  The European Space Agency's Gaia mission is scheduled for launch in 2013. It
will operate at L2 for 5 years, rotating slowly to scan the sky so that its two
optical telescopes will repeatedly observe more than one billion stars. The
resulting data set will be iteratively reduced to solve for the position,
parallax and proper motion of every observed star. The focal plane contains 106
large area silicon CCDs continuously operating in a mode where the line
transfer rate and the satellite rotation are in synchronisation.
  One of the greatest challenges facing the mission is radiation damage to the
CCDs which will cause charge deferral and image shape distortion. This is
particularly important because of the extreme accuracy requirements of the
mission. Despite steps taken at hardware level to minimise the effects of
radiation, the residual distortion will need to be calibrated during the
pipeline data processing. Due to the volume and inhomogeneity of data involved,
this requires a model which describes the effects of the radiation damage which
is physically realistic, yet fast enough to implement in the pipeline. The
resulting charge distortion model was developed specifically for the Gaia CCD
operating mode. However, a generalised version is presented in this paper and
this has already been applied in a broader context, for example to investigate
the impact of radiation damage on the Euclid dark-energy mission data.
