Geocenter : notion, definition and realization

Comparison between the geophysical theoretical variations of the geocenter and the ones observed by spatial geodesy is not trivial:the geodetic community works in a reference frame not centered to the center of mass and the initial reference sphere used by the geophysicists is not realistic. Within the frame of the french research group AGRET (Astrometrie, Geodynamique et Systemes de Reference), the geodetic, geophysical and astronomical communities worked together in order to propose a definition of the geocenter and to establish the relations between measurements and geophysical reference frame. We present in this paper the mean conclusions of this working group and we propose some recommendations.