GAIA Astrometry Mission

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- The effects of Sagittarius on the structure and movement of stars in the Milky Way have been described previously, but the new findings for the first time show that the dwarf galaxy was likely directly responsible for the build-up of the stellar mass in the Milky Way. It, however, seems possible that without the dwarf galaxy crossing paths with the Milky Way, Earth and life on it may not have been born. In fact, it seems possible that even the Sun and its planets would not have existed if the Sagittarius dwarf had not gotten trapped by the gravitational pull of the Milky Way and eventually smashed through its disc. Every collision stripped Sagittarius of some of its gas and dust, leaving the galaxy smaller after each passage.

• April 25, 2020: The Gaia Mission celebrated the second anniversary of Gaia Data Release 2 (DR2). In the previous 2 years, almost 3000 refereed papers based on Gaia Data Release 2 had been published. 

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• March 2, 2020: Astronomers have pondered for years why the Milky Way, is warped. Data from ESA's star-mapping satellite Gaia suggest the distortion might be caused by an ongoing collision with another, smaller, galaxy, which sends ripples through the galactic disc like a rock thrown into water. ⁷³⁾

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- Astronomers have known since the late 1950s that the Milky Way's disc – where most of its hundreds of billions of stars reside – is not flat but somewhat curved upwards on one side and downwards on the other. For years, they debated what was causing this warp. They proposed various theories including the influence of the intergalactic magnetic field or the effects of a dark matter halo, a large amount of unseen matter that is expected to surround galaxies. If such a halo had an irregular shape, its gravitational force could bend the galactic disc.

- A team of scientists using data from the second Gaia data release has now confirmed previous hints that this warp is not static but changes its orientation over time. Astronomers call this phenomenon precession and it could be compared to the wobble of a spinning top as its axis rotates. Moreover, the speed at which the warp processes is much faster than expected – faster than the intergalactic magnetic field or the dark matter halo would allow. The warp's speed is, however, slower than the speed at which the stars themselves orbit the galactic centre. Such insights were only possible thanks to the unprecedented ability of the Gaia mission to map the Milky Way, in 3D, by accurately determining the positions of more than one billion stars in the sky and estimating their distance from us. As impressive as the warp and its precession appear on the galactic scale, although it has no noticeable effects on life on Earth. 

• February 13, 2020: A sizable international team published findings about the discovery of a new binary star in Astronomy & Astrophysics (Ref. 79). ⁷⁵⁾

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-  Gravitational lensing is a powerful tool for space exploration. Photons deviate from their straight path when passing near a massive body due to its gravitational field, causing an increase in the brightness of the star, reflected on the light curve without changing the star's physical parameters. When a massive object crosses the line of sight between a star and an observer on Earth, it causes a brief brightening of the background star, known as a gravitational microlensing event. These events are rare and unpredictable. To detect microlensing events in the Milky Way, hundreds of millions of stars had to be tracked daily. The European Space Agency's GAIA mission reports significant brightness changes to Earth, after which international telescopes track these objects to identify their variability.

- Since 2016, Kazan Federal University and Turkish colleagues have participated in the GAIA classification program. Most variable objects are cataclysmic variables, supernovae, or active galactic nuclei, but some show temporary brightness changes. The data allowed simulation of the complex movement involving an observer on Earth and a binary system. The Gaia16aye event's light curve suggested a binary system due to the appearance of multiple peaks. Polish scientists created a geometric model of the Gaia16aye microlensing phenomenon through international collaboration and theoretical calculations.

• February 10, 2020: Scientists from Rochester Institute of Technology discovered a newborn massive planet closer to Earth than any other of similarly young age found to date. The baby giant planet, called 2MASS 1155-7919 b, is located in the Epsilon Chamaeleontis Association and lies only about 330 light years from the solar system. ⁷⁶⁾ The discovery, published in the Research Notes of the American Astronomical Society, provided researchers with an exciting new way to study how gas giants form.  The scientists used data from the Gaia space observatory to make the discovery. The giant baby planet orbits a star that is only about 5 million years old, about one thousand times younger than the sun. The planet orbits its sun at 600 times the distance of the Earth to the sun. The authors hope that follow-up imaging and spectroscopy will help astronomers understand how massive planets can end up in such wide orbits.

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• January 21, 2020: A 500-day global observation campaign spearheaded more than three years ago by ESA's galaxy-mapping Gaia provided insights into the binary system of stars that caused an unusual brightening of an even more distant star. ⁷⁷⁾

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- The brightening of the star, located in the Cygnus constellation, was first spotted in August 2016 by the Gaia Photometric Science Alerts program. This system, maintained by the Institute of Astronomy at the University of Cambridge, UK, scans daily the huge amount of data coming from Gaia and alerts astronomers to the appearance of new sources or unusual brightness variations in known ones, so that they can quickly point other ground and space-based telescopes to study them in detail. In this particular instance, follow-up observations performed with more than 50 telescopes worldwide revealed that the source – since then named Gaia16aye after Ayers Rock, the famous landmark in Australia – was behaving in a rather strange way. This brightening was caused by gravitational microlensing – an effect predicted by Einstein's theory of general relativity, caused by the bending of spacetime in the vicinity of very massive objects, like stars or black holes.

