, αelement enhancements and individual elemental abundances can additionally be derived.

Descriptions of the status of this system can be found in Bailer-Jones et al. (2013) and Recio-Blanco et al. (2016).

All the results from the analysis tasks above will also be published as part of the Gaia data releases, thus complementing the basic astrometry, photometry, and spectroscopy to provide a rich data set, ready for exploitation by the astronomical community.

Moreover, these analysis tasks feed back into the preceding processing steps and thus provide an essential and strong internal quality control of the DPAC results.

The analysis tasks above will not necessarily lead to results that are mutually consistent or consistent with the preceding processing steps.

This necessitates an additional, complex task within DPAC, called catalogue integration, which is in charge of integrating, at the end of each iterative cycle, all the results from the various processing chains into a consistent list of sources and their observational and astrophysical parameters.

This list then forms the basis for a next cycle of iterative processing and also for the public Gaia data releases.

In order to achieve the iterative improvement of the Gaia processing results, within a given processing cycle all the data collected from the start of the mission are processed again to derive the basic astrometric, photometric, and spectroscopic data (thus involving the upstream systems IDU, AGIS, AVU-GSR, photometric, and RVS processing).

To keep this process manageable, the data collected by Gaia are split into data segments, Article number, page 24 of 36
where each segment covers a certain time range.

During a given data processing cycle n, typically all the data segments treated during cycle n −1 plus the latest complete segment are processed again by the upstream systems.

During the same cycle n, the downstream systems (NSS, SSO, SEA, EO, variable star analysis, and astrophysical-parameter inference) will process the astrometric, photometric, and spectroscopic data derived from the data segments used during cycle n −1.

The Gaia intermediate data release schedule is thus driven by the lengths of the processing cycles, while the quality of the published data at each release is related to the amount of data segments treated (sky, scan-direction, and time coverage; signalto-noise) and the amount of iterative cycles completed (calibration quality).
7.3.

Simulations, supplementary data and observations, data publication Apart from the processing tasks mentioned in the previous sections, the DPAC responsibilities also include supporting tasks that are necessary for the successful preparation and execution of the Gaia data processing and the publication of its results:
Simulations.

Data simulations formed an essential element of the DPAC preparations for the operational lifetime of both Gaia and the data processing.

The simulations spanned the three levels from the pixels in the focal plane (GIBIS; Babusiaux et al. 2011), through simulated telemetry (GASS; Masana et al. 2008) to simulated DPAC data products (GOG; Antiche et al. 2014), which were used both internally and to support the astronomical community in preparing itself for the Gaia mission (Robin et al. 2012; Luri et al. 2014).

These last simulations have been made available and will continue to be available alongside the released Gaia data.

Relativistic astrometric models.

The proper modelling and interpretation of astrometric data at the micro-arcsecond level accuracy aimed for by the Gaia mission requires the data processing to be fully compatible with general relativity (for example to accurately account for the effect of light bending by solar system objects on the apparent source directions).

The relativistic model used for AGIS is described in Klioner (2003, 2004), while the model employed for AVU-GSR is documented in de Felice et al. (2004, 2006).

The relativistic modelling of the observations also requires an up-to-date solar system ephemeris.

For Gaia DR1, the DPAC employed the INPOP10e ephemeris (Fienga et al. 2013).

Auxiliary observations.

The Gaia photometric and spectroscopic data have to be tied to a physical flux and wavelength scale that is achieved through standard stars.

In preparation of the Gaia mission, therefore, extensive observational campaigns to collect the relevant spectro-photometric data took place (Pancino et al. 2012) as well as monitoring campaigns to establish the photometric stability of candidate standard stars (Altavilla et al. 2015).

For the RVS, a compilation of radial-velocity standards was undertaken (Crifo et al. 2010; Soubiran et al. 2013), while for the solar system object processing, data on solar analogues had to be compiled to calibrate the interpretation of the Gaia photometry.