Minimize GAIA (Global Astrometric Interferometer for Astrophysics)

GAIA (Global Astrometric Interferometer for Astrophysics) Mission

Spacecraft   Launch   Mission Status   Payload Module   Ground Segment   Big Data Archive   References

Gaia (mother Earth in Greek mythology) is an ESA cornerstone space astrometric mission, part of the Horizon 2000 Plus long-term scientific program, with the goal to compile a 3D space catalog of > 1000 million stars, or roughly 1% of the stars in our home galaxy, the Milky Way. Gaia will monitor each of its target stars about 70 times to a magnitude of G=20 over a period of 5 years. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission will also study about 500,000 distant quasars and will provide stringent new tests of Albert Einstein's General Theory of Relativity. 1) 2) 3) 4) 5)

Cataloguing the night sky is an essential part of astronomy. Before astronomers can investigate a celestial object, they must know where to find it. Without this knowledge, astronomers would wander helplessly in what Galileo once termed a ‘dark labyrinth'.

During the satellite's expected lifetime of five years, Gaia will observe each star about 70 times, each time recording its brightness, color and, most importantly, its position. The precise measurement of a celestial object's position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers.

By comparing Gaia's series of precise observations, today's astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia's highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies.

The primary mission objectives are:

• Measure the positions and velocity of approximately one billion stars in our Galaxy

• Determine their brightness, temperature, composition and motion through space

• Create a three-dimensional map of the Galaxy.

Additional discoveries expected:

- hundreds of thousands of asteroids and comets within our Solar System

- seven thousand planets beyond our Solar System

- tens of thousands of ‘failed' stars, called brown dwarfs

- twenty thousand exploding stars, called supernovae

- hundreds of thousands of distant active galaxies, called quasars.

Gaia objective is to provide a very accurate dynamical 3D map of our Galaxy by using global astrometry from space, complemented with multi-color multi-epoch photometric measurements. The aim is to produce a catalog complete for star magnitudes up to 20, which corresponds to more than one billion stars or about 1% of the stars of our Galaxy. The instrument sensitivity is such that distances beyond 20-100 kiloparsec (kpc) will be covered, therefore including the Galaxy bulge (8.5 kpc) and spiral arms. The measurements will not be limited to the Milky Way stars. These include the structure, dynamics and stellar population of the Magellanic Clouds, the space motions of Local Group Galaxies and studies of supernovae, galactic nuclei and quasars, the latter being used for materializing the inertial frame for Gaia measurements.

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Figure 1: Gaia measurements objectives (image credit: ESA, Airbus Defence and Space) 6)

 

Background: Gaia is ESA's second space mission dedicated to astrometry. It builds on the legacy of the successful Hipparcos mission (1989-1993). 7) Like Hipparcos, Gaia's observation strategy is based on detecting stellar positions in two fields of view separated by a 'basic angle', which for Gaia is 106.5º. This strategy allows astronomers to establish a coherent reference frame over the entire sky, yielding highly accurate measurements of stellar positions.

After a detailed concept and technology study during 1998–2000, Gaia was selected as a confirmed mission within ESA's scientific program in October 2000. It was confirmed by ESA's Science Program Committee following a re-evaluation of the science program in June 2002, and reconfirmed following another re-evaluation of the program in November 2003. The project entered Phase-B2/C/D in February 2006. As of the summer 2012, Gaia is in Phase-D (Qualification and Production) and will be launched in the second half of 2013. 8) 9) 10)

• In June 2013, ESA's billion-star surveyor, Gaia, has completed final preparations in Europe and is ready to depart for its launch site in French Guiana. The Gaia spacecraft arrived in Cayenne, French Guiana, on August 23, 2013 on board the Antonov 124 aircraft.

• On Oct. 23, 2013, ESA postponed the launch of the Gaia mission. The decision was taken due to a technical issue that was identified in another satellite already in orbit. The issue concerns components used in two transponders on Gaia that generate ‘timing signals' for downlinking the science telemetry. To avoid potential problems, they will be replaced.

The transponders were removed from Gaia at Kourou and returned to Europe, where the potentially faulty components were replaced and verified. After the replacements have been made, the transponders will be refitted to Gaia and a final verification test made. As a consequence of these precautionary measures, it will not be possible to launch Gaia within the window that includes the previously targeted launch date of 20 November. The next available launch window is 17 December to 5 January 2014. 11)

• Update Oct. 20, 2013: The upcoming launch manifest of Arianespace has now been established. Gaia is scheduled for launch on 20 December.