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- This sudden and sharp drop in brightness suggested that the gravitational lens causing the brightening must consist of a binary system – a pair of stars, or other celestial objects, bound to one another by mutual gravity. The combined gravitational fields of the two objects produce a lens with a rather intricate network of high magnification regions. When a background source passes through such regions on the plane of the sky, it lights up, and then dims immediately upon exiting it. From the pattern of subsequent brightenings and dimmings, the astronomers were able to deduce that the binary system was rotating at a rather fast pace. The long period of observations, which lasted until the end of 2017, and the extensive participation of ground-based telescopes from around the globe enabled the astronomers to gather a large amount of data – almost 25,000 individual data points. 

• January 7, 2020: Astronomers at Harvard University have discovered a monolithic, wave-shaped gaseous structure — the largest ever seen in this galaxy — made up of interconnected stellar nurseries. Dubbed the "Radcliffe Wave" in honor of the collaboration's home base, the Radcliffe Institute for Advanced Study, the discovery transforms a 150-year-old vision of nearby stellar nurseries as an expanding ring into one featuring an undulating, star-forming filament that reaches trillions of miles above and below the galactic disk. ⁸⁰⁾

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- The researchers discovered a long, thin structure, about 9,000 light-years long and 400 light-years wide, with a wave-like shape, cresting 500 light-years above and below the mid-plane of the Milky Way's disk. The Wave includes many of the stellar nurseries that were thought to form part of "Gould's Belt," a band of star-forming regions believed to be oriented in a ring around the sun. The new, 3D map showed the Milky Way in a new light, giving researchers a revised view of the Milky Way and opening the door to other major discoveries. In earlier studies, the research group at Harvard, pioneered advanced statistical techniques to map the 3D distribution of dust using vast surveys of stars' colours. Before Gaia, there was no data set expansive enough to reveal the galaxy's structure on large scales. Since its launch in 2013, the space observatory has enabled measurements of the distances to one billion stars in the Milky Way.

PLM (Payload Module)

The payload module is housed inside a geometrical, dome-like structure called the 'Thermal Tent' (Figures 14 and 15). The payload consists of a single integrated instrument (Figure 56) that comprises three major functions. In the earlier spacecraft designs, the three functions were distributed over three separate instruments. Now the three functions are built into a single instrument by using common telescopes and a shared focal plane: ^{82) 83) 84) 85) 86)}

1) The Astrometric instrument (ASTRO) is devoted to star angular position measurements, providing the five astrometric parameters:

- Star position (2 angles)

- Proper motion (2 time derivatives of position)

- Parallax (distance)

ASTRO is functionally equivalent to the main Hipparcos instrument.

2) The Photometric instrument provides continuous star spectra for astrophysis in the band 320-1000 nm and the ASTRO chromaticity calibration

3) The RVS (Radial Velocity Spectrometer) provides radial velocity and high resolution spectral data in the narrow band 847-874 nm.

Each function is achieved within a dedicated area on the focal plane. Afocal elements are located close to the focal plane for the photometric and spectroscopic functions, providing dispersion of the star's spectrum along the scan. This allows both functions to take benefit from the two viewing directions and from the large ASTRO aperture, and to operate in densely populated sky areas. RVS is implemented as a grating plate, combined with four prismatic spherical lenses. This allows the necessary dispersion value to be met while correcting most of the telescope aberrations. ⁸⁷⁾

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Legend to Figure 56: The focal plane is hanging on the ‘optical bench torus' made of silicon carbide. The optics consist of 10 mirrors and the refractive optical elements. Mirrors M1, M2 and M3 form one telescope and M1', M2', M3' the other telescope. The subsequent set of mirrors M4/M4', M5 and M6 combine the light from both telescopes and direct it to the focal plane assembly. The fields of view of the two telescopes are 106.5º apart (Astrium SAS).

Design:

The payload design is characterised by:

• A dual telescope concept, with a common structure and a common focal plane. Both telescopes are based on a TMA (Three Mirror Anastigmat) design. The beam combination is achieved in image space with a small beam combiner, rather than in object space as was done in the Hipparcos satellite. This saves the mass of the beam combiner, simplifies the accommodation and eliminates the directional ambiguity of the detected targets.

• The use of SiC (Silicon Carbide) ultra-stable material for mirrors and telescope structure provides low mass, isotropy, thermo-elastic stability and dimensional stability in a space environment. This allows to meet the stability requirements for the basic angle between the two telescopes with a passive thermal control instead of an active one.

• A highly robust BAM (Basic Angle Measurement) system.

• A large common focal plane shared by all instruments.

Optics:

Gaia contains two identical telescopes, pointing in two directions separated by a 106.5º basic angle and merged into a common path at the exit pupil. The optical path of both telescopes is composed of six reflectors (M1-M6), the last two of which are common (M5-M6). Both telescopes have an aperture of 1.45 m x 0.5 m and a focal length of 35 m. The telescope elements are built around the hexagonal optical bench with a ~3 m diameter, which provides the structural support.

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Although the optical design is fully reflective, based on mirrors only, diffraction effects with residual aberrations induce systematic chromatic shifts of the diffraction images and thus of the measured star positions. This effect, usually neglected in optical systems, is also critical for Gaia. These systematic chromatic displacements will be calibrated as part of the on-ground data analysis using the colour information provided by the photometry of each observed object.

Astrometry

The main objective of the astrometric instrument (ASTRO) is to obtain accurate measurements of the relative positions of all objects that cross the fields of view of Gaia's two telescopes. The two fields of view are combined onto the single focal plane.

During its five-year mission, Gaia will systematically scan the whole sky and will have obtained some 70 sets of relative position measurements for each star. These permit a complete determination of each star's five basic astrometric parameters: two specifying the angular position, two specifying the proper motion, and one - the parallax - specifying the star's distance. The five-year long mission also permits the determination of additional parameters, for example those relevant to orbital binaries, extra-solar planets, and solar-system objects.

