  Gaia is ESA's ambitious space astrometry mission the main objective of which
is to astrometrically and spectro-photometrically map 1000 Million celestial
objects (mostly in our galaxy) with unprecedented accuracy. The announcement of
opportunity 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. The satellite will downlink close to 100 TB of raw
telemetry data over 5 years. To achieve its required accuracy of a few 10s of
Microarcsecond astrometry, a highly involved processing of this data is
required.
  In addition to the main astrometric instrument Gaia will host a Radial
Velocity instrument, two low-resolution dispersers for multi-color photometry
and two Star Mappers. Gaia is a flying Giga Pixel camera. The various
instruments each require relatively complex processing while at the same time
being interdependent. We describe the overall composition of the DPAC and the
envisaged overall architecture of the Gaia data processing system. We shall
delve further into the core processing - one of the nine, so-called,
coordination units comprising the Gaia processing system.
