Commissioning review: Gaia ready to start routine operations

The commissioning of Gaia came to its formal end on 18 July 2014 when the board members of the mission in-orbit commissioning review (MIOCR) confirmed the readiness of the space and ground segments to start routine operations. The review summarised the commissioning activities both on ground and in orbit. New scientific performance estimates have been calculated since using in-orbit commissioning data.
 
Spacecraft performance
All Gaia subsystems were operated during commissioning and can be used in the routine phase of the mission. Facts with a direct scientific impact are:
 
- all 106 CCDs and associated electronics modules are working and are collecting data;
- the data collection hardware is fully operational and the software has been tuned to match the in-orbit performances;
- the science data from the CCDs is correctly transferred into the mass memory with an assigned priority;
- the priority scheme on board is working correctly with low-priority data being overwritten in case of mass-memory overflow and high-priority data getting precedence in the downlink;
- the telescopes have been aligned and focused for the full focal plane;
- the spacecraft spin rate has been matched to the clocking speed of the CCDs;
- the phased-array antenna has a good link margin allowing high throughput of data to ground;
- the power budget on board is very healthy;
- the launch and all orbit corrections have been very accurate and efficient leaving a good margin of chemical propellant for all future orbit manoeuvres;
- the science-mode attitude control is working very well, including the determination of the spin rate from the stars observed with the payload and the continuous adjustment of the spin rate with the micro-propulsion system;
- the cold-gas consumption of the micro-propulsion system is low enough to allow for a mission extension exceeding the nominal 1-year extension after the 5-year routine phase;
- the rubidium atomic clock on-board and the time-correlation procedure on ground provide the necessary accuracy for Gaia's science.
 
Ground-segment performance
During commissioning the ground segment was experiencing long periods of data reception from the spacecraft, representative of the routine phase. The data were collected by the ESA deep-space ground stations in Cebreros, New Norcia, and Malarg├╝e and were transferred to the Mission Operations Centre (MOC) in ESOC, Darmstadt. The science data were then copied to the Science Operations Centre (SOC) in ESAC, Madrid, for the first step of data processing. After execution of the Initial Data Treatment and First Look analysis, the data were stored in databases to subsequently be picked up for specific processing by five additional Data Processing Centres operational in the Gaia Data Processing and Analysis Consortium (DPAC). Many of the scientists in DPAC have already been involved in processing tasks with algorithms running in the Barcelona, Cambridge, Geneva, Torino and Toulouse centres. The CNES data centre in Toulouse also has the special task of maintaining a full copy of all Gaia data, ensuring a backup in a different geographical location.
 
The commissioning phase has been an intensive period in which MOC, SOC, DPAC and the industrial prime contractor Airbus Defence & Space, sometimes with support of its sub-contractors, have worked together to make Gaia ready for the routine phase. This collaboration has also ensured the necessary knowledge transfer from industry to MOC, SOC and DPAC who will be responsible for the nominal mission phase. The mission in-orbit commissioning review concluded that the ground segment is ready to start routine observations.
 
Scientific performance
The basic requirement to allow Gaia to deliver its scientific promise is met by the properly functioning spacecraft and ground segment, as detailed above. A number of issues and anomalies had to be dealt with during Gaia development and in-orbit commissioning. The status of these issues is outlined below.
 
Radiation damage on the CCDs: Before launch, extensive testing was done to prepare calibration procedures for data from CCDs having experienced radiation damage. Despite the Sun having been active during the past months, we have not seen, with the current measurement accuracy, radiation-damage effects in the data. Therefore, the validity of the radiation-calibration work done on ground can only be ensured at a later stage.
 
Ground-Based Optical Tracking (GBOT) of Gaia: When Gaia achieved its nominal 45 degrees Solar aspect angle at the L2 Lagrange point, the spacecraft turned out to be at the faintest end of its estimated brightness range. The GBOT group in DPAC has subsequently rearranged the tracking of Gaia through modified observing sequences, sometimes even with different telescopes. These new observing strategies, complemented by high-resolution radio observations, ensure that Gaia's orbit in the Solar system can be reconstructed with sufficient accuracy to achieve its science goals.
 