By measuring the instantaneous image centroids from the data sent to ground, Gaia measures the relative separations of the thousands of stars simultaneously present in the combined two fields. The spacecraft operates in a continuously scanning motion, such that a constant stream of relative angular measurements is built up as the fields of view sweep across the sky. High angular resolution (and hence high positional precision) in the scanning direction is provided by the large primary mirror of each telescope. The wide-angle measurements provide high rigidity of the resulting reference system.

Design: The astrometric instrument (ASTRO) comprises the two telescopes and the dedicated area of 62 CCDs in the focal plane, where the two fields of view are combined onto the AF (Astrometic Field). Each CCD is read out in TDI (Time Delay Integration) mode, synchronised to the scanning motion of the satellite. In practice, stars entering the combined field of view first pass across the column of the SM (Sky Mapper) CCDs, where each object is detected. Information on an object's position and brightness is processed on board in real-time to define the windowed region around the object to be read out by the following CCDs.

On-ground Data Processing: The a posteriori on-ground data processing is a highly complex task, linking all relative measurements and transforming the location (centroiding) measurements in pixel coordinates to angular field coordinates through a geometrical calibration of the focal plane, and subsequently to coordinates on the sky through calibrations of the instrument attitude and basic angle.

Further necessary corrections to be performed include those for optics effects (systematic chromatic shifts and aberration) and general-relativistic effects (light bending due to the Sun, the major planets plus some of their moons, and the most massive asteroids).

Accuracy: The accuracy of the measurements depends on the stellar type and relies on the stability of the basic angle of 106.5° between the two telescopes. This angle is monitored by the BAM (Basic Angle Monitoring) system.

Photometry

The photometer will measure the SED (Spectral Energy Distribution) of all the detected objects. This will serve two goals:

• From the SED measurements, astrophysical quantities such as luminosity, effective temperature, mass, age, and chemical composition are derived

• In order to meet the astrometric performance requirements, the measured centroid positions must be corrected for systematic chromatic shifts induced by the optical system. This is only possible with the knowledge of the spectral energy distribution of each observed target in the wavelength range covered by the CCDs of the main astrometric field (~320-1000 nm).

Design: The photometer (like the spectrometer) is merged with the astrometric function, using the same large collecting apertures of the two telescopes. The photometry function is achieved by means of two low dispersion optics located in the common path of the two telescopes: one for the short wavelengths (BP) and one for the long wavelengths (RP).

- BP (Blue Photometer): Measurement in the 320-660 nm spectral range

- RP (Red Photometer): Measurement in the 650-1000 nm spectral range.

The baseline design uses only one prismatic element in fused silica for each photometer, to disperse the collected light along scan prior to detection.

The prisms are located at the nearest possible position from the focal plane, in order to facilitate the mechanical holding and moreover reduce the shadowed areas. Both prisms are attached to the box shaped CCD radiator directly in front of the detector array. Both photometers, BP and RP, have a dedicated CCD strip that covers the full astrometric field of view in the across-scan direction.

Accuracy: The spectral resolution is a function of wavelength as a result of the natural dispersion curve of fused silica; the dispersion is higher at short wavelengths.

The BP and RP dispersers will be designed in such a way that BP and RP spectra have similar sizes (on the order of 30 pixels along scan). BP and RP spectra will be binned on-chip in the across-scan direction; no along-scan binning is foreseen. For bright stars, single-pixel-resolution windows are foreseen to be used, in combination with TDI gates.

The end of mission sky averaged magnitude standard error will depend on the star type, magnitude and wavelength band and it will typically be in the range of 10-200 x 10^-3 in magnitude.

Spectrometry

Objectives: The primary objective of Gaia's RVS (Radial Velocity Spectrometer) instrument is the acquisition of radial velocities. These LOS (Line-of-Sight) velocities complement the proper-motion measurements provided by the astrometric instrument. To this end, the instrument will obtain spectra in the narrow near infrared band (847-874 nm) with a spectral resolution λ/Δλ of ~ 11,500.

The RVS wavelength range, 847-874 nm, has been selected to coincide with the energy-distribution peaks of G- and K-type stars which are the most abundant RVS targets. For these late-type stars, the RVS wavelength interval displays, besides numerous weak lines mainly due to Fe, Si, and Mg, three strong ionised calcium lines (at around 849.8, 854.2, and 855.2 nm). The lines in this triplet allow radial velocities to be derived, even at modest SNRs (Signal-to-Noise Ratios). In early-type stars, the RVS spectra may contain weak lines such as Ca II, He I, He II, and N I, although they will generally be dominated by Hydrogen Paschen lines.

Design and operations: The RVS instrument is a near-infrared, medium-resolution, integral-field spectrograph dispersing all the light entering the field of view. It is integrated with the astrometric and photometric functions and uses the common two telescopes.

- Wavelength range: 847 - 874 nm

- Resolution (R=λ/Δλ): ~ 11,500.

The RVS uses the SM (Sky Mapper) function for object detection and confirmation. Objects will be selected for RVS observations, based on measurements made slightly earlier in the RP (Red Photometer). Light from objects coming from the two viewing directions of the two telescopes is superimposed on the RVS CCDs.

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The spectral dispersion of objects in the field of view is achieved by means of an optical module physically located between the last telescope mirror (M6) and the focal plane. This module contains a grating plate and four dioptric, prismatic, spherical, fused-silica lenses which correct the main aberrations of the off-axis field of the telescope. The RVS module has unit magnification, which means that the effective focal length of the RVS equals 35 m.

