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The European Space Agency (ESA) Gaia satellite has 106 CCD image sensorswhich will suffer from increased charge transfer inefficiency (CTI) as a resultof radiation damage. To aid the mitigation at low signal levels, the CCD designincludes Supplementary Buried Channels (SBCs, otherwise known as `notches')within each CCD column. We present the largest published sample of Gaia CCD SBCFull Well Capacity (FWC) laboratory measurements and simulations based on 13devices. We find that Gaia CCDs manufactured post-2004 have SBCs with FWCs inthe upper half of each CCD that are systematically smaller by two orders ofmagnitude (<50 electrons) compared to those manufactured pre-2004 (thousands ofelectrons). Gaia's faint star (13 < G < 20 mag) astrometric performancepredictions by Prod'homme et al. and Holl et al. use pre-2004 SBC FWCs asinputs to their simulations. However, all the CCDs already integrated onto thesatellite for the 2013 launch are post-2004. SBC FWC measurements are notavailable for one of our five post-2004 CCDs but the fact it meets Gaia's imagelocation requirements suggests it has SBC FWCs similar to pre-2004. It is toolate to measure the SBC FWCs onboard the satellite and it is not possible totheoretically predict them. Gaia's faint star astrometric performancepredictions depend on knowledge of the onboard SBC FWCs but as these arecurrently unavailable, it is not known how representative of the whole focalplane the current predictions are. Therefore, we suggest Gaia's initialin-orbit calibrations should include measurement of the onboard SBC FWCs. Wepresent a potential method to do this. Faint star astrometric performancepredictions based on onboard SBC FWCs at the start of the mission would allowsatellite operating conditions or CTI software mitigation to be furtheroptimised to improve the scientific return of Gaia.

Digging supplementary buried channels: investigating the notch architecture within the CCD pixels on ESA's Gaia satellite