  The European Space Agency's Gaia satellite was launched into orbit around L2
in December 2013 with a payload containing 106 large-format scientific CCDs.
The primary goal of the mission is to repeatedly obtain high-precision
astrometric and photometric measurements of one thousand million stars over the
course of five years. The scientific value of the down-linked data, and the
operation of the onboard autonomous detection chain, relies on the high
performance of the detectors. As Gaia slowly rotates and scans the sky, the
CCDs are continuously operated in a mode where the line clock rate and the
satellite rotation spin-rate are in synchronisation. Nominal mission operations
began in July 2014 and the first data release is being prepared for release at
the end of Summer 2016.
  In this paper we present an overview of the focal plane, the detector system,
and strategies for on-orbit performance monitoring of the system. This is
followed by a presentation of the performance results based on analysis of data
acquired during a two-year window beginning at payload switch-on. Results for
parameters such as readout noise and electronic offset behaviour are presented
and we pay particular attention to the effects of the L2 radiation environment
on the devices. The radiation-induced degradation in the charge transfer
efficiency (CTE) in the (parallel) scan direction is clearly diagnosed;
however, an extrapolation shows that charge transfer inefficiency (CTI) effects
at end of mission will be approximately an order of magnitude less than
predicted pre-flight. It is shown that the CTI in the serial register
(horizontal direction) is still dominated by the traps inherent to the
manufacturing process and that the radiation-induced degradation so far is only
a few per cent. Finally, we summarise some of the detector effects discovered
on-orbit which are still being investigated.
