# 1.2.3 Overview of the data processing

## System architecture

The system architecture of Gaia is documented in O’Mullane et al. (2011).

## Common tools

A common set of (Java) tools are in place for DPAC data processing. These ensure consistency of the data processing and avoid multiple, independent developments of similar concepts. The most important common tools are:

Configuration DataBase (CDB)

this provides data related to the configuration of the spacecraft that could impact on data processing. Included are information such as the along-scan phasing offset of each CCD row and strip; the charge-injection configuration for each CCD (Section 1.3.3); the CCD window geometry (Table 1.1); TDI-gate information for each CCD; etc.

Gaia Parameter DataBase (GPDB)

this provides useful mathematical, physical, and astronomical constants (for instance the value of $\pi$, the Solar System ephemeris, and the speed of light in a vacuum to a significant number of digits), as well as mission-specific information and nominal, pre-flight spacecraft and payload design parameters.

GaiaTools (GT)

this is the main common software library that provides utilities such as reading / writing data in accordance to the common data model (see Section 1.2.3) and related database querying facilities; plotting tools; mathematical tools (vector / array manipulation and quaternion operations); fitting algorithms; scanning law integration; timescale and calendaric transformations; and so on.

There is also a common acronym list, available on https://gaia.esac.esa.int/gpdb/glossary.txt.

## Main database and data model

The Gaia data-processing centres (DPCs) exchange data through the main database (MDB) located at DPCE (Section 1.3.4) following a hub-and-spokes topology. The MDB also houses the DPAC end data products from which the public data releases are produced. For more details, see Gaia Collaboration et al. (2016).

## Data flow

The data flow in the Gaia science ground segment is described in Hernandez and Hutton (2015).

## Daily processing

The daily processing of Gaia data includes the Initial Data Treatment (IDT; Fabricius et al. 2016, see also Section 2.4.2) and First Look (FL; Fabricius et al. 2016, see also Section 2.5.2), the Astrometric Verification Unit (AVU; Section 2.5.1) processing, the RVS daily processing (Sartoretti et al. 2018), the Photometric Science Alerts (PSAs), and the solar-system science alerts. More details are provided in Gaia Collaboration et al. (2016).

## Cyclic processing

The cyclic processing, encompassing the Intermediate Data Update (IDU; Section 2.4.2), the Astrometric Global Iterative Solution (AGIS; Section 3.4.2), the Global Sphere Reconstruction (AVU-GSR; Section 2.5.1), the Photometric Pipeline (PhotPipe; Section 5.2.1), and the RVS pipeline (Chapter 6), is detailed in Gaia Collaboration et al. (2016).

## Astrometric and photometric pre-processing

The astrometric and photometric pre-processing is summarised in Gaia Collaboration et al. (2016) and detailed in Fabricius et al. (2016). Further information is provided in Chapter 2.

## Astrometry

The astrometric processing and validation is summarised in Gaia Collaboration et al. (2016) and detailed for Gaia DR2 in Lindegren et al. (2018) and Mignard et al. (2018). Further documentation is provided in Chapter 3.

## Photometry

The photometric processing and validation is summarised in Gaia Collaboration et al. (2016) and detailed for Gaia DR2 in Evans et al. (2018) and Riello et al. (2018). Further documentation is provided in Chapter 5.

## Spectroscopy

The spectroscopic processing and validation is summarised in Gaia Collaboration et al. (2016) and detailed for Gaia DR2 in Sartoretti et al. (2018). Further documentation is provided in Chapter 6.

## Non-single stars and exoplanets

Gaia DR2 does not cover non-single stars (NSSs) and/or exoplanets. The processing of these objects is summarised in Gaia Collaboration et al. (2016).

## Extended objects

Gaia DR2 does not cover extended objects (EOs). The processing of these objects is summarised in Gaia Collaboration et al. (2016).

## Solar-system objects

The processing and validation of asteroid data is summarised in Gaia Collaboration et al. (2016) and detailed for Gaia DR2 in Gaia Collaboration et al. (2018f). Further documentation is provided in Chapter 4.

## Variability

The variability processing and validation is summarised in Gaia Collaboration et al. (2016) and detailed for Gaia DR2 in Holl et al. (2018). Further documentation is provided in Chapter 7.

## Astrophysical parameters

The processing and validation of astrophysical parameters is summarised in Gaia Collaboration et al. (2016) and detailed for Gaia DR2 in Andrae et al. (2018). Further documentation is provided in Chapter 8.