GEPI

A central component of the scientific data processing for Gaia is the astrometric core solution, which is a simultaneous least-squares estimation of a very large number of parameters describing the satellite attitude, the geometric calibration of the instrument, and the astrometric parameters for a subset of the stars. Although the mathematical formulation as a general minimization problem is simple and has been known for a long time, it is only recently that the practical solution has developed from an approximate ad hoc procedure into a rigorous and well-understood method. After outlining the background to this problem, I highlight some of the interesting developments and current status.