of the day). These distributions of masses can be measured by space owing to artificial satellites, the orbitography of which provides the Earth gravity field determination. Then, the temporal variations of the Earth gravity field can be related to the variations of the Earth Orientation Parameters (EOP) (with the Inertia Tensor). Nowadays, the Earth orientation measurements in space, obtained with Very Long Baseline Interferometry (VLBI), have a precision better than the milliarcsecond level. It is then necessary to consider all the geophysical sources that can improve the models precision. The goal of my PhD Thesis was to use the Earth gravity field measurements, as well as its variations, as a tool to improve the Earth orientation modelisation. We present here the results obtained as well as various proposals to extend these investigations (numerical studies, the use of J2 geophysical series, integration of GRACE data ...).

astro-ph↗

arXiv2007

Earth Orientation and Temporal Variations of the Gravity Field

The high accuracy now reached in the VLBI Earth Orientation Parameters (EOP) determination requires looking further at the various geophysical contributions to variations in EOP. The determination of the Earth gravity field from space geodetic techniques now allows us to obtain the temporal variations of the low degree coefficients of the geopotential, combining the treatment of different satellites (e.g. Lageos1, Lageos2, Starlette ...). We present a new computation of the degree 2 coefficients of the variable Earth gravity field. This study is based upon the using of (i) new orbit standards, (ii) the GRACE mean gravity field and (iii) Lageos1 data from 1985 until 2004 (merged with Lageos2 data from 1993). These temporal variations of the Earth gravity field can be related to the Earth Orientation Parameters through the inertia tensor. This paper shows these relations and discusses how such geodetic data can contribute to the understanding of the variations in EOP. This paper also studies if these refined 20-yr of Lageos data can give us refined value of the 18.6-yr tidal term, as well as of the secular drift of the C20 geopotential coefficient.

astro-ph↗

arXiv2007

Precession, nutation, and space geodetic determination of the Earth's variable gravity field

Precession and nutation of the Earth depend on the Earth's dynamical flattening, H, which is closely related to the second degree zonal coefficient, J2 of the geopotential. A small secular decrease as well as seasonal variations of this coefficient have been detected by precise measurements of artificial satellites (Nerem et al. 1993; Cazenave et al. 1995) which have to be taken into account for modelling precession and nutation at a microarcsecond accuracy in order to be in agreement with the accuracy of current VLBI determinations of the Earth orientation parameters. However, the large uncertainties in the theoretical models for these J2 variations (for example a recent change in the observed secular trend) is one of the most important causes of why the accuracy of the precession-nutation models is limited (Williams 1994; Capitaine et al. 2003). We have investigated in this paper how the use of the variations of J2 observed by space geodetic techniques can influence the theoretical expressions for precession and nutation. We have used time series of J2 obtained by the "Groupe de Recherches en Géodésie spatiale" (GRGS) from the precise orbit determination of several artificial satellites from 1985 to 2002 to evaluate the effect of the corresponding constant, secular and periodic parts of H and we have discussed the best way of taking the observed variations into account. We have concluded that, although a realistic estimation of the J2 rate must rely not only on space geodetic observations over a limited period but also on other kinds of observations, the monitoring of periodic variations in J2 could be used for predicting the effects on the periodic part of the precession-nutation motion.

astro-ph↗