Stray light: Gaia is encountering undesired stray light both from the astronomical sky and the Sun. The stray light is not constant across the focal plane and is variable in time. The component from the Sun is repeating synchronously with the spin period. The impact is negligible for bright objects, but increases towards lower flux levels. New scientific performance estimates have been calculated with the observed stray-light levels. Further work will take place during the remainder of 2014 to optimise the on-board software for the Radial Velocity Spectrometer (RVS) to mitigate part of the impact caused by the higher background levels. One change has already been implemented and the RVS will be operated permanently in the high-resolution mode (resolving power around 11,000).
 
Basic-angle variations: By design of the spacecraft, the angle between the two lines of sight is kept as constant as possible. However, it has always been known that inherent stability is not possible at micro-arcsec level and therefore a specific device, the Basic Angle Monitor (BAM), has been included in the payload. The BAM works very well and measures spin-synchronous variations, as anticipated. However, the 1-milli-arcsec amplitude of these variations is much larger than expected. Significant effort has been invested during commissioning to ensure that the basic-angle variation is correctly measured by the BAM. This has been confirmed and future calibration work will focus on achieving the highest possible accuracy for the basic-angle-variation measurements.
 
Contamination: A decontamination activity was included in the timeline and exercised immediately after launch. However, during commissioning, a transmission degradation caused by water freezing on cold optical parts has been observed. Three additional decontamination activities have shown that transmission can be recovered by temporarily heating the optical parts. The rate of contamination seems to be decreasing with time. Nevertheless, at least one or two further decontamination operations will be required during the routine phase.
 
Micrometeoroids: The angular-momentum transfer from micrometeoroids was one of the mission elements taken into account in the design of Gaia. The commissioning data now show that the attitude control system on Gaia identifies and corrects any changes in the spin rate extremely well. Such attitude corrections had already been anticipated in the scientific software which will produce the Gaia catalogue. While the number of impact events in the 1-10 milli-arcsec/s spin-rate-change range is once a day, as expected, there seem to be two orders of magnitude more impacts in the 0.1-1 milli-arcsec/s range. These minute events can be detected as a result of the excellent performance of the attitude control system. However, before starting to correct micrometeoroid flux models, further work needs to be conducted to ensure that these small spin-rate changes are indeed caused by micrometeoroid hits.
 
Scientific performance estimates have been calculated on the basis of the commissioning data. During commissioning, tests have been conducted to explore Gaia's performance for the ~6000 brightest stars in the sky which were originally thought to be too bright for Gaia. The on-board detection parameters are now tuned in such a way that the bright limit for Gaia has been extended from 6 to 3 mag. The remaining ~230 stars in the sky brighter than this limit are actually also reachable by Gaia but this will require activating a special mode which has been tested successfully during commissioning and which will be used. For this mode, the on-ground processing or calibration software does not yet exist, but the end-of-mission astrometric precision for these very brightest stars is expected to be of the order of some tens of micro-arcsec.
 
The Gaia data-release scenario has been revised taking into account the new software development required to cope with the commissioning findings. The revised scenario anticipates the first intermediate catalogue in the summer of 2016. The work on the science-alerts pipelines is progressing nominally with the aim to have the first alerts released later this year.
 
Last week Friday, 25 July 2014, Gaia started its routine phase by scanning the sky for 28 days using the so-called ecliptic-poles scanning law. This is useful to bootstrap the basic calibrations of the data. After these 28 days, the nominal scanning law will be used to determine how Gaia is scanning the sky. Although the commissioning phase has ended, some activities remain to be completed. The root causes of the stray light and the basic-angle variations have not been found yet. A dedicated working group will hence address these topics during the remainder of 2014. Nonetheless, Gaia has started its 5-year journey today to produce a map of the Galaxy in three dimensions for one billion stars with unprecedented precision!