Spectral dispersion is oriented in the along-scan direction. A dedicated passband filter restricts the throughput of the RVS to the desired wavelength range.

The RVS-part of the Gaia FPA (Focal Plane Assembly) contains 3 CCD strips and 4 CCD rows. Each source will typically be observed during ~40 FOV transits throughout the 5-year mission. On the sky, the RVS CCD rows are aligned with the astrometric and photometric CCD rows; the resulting semi-simultaneity of the astrometric, photometric, and spectroscopic transit data will be advantageous for variability analyses, scientific alerts, spectroscopic binaries, etc. All RVS CCDs are operated in TDI (Time Delay Integration) mode.

The RVS spectra will be binned on-chip in the across-scan direction. All single-CCD spectra are foreseen to be transmitted to the ground without any further on-board (pre-)processing. For bright stars, single-pixel-resolution windows are foreseen to be used, possibly in combination with TDI gates. It is currently foreseen that the RVS will be able to reach object densities on the sky of up to 40,000 objects/ degree².

On-ground Data Processing: Radial velocities will be obtained by cross-correlating observed spectra with either a template or a mask. An initial estimate of the source atmospheric parameters derived from the astrometric and photometric data will be used to select the most appropriate template or mask. Iterative improvements of this procedure are foreseen. For stars brighter than ~15^th magnitude, it will be possible to derive radial velocities from spectra obtained during a single field-of-view transit. For fainter stars, down to ~17^th magnitude, accurate summation of the ~40 transit spectra collected during the mission will allow the determination of mean radial velocities.

Atmospheric parameters will be extracted from observed spectra by comparison of the latter to a library of reference-star spectra using, for example, minimum-distance methods, principal-component analyses, or neural-network approaches. The determination of the source parameters will also rely on the information collected by the other two instruments: astrometric data will constrain surface gravities, while photometric observations will provide information on many astrophysical parameters.

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As the spacecraft slowly rotates, the light from the celestial object (that is, the image of the object) passes across the focal plane. In this way, Gaia steadily scans the whole sky as the satellite spins and gradually precesses, with each part being observed around 70 times in the course of the operational lifetime.

FPA (Focal Plane Assembly) ^{88) 89)}

The Gaia focal-plane assembly is the largest ever developed for a space application, with 106 CCDs, a total of 937 Mpixels (almost 1 Gpixel) , which are around 90% light efficient (c.f. 20% typical terrestrial camera efficiency). The FPA has a physical dimension of 1.0 m x 0.4 m (Figure 61). The focal-plane assembly is common to both telescopes and serves five main functions:

1) The WFS (Wave-Front Sensor) and basic-angle monitor, covering 2+2 CCDs: a five-degrees-of-freedom mechanism is implemented behind the M2/M2' secondary mirrors of the two telescopes for re-aligning the telescopes in orbit to cancel errors due to mirror micro-settings and gravity release. These devices are activated following the output of two Shack–Hartmann-type wave-front sensors at different positions in the focal plane. The BAM (Basic Angle Monitor) system (2 CCDs in cold redundancy) consists of a Youngs-type interferometer continuously measuring fluctuations in the basic angle between the two telescopes with a resolution of 0.5 µarcsec per 15 minutes.

2) The SM (Sky Mapper), containing 14 CCDs (seven per telescope), which autonomously detects objects down to 20^th magnitude entering the fields of view and communicates details of the star transits to the subsequent CCDs.

3) The main AF (Astrometric Field), covering 62 CCDs, devoted to angular-position measurements, providing the five astrometric parameters: star position (two angles), proper motion (two time derivatives of position), and parallax (distance) of all objects down to 20^th magnitude. The first strip of seven detectors (AF1) also serves the purpose of object confirmation.

4) The blue and red photometers (BP and RP), providing low-resolution, spectro-photometric measurements for each object down to 20^th magnitude over the wavelength ranges 330–680 nm and 640–1050 nm, respectively. The data serves general astrophysics and enables the on-ground calibration of telescope-induced chromatic image shifts in the astrometry. The photometers contain seven CCDs each.

5) The RVS (Radial Velocity Spectrometer), covering 12 CCDs in a 3 x 4 arrangement, collecting high-resolution spectra of all objects brighter than 17^th magnitude, allowing derivation of radial velocities and stellar atmospheric parameters.

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Observation Sequence in the Focal Plane: All CCDs, except those in the SM (Sky Mapper), are operated in windowing mode: only those parts of the CCD data stream, which contain objects of interest, are read out; remaining pixel data is flushed at high speed. The use of windowing mode reduces the readout noise to a handful of electrons while still allowing reading up to 20 objects simultaneously.

- Every object, crossing the focal plane, is first detected either by SM1 or SM2. These CCDs record, respectively, the objects only from telescope 1 or from telescope 2. This is achieved by a physical mask that is placed in each telescope's intermediate image, at M4/M41 beam-combiner level.

- Next,a surrounding window is allocated to the object, which is propagated through the following CCDs of the CCD row as the imaged object crosses the focal plane; the actual propagation uses input from the spacecraft's attitude control system, which provides the predicted position of each object in the focal plane versus time. After detection in SM, each object is confirmed by the CCD detectors in the first strip of the AF1 (Astrometric Field); this step eliminates false detections such as cosmic rays.

