in this configuration, a new AOCS survival mode was developed and implemented in the central software; this mode uses the torque authority provided by a slight misalignment of thrusters (originally designed only for attitude-control manoeuvres) to maintain three-axis spacecraft control in case of thruster 3A failure.

Second, the biases of the mass-flow sensors of the micropropulsion thrusters, which means the offsets achieved with zero cold-gas mass flow, were found to be drifting.

Such a drift in itself can be calibrated, although initially at the expense of science time because such a calibration initially required switching to chemical-propulsion control.

Although the fear was that the drift would exceed the dynamic range of the offset measurement circuit, which is ±400 mV.

With time progressing and constant operation of the B branch, however, the offset drifts of the various thrusters have stabilised to values within the range that can be calibrated.

The most probable root cause of the drift is incomplete pre-launch annealing of a (or some) resistor(s) in the mass-flow measurement chain.

Third, the spacecraft rotation rate was frequently found, of order once per minute, to be changing rapidly by typically up to a few milliarcsec per second and then quickly back to the rate before the excursion.

From the characteristics of the ratechange signature, it is clear that these events are caused by sudden, minute structural changes (mass displacements) within the spacecraft causing a quasi-instantaneous discontinuity in the spacecraft attitude; these rate spikes are hence named microclanks in contrast to micro-meteoroid hits, which cause a sudden input of angular momentum and hence permanently change the spin rate of the spacecraft.

Micro-clanks are observed both in the along-scan and across-scan direction, and they are often repeated (quasi-)periodically with the spin period of the spacecraft.

In the along-scan direction, the vast majority of micro-clanks affect both fields of view equally and simultaneously, with no discernible effect in the BAM data, suggesting their origin is outside of the optical instrument.

For a small fraction of events, however, the occurrence times coincide with jumps in the BAM fringeposition data (see below), suggesting that these events originate within the mechanical structure of the optics.

Micro-clanks have also been detected in Hipparcos data (van Leeuwen 2007) and, for that mission, have been attributed to small mechanical adjustments in the hinges of the solar-panel wings created by the vary-
ing amount of sunlight falling upon the wings over the rotation period of the Hipparcos spacecraft.

For Gaia, something similar is likely happening but then primarily involving the bottom of the spacecraft (i.e. sunshield, launcher-interface ring, and/or phased-array antenna).

Micro-clanks are easy to identify from data derived from CCD transit times and will be calibrated out in the preprocessing of the attitude data (see Lindegren et al. 2016).

In addition to the above, three issues affecting payload performance were uncovered during commissioning: contamination (Sect. 4.2.1), straylight (Sect. 4.2.2), and periodic basic angle variations (Sect. 4.2.3).
4.2.1.

Contamination Soon after launch, it was discovered that the optics have timevariable, degrading transmission because of continued contamination by water ice.

The transmission loss is wavelength dependent and, hence, different in the different instruments and also this loss varies with detector position in the focal plane within a given instrument.

To restore the telescope throughput, three payload decontaminations were performed during commissioning (7 February, 13 March, and 30 June 2014) in which the focal plane and/or (selected) telescope mirrors were actively heated to sublimate the contaminant and let it escape through the apertures to space.

With each decontamination, the rate of contamination was reduced and the stable period without noticeable contamination build-up lasted longer (see also Sect. 6.4 and Fig. 8).

This indicates that the source of the contamination, suspected to be slowly releasing trapped air (water vapour) within multilayer insulation blankets and/or carbon fibre-reinforced polymer structural parts, is drying up.
4.2.2.

Straylight Soon after payload was switched on, it was discovered that straylight levels are modulated with the spacecraft rotation and some two orders of magnitude higher than expected.

The origin of the straylight has been traced back foremost to scattered sunlight and secondly to the integrated brightness (and extremely bright stars) of the Milky Way, the light of which can reach the focal plane through a few unbaffled straylight paths.