Title: "GSR - The problem of the Global Astrometric Sphere Reconstruction in Gaia"

　　Speaker: Alberto Vecchiato (Turin astronomical observatory, Italy)

　　Time & Place: Tuesday, 2:30pm, March 18th, 3rd floor big conference room

　　Abstract:

　　"The main goal of the ESA Gaia space mission is to produce a 6D catalog of about 1 billion objects with a measurement accuracy which will make their parallaxes up to the Milky Way scale accessible for the first time. One of the main steps of the data reduction process is the materialization of the catalog reference, that requires the solution of the so-called "Global Astrometric Sphere Reconstruction" problem. As a result of absolute astrometry measurements this task can be conceived as the definition of a unit of measure, with two potentially serious hazards. First, a complete scientific verification of these results can be done only at the price of an experimental effort similar to that of Gaia. Second, any error which might plague the Gaia catalog will likely be transferred to the many scientific results coming from the catalog itself and from its future exploitation, with little or no possibility to be aware of their presence. For this reasons, the consortium in charge for the data reduction, following the previous experience of HIPPARCOS, decided to somewhat "duplicate" the pipeline at some of the most critical points, one of which is the Global Sphere Reconstruction. In this talk we will review the concepts laying at the foundations of this classical astrometric problem, describing the principles used to implement it into the Gaia pipeline, and giving a glance of its two implementations, i.e. AGIS and GSR.