- The object then progressively crosses the eight next CCD strips in AF, followed by the BP, RP, and RVS detectors (the latter ones are present only for four of the seven CCD rows).

- The nominal integration time per CCD is 4.42 seconds, corresponding to 4500 pixels along scan. For bright saturating objects, the integration time in AF1-AF9, BP and RP is reduced by activating electronic TDI gates in the detector over a short period corresponding to the bright star window. The purpose of the TDI gates is to lower the effective number of pixels along scan. Twelve gates are available in the detector and allow optimising the signal collection for bright stars at the minimum expense for faint stars.

CCD characteristics: All CCDs have the same format and are derived from e2V Technologies (UK) design and are large-area, back-illuminated, full-frame devices. They are operated in TDI (Time Delay Integration) mode with a TDI period of 982.8 µs. The focal plane is passively cooled to 170 K for reducing its sensitivity to radiation. The box shaped CCD radiator provides the radiative surface with the colder internal payload cavity (120 K) as well as CCD shielding against radiation and support for the photometer prisms.

The Gaia CCDs are fabricated in three variants, AF-, BP-, and RP-type, to optimise quantum efficiency corresponding to the different wavelength ranges of the scientific functions. The AF-type variant is built on standard silicon with broadband anti-reflection coating. It is the most abundant type in the focal plane, used for all but the photometric and spectroscopic functions. The BP-type only differs from the AF-type through the blue-enhanced backside treatment and anti-reflection coating, and it is exclusively used in BP. The RP-type is built on high-resistivity silicon with red-optimised anti-reflection coating to improve near-infrared response. It is used in RP as well as in RVS.

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Payload module data handling ⁹⁰⁾

Time reference: The star localisation measurement is performed by transit detection and time measurement, which calls for a very accurate datation of object transits. For this purpose, a CDU (Clock Distribution Unit) provides all necessary timing signals and clock functions for video sample time tagging and ground based time scale correlation. All signals are generated on the basis of the highly stable central master 10 MHz Rubidium atomic clock.

Time correlation: In addition to the classical corrected one-way path technique, it is proposed that on-board-to-UTC time correlation be performed by a specific two way process. This technique allows to cancel symmetrical delays of ionospheric or tropospheric origin, or delays of relativistic origin. This correlation performance is furthermore independent of the orbit.

On-board Payload Processing: The PDHS (Payload Data Handling System) is implemented as a set of 7 VPUs (Video Processing Units), one for each detector row of the focal plane, feeding a common 960 GB solid state mass memory at PDHU. The processing part of the PDHS has a modular architecture which follows the FPA architecture and eases the accommodation. In the case of failure of one channel, this would have little impact on the science performance. The file-organised mass memory is a standard stand-alone unit.

Although Gaia has the biggest camera that has ever flown in space, Gaia does not actually take pictures in the conventional sense. Instead, it rather tracks the stars across its sensors as the telescopes rotate and the field of view moves across the star-filled sky. In order to do so, a constant readout of the onboard CCDs is done, and this takes a lot of computing power. For this, the seven high performance VPUs (Video Processing Units) are used which interface with the 'camera'. A VPU incorporates a dedicated Astrium-developed pre-processing board, and for the bulk of the processing, a SCS750 PowerPC board from Maxwell Technologies, Inc., of San Diego, USA. Each of the VPUs exhibits a processing power of more than 1000 MIPS (>1 GIPS). A VPU has a mass of 3.2 kg and a size of 195 x 120 x 253 mm. ^{91) 92) 93)}

On-board data processing algorithms allow computations to be made in real time, without data buffering. The hardware-software share offers full flexibility and algorithms may be modified in-flight following first in-orbit results.

A precise centroiding of bright stars is made with the measurements performed in the first 2 columns of detectors in order to determine the star velocities for monitoring the spacecraft attitude control.

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Metrology and alignment: Gaia embarks two specific devices, for recovery of the optical quality after launch and for continuous monitoring of the angle between the two telescope lines of sight. All these devices are operated at 130 K.

1) A five-degree-of-freedom static mechanism is implemented behind the secondary mirrors of the two telescopes , based on the TMA (Three Mirror Anastigmat) design, for securing the optical performance in orbit and cancelling static residual errors due to mirror micro-settings and gravity release. The BAM (Basic Angle Monitoring) system consists of a Fizeau interferometer, measuring fluctuations in the basic angle between the two telescopes. The image quality is measured by two Shark-Hartmann Wave Front Sensors using two dedicated CCDs in the focal plane. The signal is coming from bright stars.

2) During science operations, a Fizeau laser interferometer measures the in orbit fluctuation of the basic angle between the two input telescopes with an accuracy better than 0.5 µas. The fringe motion with respect to the detector frame provides the line-of-sight of the telescopes along scan, and therefore the basic angle variations.

Ground Segment

ESA's most powerful ground stations, the 35 m deep-space stations in New Norcia, Australia (DSA 1), Cebreros, Spain (DSA 2), and Malargüe, Argentina (DSA 3), will be used to send commands to Gaia and receive the high volume of science data that must be returned to Earth to create Gaia's Galactic Map. During the critical LEOP phase, additional ground station support will be provided by ESA's 15 m diameter Kourou, Maspalomas and Perth stations. ^{94) 95)}

Communications and Orbit Tracking

• The end-to-end timing of the measurements must be highly precise. To this end, Gaia carries an atomic clock to time stamp the science data, which has to be matched by ultra-precise time stamping on ground at data reception. In fact, the CCSDS ground time-stamping standard that the space community uses was extended to a picosecond (10^-12 s) resolution to meet the Gaia requirements, and this capability was added to ESA's Estrack Deep Space Antennas.

