Gaia CCD-level measurements need to be processed, i.e. cal-
ibrated and transformed into astrophysically meaningful quan-
tities. This is being carried out under the remit of the Gaia
Data Processing and Analysis Consortium (DPAC). The DPAC
Article number, page 21 of 36 evolved out of the Gaia community working groups that were formed after the selection of Gaia in 2000, and formally started its activities in 2006.

The DPAC subsequently responded to the ESA announcement of opportunity for the Gaia data processing, and was officially entrusted with this responsibility in 2007.

The DPAC currently consists of some 450 astronomers, software engineers, and project management specialists, based in approximately 25, mostly European countries.

The remainder of this section provides a summary description of the Gaia data processing, which serves as the context for more detailed expositions of how specific parts of a Gaia data release are derived from the raw observations.

The Gaia data processing, which is summarised below, is a very complex task, serving a wide variety of scientific goals.

The data processing can be split into two broad categories: daily and cyclic.

The daily tasks produce the preprocessed data that are needed by the cyclic systems, provide payload health monitoring, and feed the alerts systems.

The daily systems process the Gaia telemetry in near-real time as it comes down from the spacecraft.

In contrast, the cyclic processing iterates between calibration and the determination of source parameters (to be interpreted here in a broad sense, ranging from astrometry to astrophysical characteristics), by repeatedly processing all the Gaia data until the system converges.

This is a consequence of the self-calibrating nature of Gaia (Sect. 3.1).

In addition, the DPAC is responsible for the data simulations that were used to support the mission preparations, the provision of ground-based data needed for the calibration of the Gaia data, and the validation, documentation, and publication (through the Gaia Archive, developed, hosted, and operated by ESA) of the data processing results.
7.1.

Daily data processing The following daily tasks run within the DPAC: Initial Data Treatment and First Look.

Following the reception and reconstruction of the telemetry stream at the SOC (see Sect. 6.2), the IDT and First-Look systems together take care of the preprocessing of the incoming raw Gaia observations (so that they can be treated in the subsequent cyclic processing) and the daily payload health monitoring.

These systems run at the DPAC data processing centre hosted at the Gaia SOC and are briefly described in Sects. 6.2 and 6.3 above, while the detailed description is provided by Fabricius et al. (2016).

Astrometric Verification Unit.

Two systems that independently treat the raw data served by IDT are run as part of the astrometric verification unit, which aims to independently verify all the critical components contributing to the Gaia astrometric error budget.

The basic angle monitor unit (Riva et al. 2014) provides an independent monitoring of the BAM data and calibrated measurements of the basic angle variations.

The astrometric instrument model unit (Busonero et al. 2014) is a scaled-down counterpart of IDT and First Look that is restricted to a subset of the astrometric elements of the daily processing (see Fabricius et al. 2016), and it provides calibrations of the point spread function that are independent from the IDU system described below.

RVS daily.

Although IDT does some very basic processing of RVS data (see Fabricius et al. 2016), and First Look produces diagnostic plots that permit a first verification of the health of the RVS instrument, a special pipeline handles the full daily preprocessing of RVS data, including the establishment of initial calibrations and health monitoring of the RVS Article number, page 22 of 36
instrument.

For further information on the RVS processing pipeline, see Katz et al. (2011).

Science alerts.

Because Gaia repeatedly scans the entire sky and quasi-simultaneously measures the position (to a spatial resolution of 50–100 mas), apparent brightness, and spectral energy distribution of sources, it forms a unique transient survey machine, for example capable of discovering many thousands of supernovae over the course of its lifetime (e.g.

Hodgkin et al. 2013).

Therefore, DPAC runs a system that uses the IDT outputs, including source positions (already accurate to better than 100 mas) and fluxes, to build up a history of the observed sky to enable the discovery of transient phenomena, for which spectro-photometry is immediately available at each epoch.

The candidate transients are then filtered and the most interesting candidates are published as alerts, including the relevant Gaia data to enable rapid follow-up with ground-based telescopes.