AGRET

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ASTROMETRY,GEODYNAMICS

AND REFERENCE SYSTEMS

- Summary -

The French Centre National de la Recherche Scientifique (CNRS) created an interdisciplinary research group (Groupement de Recherches, GDR) that will foster and coordinate studies of topics in geodynamics topics that are playing a role in the realisation and maintenance of space reference frames.

Understanding the dynamics of natural phenomena implies that abstract metrological conceptions be applied to real objects in the universe that are both moving and deforming. The informations and the know-how that are necessary for achieving an optimal trade-off inside this contradicting situation are developped independently in various laboratories. Every one knows one aspect of the problem and tries to elaborate reasonable solutions that are optimized from the viewpoint of his own needs. The need for establishing close cooperation is more often stressed than met. The new GDR, "Astrometry, Geodynamics and Reference Systems (AGRET)" will be devoted to establishing such working interactions.

About twenty institutes and laboratories will be involved. They belong to a variety of administrations: CNRS, Universities, Agencies (CNES, IGN, SHOM, IRD). The AGRET activities will be somewhat parallel to, and coordinated with, the CNES-related GRGS (Groupe de Recherches de Géodésie Spatiale), with particular emphasis put on the ground-based activities.

Studies will concentrate on the following three major topics.

Unification of terrestrial and celestial reference frames, with topics such as the tie of the extragalactic and solar system-based celestial reference frames (Lunar Laser ranging, ms pulsars), the time evolution of the tie of the galactic celestial reference frame with the extragalctic one, and the unification of space references derived from satellite geodesy and Very Long Baseline radio Interferometry.

Vertical motions and references.. The precise and accurate vertical land motions are key parameters for sea level rise estimations. They are complex products of satellite geodesy analysis. They require the knowledge of global deformations of the Earth under effects such as ocean or atmospheric loading, as well as clear understanding of local effetcs. Achieving accuracy, time stability and geophysical reliability of the results requires a close interdisciplinary cooperation.

Motions of the geocenter. By the very nature of the physics involved in satellite dynamics, satellite geodesy data analysis gives access to an instantaneous geocenter , while terrestrial frame analysis relies on an origin of axes related to another, less perturbed, location of the geocenter relative to the crust. Considering the high precision of the satellite geodesy results and the availability of models to predict the effects of mass redistributions on the surface of the Earth, this problem offers a unique opportunity to crosscheck global dynamics and reference frame definitions.

In addition to these topics, special attention will be given to the scientific programs that should be supported by mobile instruments such as the portable absolute gravimeter and laser ranging station, and to the French participations in the international programs and services related to astrometry, geodynamics and reference systems.