• The orbit of Gaia must also be determined to very high accuracy (to within 150 m at 1.5 million km). The traditional radiometric methods are supplemented by optical observations from ground-based telescopes, which take pictures of Gaia against the background stars, and ΔDOR (delta-Differential One-way Ranging) measurements in the commissioning phase (a method where multiple DSA stations are used to precisely determine the spacecraft position with respect to a Quasar).

• GBOT (Ground Based Orbit Tracking) campaign: GBOT utilises a network of small-to-medium telescopes aiming to track the Gaia observatory. GBOT is committed to deliver one set of data per day, which allows the determination of Gaia's position good to 20 m arcsec. The GBOT data on Gaia will be included in the orbit reconstruction performed at ESOC in order to increase the accuracy of this undertaking to a level of 150 m in position and 2.5 mm/s in motion. These tight constraints are needed, to ensure that Gaia's measurements of the stars and Solar System objects are as accurate as possible. ⁹⁶⁾

Astronomers within the DPAC community – first set up the GBOT project in early 2008 and trialled it on missions already in the same orbital location that Gaia will operate from – L2 – including NASA's WMAP and ESA's Planck satellites. This allowed the project to test their methods, and also get some clues about the probable magnitude that Gaia will have once in orbit. It is assumed that it will be around magnitude 18, but that it is still a big unknown.

Since then, a whole infrastructure was set up, developing observing techniques, a dedicated software pipeline, a database, and observatories were recruited to deliver the GBOT project data. The backbone of the data will be supplied by the 2 m Liverpool telescope, located on La Palma, Canary Islands, Spain, and the Las Cumbres Optical Global Telescope Network (LCOGT.net), which operates 1 m telescopes in Chile, South Africa, Australia and Texas. The project will also have some support from ESO's VST (2.6 m telescope at Paranal, Chile) and additional facilities will also provide data when needed.

In 2012, the project started a new fork of GBOT, radio-GBOT, which involves VLBI observations of Gaia. These are much more precise than the optical observations, but because they use more resources, the project will use this technique less often and therefore the radio data will be used only to complement the optical measurements.

The coordination of the GBOT activities is done from Heidelberg. The data reduction, analysis and storage, will be done at the Observatoire de Paris (with a mirror of the database in Heidelberg). The pipeline software, which has been developed by the GBOT group in Paris, imports and harmonises the data obtained from the partner observatories, processes the data and finally outputs the position of Gaia. The data is then delivered to ESA's MOC (Mission Operations Center) in Darmstadt via the SOC (Science Operations Center) in Villafranca near Madrid. Likewise, the reconstructed orbit files from ESOC are retrieved by GBOT, converted into data on Gaia's position, with finder charts, and then supplied to the partner observatories.

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Now, shortly before the launch of Gaia, GBOT is ready for action. GBOT's observations commence about 10 days after launch; any earlier and Gaia is too bright for the instruments of the partner institutes. The project hopes to obtain motion clips of the spacecraft moving in front of star fields as the satellite journeys towards L2. It will be a challenging task for the small team, but we will do our very best to deliver!

MSC (Mission Control System)

Mission operations will be conducted by the Flight Control Team at ESOC (European Space Operations Center) in Darmstadt Germany, comprising spacecraft operations (mission planning, spacecraft monitoring and control, and all orbit and attitude determination and control) as well as scientific instrument operations (quality control and collection of the science telemetry). The ground segment at ESOC will comprise all facilities, hardware, software and documentation required to conduct mission operations.

The ground operations facilities consist of:

• Ground stations and the communications network

• Mission control center

• FCS (Flight Control System)

• Software-based spacecraft simulator

All mission and flight control facilities, except the ground stations, are located at ESOC, including the interfaces for the provision of science telemetry to the SOC (Science Operations Center) at ESA/ESAC (European Space Astronomy Center), ESA facility in Villafranca, Spain, located about 30 km west of Madrid. ⁹⁷⁾

The science data will be distributed to ESAC after being stored in dedicated Science Data Servers at ESOC, via high-speed communication lines.

​​

The science data processing requirements for Gaia are among the most challenging of any scientific endeavor to date. Due to the immense volume of data that will be collected, for 1 billion stars, it will be a major challenge, even by the standards of computational power in the next decade, to process, manage and extract the scientific results necessary to build a 3-dimensional view of our Galaxy, the Milky Way.

A total of some 100 TB of science data will be collected during Gaia's lifetime. The estimated total data archive will surpass 1 PB (Petabyte or 10¹⁵ bytes), roughly equivalent to 1000 1 TB hard drives from a top-end home PC.

DPAC (Data Processing & Analysis Consortium)

Unlike a mission such as the Hubble Space Telescope, Gaia does not produce data that is immediately scientifically useful. The raw telemetry must first be processed before the sought after distances can be obtained, motions, and properties of the stars observed by Gaia. This immense task will be undertaken by a pan-European collaboration, the Gaia DPAC (Data Processing and Analysis Consortium). DPAC is responsible for the processing of Gaia‘s data with the final objective of producing the Gaia Catalogue. Drawing its membership from over 20 countries (Figure 68), the consortium brings together skills and expertise from across the continent, reflecting the international nature and cooperative spirit of ESA itself.

