GINS: a new tool for VLBI Geodesy and Astrometry

In the framework of the "Groupe de Recherches de G\'eod\'esie 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

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 G\'eod\'esie 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

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

International audienceThe "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 ...)

Earth Orientation and Temporal Variations of the Gravity Field

National audienceThe 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

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

National audienceThe 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)

Gravitational potential, inertia and Earth rotation

International audienceSeveral 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