Summary: It is known that the photon flux and the density of stars on the sky are in principle more than sufficient to calibrate the geometry of the CCDs to the required accuracy in Gaia, even considering that each pixel column must be separately calibrated. However, this assumes that the mean centroid shift can be calibrated, which becomes in practice impossible...
 

Bibtex entry for this abstract:

@UNPUBLISHED{LL:SAG-LL-022,
author = {L.~Lindegren},
title={{C}harge trapping effects in {C}{C}{D}s for {G}aia astrometry},
institution={},
year={1998},
month={September},
url={http://www.rssd.esa.int/doc_fetch.php?id=2844371},
note={SAG-LL-022},
type={Technical note}
}