The DPAC consists of about 450 persons, spread over academic institutes and space agencies throughout Europe and beyond, who are actively contributing to writing the millions of lines of code needed for the data processing and to subsequently operate the software systems and validate the resulting output. Each DPAC is responsible for a different aspect of the Gaia data processing. ^{98) 99) 100) 101)}

​​

Legend to Figure 68: Next to the European country DPACs, there are also members in Brazil, Canada, Chile, Israel, and the USA.

To organise the large amount of tasks to be carried out, the DPAC has been subdivided into nine specialist units known as CUs (Coordination Units). Each CU takes the responsibility for the development of a specific part of the Gaia data processing: system architecture, simulations, astrometry, photometry, spectroscopy, object processing, variability processing, astrophysical parameters, and catalog publication. The CUs draw their membership from multiple countries.

​​

The astronomers in the CUs conceive the scientific algorithms for the data processing and also carry out a large fraction of the software development. The software is then run at one of the six DPCs (Data Processing Centers). The personnel at the data processing centers also provide the much needed software engineering expertise. Such a large software system cannot be developed and operated by astronomers alone!

The schematic of Figure 69 shows how each CU is supported by a specific DPC (indicated in red). The data exchange within DPAC will take place through the so-called MBD (Main Data Base), housed at ESAC. After the completion of a processing cycle, data is then extracted from the MDB and prepared for release.

Note that the Gaia project is unique in that the scientific data produced by DPAC are not subject to a proprietary period. On completion of a processing cycle the results are immediately available to the scientific community and also to the general public. The nine CUs and six DPCs are coordinated by an executive committee, the DPACE (DPAC Executive) as shown in Figure 70. ¹⁰²⁾

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Gaia's VO (Virtual Observatory) Big Data Archive

ESA's Gaia mission will survey the sky for at least 5 years providing high accuracy astrometry, radial velocities and multi-colour photometry. The DPAC (Data Analysis and Processing Consortium) efforts will result in an astronomical catalog with unprecedented accuracy and completeness of at least 1 billion (10⁹) sources, and over 1PB of associated data products. ¹⁰³⁾

This brings big data challenges in storing, querying and distributing all the associated data and meta data, comparing them with other astronomical catalogues, enabling analysis, visualisation, data mining and then sharing these results with other scientists. The amount of data involved forces a change of paradigm in dealing with astronomy archives. The usual usage of downloading the data to the users for her/him to work further on it needs towards evolve to a new way of working where the users' can send her/his code to the data, run it there on computing and storage services provided directly by the archive, where the data reside. The Gaia archive will provide an infrastructure to run added value interfaces and software on top of the Gaia data.

The first Gaia catalog will be publicly released to the scientific community around summer 2016, but the development of the Gaia archive has long started and an internal version is already available for the Gaia Consortium.

The term Big Data can be used and understood differently by people, but today industry widely refers to the five "Vs" (Volume, Velocity, Variety, Veracity and Value) when speaking about Big Data. From the Volume perspective, with "only" 1 PB of data produced by the end of its lifetime, Gaia could hardly be considered as a big data mission, as many other astronomy projects (in particular ground based telescope) will produce way more data volume in a shorter timescale.

The final delivery will also include all the single epoch CCD transit data that was used in the catalog computation, reaching an estimate of around 1PB at the end of the mission in 2022 (Table 5).

Nonetheless, the billions of CCD transits, measurements and spectra (Table 5) that will result into massive sources catalogs (Figure 71) for sure makes Gaia a big data challenge, from the data Velocity and Variety points of view.

​​

Ensuring the Veracity of the Gaia data represents one of the main big data challenge of the Gaia data processing. And to finish, by performing astrometry, photometry and spectroscopy of about one billion objects in our Milky Way galaxy and beyond, the extent and content of the Gaia Catalogs will enable major progress to be made in many fields of galactic and stellar astronomy, hence its Value definitely places Gaia as a major Big Data project in astronomy.

Standard Archive Architecture

Standard ESA space science archive architecture (Figure 3) is based on the OAIS (Open Archival Information System) architecture. Users and scientists are used to interact with data or catalogues, either through a browser user interface or scriptable command line interface. They download the data and the full catalogue, usually via FTP, to their local disk and perform their science analysis on their local computer.

This working model works fine for small amount of data, but becomes difficult as data volume grows. To reduce the data transfer burden, the users try to select region of the sky and download only part of the catalogue, and then combine it with their own datasets or catalogues already stored on their disk. This can be described as "move the data to the computing facility".

​​

The VO (Virtual Observatory) provides an unified framework which enables transparent access to astronomical science data holdings coming from various different archives. The very same command can be sent to archives located in different locations (and using their own internal storage and database systems) and present results in a consistent way to the end user or applications. Early developed interoperability VO protocols (e.g., ConeSearch, Simple Image and Simple Spectra Access) greatly facilitates access to multiple datasets, but still assumes that data are finally being downloaded to the user's computer.

Usually, science archives implements a "VO layer" on top of the existing archive infrastructure, so all the data holdings can be accessible through these VO protocols. This ensures the interoperability of the archives with other VO compliant archives and applications.

New Archive Paradigm for Gaia

With the avalanche of data in astronomy, the archive model previously described reaches its limit and a new paradigm needs to be established, the so called "move the code to the data". For Gaia, the amount of data and meta data is so big that special computing infrastructure is required to efficiently handle Gaia data. For example, a query of a cone search on the ~1 billion Gaia catalog might return 10 million sources. Another typical use case is to upload a table with sources and to cross match these with the Gaia catalog. This operation is made possible with the VO TAP (Table Access Protocol), coupled with ADQL (Astronomical Data Query Language, SQL with specialised astronomical searches). TAP also supports asynchronous query, as such a cross match can require too much time to be perform interactively. UWS (Universal Worker Service) enters in action to manage these asynchronous jobs. This back-end infrastructure serves all the front-end interfaces available at the Gaia archive.

