  The European Space Agency Gaia satellite was launched into orbit around L2 in
December 2013. This ambitious mission has strict requirements on residual
systematic errors resulting from instrumental corrections in order to meet a
design goal of sub-10 microarcsecond astrometry. During the design and build
phase of the science instruments, various critical calibrations were studied in
detail to ensure that this goal could be met in orbit. In particular, it was
determined that the video-chain offsets on the analogue side of the
analogue-to-digital conversion electronics exhibited instabilities that could
not be mitigated fully by modifications to the flight hardware. We provide a
detailed description of the behaviour of the electronic offset levels on
microsecond timescales, identifying various systematic effects that are known
collectively as offset non-uniformities. The effects manifest themselves as
transient perturbations on the gross zero-point electronic offset level that is
routinely monitored as part of the overall calibration process. Using in-orbit
special calibration sequences along with simple parametric models, we show how
the effects can be calibrated, and how these calibrations are applied to the
science data. While the calibration part of the process is relatively
straightforward, the application of the calibrations during science data
processing requires a detailed on-ground reconstruction of the readout timing
of each charge-coupled device (CCD) sample on each device in order to predict
correctly the highly time-dependent nature of the corrections. We demonstrate
the effectiveness of our offset non-uniformity models in mitigating the effects
in Gaia data. We demonstrate for all CCDs and operating instrument and modes on
board Gaia that the video-chain noise-limited performance is recovered in the
vast majority of science samples.
