Gaia is the European Space Agency's (ESA's) ambitious space astrometry mission with a main objective to map astrometrically and spectrophotometrically not less than 1000 million celestial objects in our galaxy with unprecedented accuracy. The announcement of opportunity (AO) for the data processing will be issued by ESA late in 2006. The Gaia Data Processing and Analysis Consortium (DPAC) has been formed recently and is preparing an answer to this AO. The satellite will downlink around 100 TB of raw telemetry data over a mission duration of 5-6 years. To achieve its required astrometric accuracy of a few tens of microarcseconds, a highly involved processing of this data is required. In addition to the main astrometric instrument Gaia will host a radial-velocity spectrometer and two low-resolution dispersers for multi-color photometry. All instrument modules share a common focal plane consisting of a CCD mosaic about 1 m2 in size and featuring close to 10 9 pixels. Each of the various instruments requires relatively complex processing while at the same time being interdependent. We describe the composition and structure of the DPAC and the envisaged overall architecture of the system. We shall delve further into the core processing—one of the nine so-called coordination units comprising the Gaia processing system.
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