  The Gaia satellite is a high-precision astrometry, photometry and
spectroscopic ESA cornerstone mission, currently scheduled for launch in late
2011. Its primary science drivers are the composition, formation and evolution
of the Galaxy. Gaia will achieve its scientific requirements with detailed
calibration and correction for radiation damage. Microscopic models of Gaia's
CCDs are being developed to simulate the charge trapping effect of radiation
damage, which causes charge transfer inefficiency. The key to calculating the
probability of a photoelectron being captured by a trap is the 3D electron
density within each CCD pixel. However, this has not been physically modelled
for Gaia CCD pixels. In this paper, the first of a series, we motivate the need
for such specialised 3D device modelling and outline how its future results
will fit into Gaia's overall radiation calibration strategy.
