Astrometric instrument model software tool for Gaia data reduction: challenges and implementation

The Astrometric Instrument Model system comprises several monitoring and diagnostic tasks for the astrometric instrument aboard Gaia. It is a hierarchy of dedicated software modules aimed at decreasing the parameter degeneration of the relation linking the observations to the instrumental behavior, and optimize the estimation process at the CCD and field-of-view crossing level. Critical for the system is the definition and maintenance of a physical instrument model fitting the science data, and able to accommodate non nominal configurations. Precise modeling of the astrometric response is required for optimal definition of the data reduction and calibration algorithms, and to ensure high sensitivity to both instrumental and astrophysical source parameters.