Special emphasis has been put on the database design (based on PostgreSQL and pgSphere add-on which provides spherical data types, functions, and operators for PostgreSQL) and associated indexing. Furthermore, the Gaia archive is hosted on a powerful server and the most popular catalogs are stored on PCIe SSDs disks to ensure good performance of these crossmatch functions. Some examples of time required for some of the complex crossmatches are given in Table 6.
Note: PCIe (Peripheral Component Interconnect Express); SSD (Solid State Drive).

Legend to Table 6: Tycho2 vs. IGSL (Initial Gaia Source List) crossmatches are even faster than the ones with 2MASS as IGSL is located in the fastest local storage (PCIe), even when IGSL (similar to the final Gaia catalog) is around 3 times bigger than 2MASS.

End user can then interact with the Gaia through a standard GUI (Graphical User Interface) from any standard web browser. This offers a full ADQL query interface, with example to help the user familiarise with the Gaia catalog content and structure.

In addition, it is expected that many of the users will interface with Gaia data directly through scriptable interface. All operations available from the Gaia archive GUI (ie TAP/ADQL queries) can also be included directly in user's scripts. Various examples of such scripts are provided in the most commonly used programming language in astronomy (Python, C, Java).

VO applications (e.g., Topcat, VOSpec) can access directly the Gaia data through the corresponding VO protocol, TAP for table, Simple Access Protocol for spectra, without the need to develop a "VO layer" as seen in the standard ESA archive architecture.

When the user retrieves big volume of Gaia data, she/he would be able to download it to her/his local disk via FTP, but it will probably be more efficient to leave it on the archive disk itself to avoid the burden of the network transfer. This can be done with VOSpace, virtual disk accessible by VO data access protocols. By keeping the data on her/his VOSpace, the user can continue to interact with it and as well share it with any other Gaia archive user, to facilitate scientific collaboration.

​​

A similar mechanism is provided for meta data storage. If the user would download the results of a complex search (i.e., returning millions of entries) on her/his computer, she/he will probably need to ingest these into a local database to continue to work on them. It appears then more efficient to provide to the user some space directly within the Gaia archive database to store the results of her/his searches. As such, the user "database" becomes VO compliant and shareable as well with any other Gaia archive user.

Overall, the Gaia archive architecture depicted in Figure 73 has been built around these VO protocols and has become the first "VO built-in" archive. By design, the Gaia archive also becomes immediately interoperable with any other VO compliant archive and application.

Gaia AVIs (Added Value Interfaces)

The user workspace can be brought one step further to enable the user to also run her/his own code directly on the Gaia archive through so called "Gaia Added Value Interfaces" (Gaia AVIs, Figure 74). Four AVI demonstrators are currently being developed for transient alerts, advanced visualisation, spectral classification and temporal analysis. These AVIs will run using containers (Docker) and will make use of the Gaia Archive VO built-in protocols (TAP, VOSpace). A Gaia AVI Portal will be created and users will be able deposit their own code and can run it on their data located into their user workspace (VOSpace and database). AVI templates will be provided to help the user to develop their own AVIs.

The Gaia AVI project is currently being developed as a proof of concept project to be delivered in 2017 and could become the new framework for collaborative "Archive 2.0".

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Big Data Visualisation

Before being able to search for Gaia data, it might be really helpful to provide visualisation of the Gaia data in various ways, such as density maps, 1D histograms (Figure 75) or again interactive visualisation through VO application (Aladin Lite), integrated into the archive GUI, through another VO protocol SAMP (Simple Application Messaging Protocol). The production of such graphs requires the use of big data reduction techniques, such as Map / Reduce. With 10 parallel threads on a powerful machine with big RAM (1TB), fast disks (PCIe SSDs with fast random IO) and efficient database (PostgreSQL), the production of the density maps for the GUMS simulated catalogue (2.14 billion rows) took less than 2 minutes and the ones for the IGSL (1.22 billion rows) as little as 65 seconds.

​​

Another way to visualise Gaia data will be through the recently released science-driven discovery portal for all the ESA Astronomy Missions called "ESA Sky" that allow users to explore the multi-wavelength sky and to seamlessly retrieve science-ready data in all ESA Astronomy mission archives from a web application without prior-knowledge of any of the missions. Among other things, the system offers progressive multi-resolution all-sky projections of full mission datasets using a new generation of HiPS (Hierarchical Progressive Survey) files. HiPS is based on the HEALPix sky tessellation and is essentially a mapping of survey data at various spatial resolutions into a collection of HEALPix tiles. It is particular adapted to big data visualisation as it allows a dedicated client/browser tool to access and display a survey progressively, based on the principle that "the more you zoom in on a particular area the more details show up".

Summary: Gaia, ESA's cornerstone mission currently in operations, represents one of the major big data challenge in astronomy to date. A totally new archive architecture (both hardware and software) has been developed to tackle this challenge. It results into one of the first VO built-in science archive, paving the way towards flexible, open and interoperable archive services. The user will work directly with the data in the archive through dedicated user workspace, without the need to transfer it to her/his location, She/he will be able to become an actor of the archive with the possibility to bring her/his code to the data and share it with other archive users. This new "Archive 2.0" concept will be the mean to fully exploit the science legacy of the Gaia mission.

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