• Update Nov. 22, 2013: The checks on the Gaia satellite are proceeding well, enabling the launch to take place on December 19, 2013 (Ref. 11).

 

Some astrometry basics:

The precise measurement of a celestial object's position is known as astrometry, and since humans first started studying the sky, astronomers have devoted much of their time to this art. However, Gaia will do so with extraordinary precision, far beyond the dreams of those ancient astronomers (Ref. 21). 12)

By comparing Gaia's series of precise observations, today's astronomers will soon be able to make precise measurements of the apparent movement of a star across the heavens, enabling them to determine its distance and motion through space. The resulting database will allow astronomers to trace the history of the Milky Way.

In the course of charting the sky, Gaia's highly superior instruments are expected to uncover vast numbers of previously unknown celestial objects, as well as studying normal stars. Its expected haul includes asteroids in our Solar System, icy bodies in the outer Solar System, failed stars, infant stars, planets around other stars, far-distant stellar explosions, black holes in the process of feeding and giant black holes at the centers of other galaxies. Gaia will be a discovery machine.

Stars as individuals and collectives:

To understand fully the physics of a star, its distance from Earth must be known. This is more difficult than it sounds because stars are so remote. Even the closest one is 40 trillion km away, and we cannot send spacecraft out to them to measure as they go. Nor can we bounce radar signals off them, which is the method used to measure distances within the Solar System. Instead, astronomers have developed other techniques for measuring and estimating distances.

The most reliable and only direct way to measure the distance of a star is by determining its 'parallax'. By obtaining extremely precise measurements of the positions of stars, Gaia will yield the parallax for one billion stars; more than 99% of these have never had their distances measured accurately. Gaia will also deliver accurate measurements of other important stellar parameters, including the brightness, temperature, composition and mass. The observations will cover many different types of stars and many different stages of stellar evolution.

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Figure 2: Distance to a star can be calculated with simple trigonometry from the measured parallax angle (1 a.u. is 1 Astronomical Unit, or 149.6 million km), image credit: ESA/Medialab

The principles of Gaia:

At its heart, Gaia is a space telescope – or rather, two space telescopes that work as one. These two telescopes use ten mirrors of various sizes and surface shapes to collect, focus and direct light to Gaia's instruments for detection. The main instrument, an astrometer, precisely determines the positions of stars in the sky, while the photometer and spectrometer spread their light out into spectra for analysis.

Gaia's telescopes point at two different portions of the sky, separated by a constant 106.5º. Each has a large primary mirror with a collecting area of about 0.7 m2. On Earth we are used to round telescope mirrors, but Gaia's will be rectangular to make the most efficient use of the limited space within the spacecraft. These are not large mirrors by modern astronomical standards, but Gaia's great advantage is that it will be observing from space, where there is no atmospheric disturbance to blur the images. A smaller telescope in space can yield more accurate results than a large telescope on Earth.

Gaia is just 3.5 m across, so three curved mirrors and three flat ones are used to focus and repeatedly fold the light beam over a total path of 35 m before the light hits the sensitive, custom-made detectors. Together, Gaia's telescopes and detectors will be powerful enough to detect stars up to 400,000 times fainter than those visible to the naked eye.

Gaia uses the global astrometry concept demonstrated by Hipparcos. The principle is to link stars with large angular distances in a network where each star is connected to a large number of other stars in every direction. The condition of closure of the network ensures the reduction of the position errors of all stars. This is achieved by the simultaneous observation of two fields of views separated by a very stable basic angle. The spacecraft is slowly rotating at a constant angular rate of 1º/min around a spin axis perpendicular to both fields of view, which describe a great circle on the sky in 6 hours. The spacecraft rotation axis makes an angle of 45º with the Sun direction (Figure 3). A slow precession around the Sun-to-Earth direction, with a 63.12 days period, enables to repeat the observation of sky objects with 86 transits on average over the 5 years of mission.

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Figure 3: Illustration of the sky scanning principle (image credit: ESA)

The resulting performance will enable a breakthrough in the astrometry field, as well regarding star position and velocity performance as for the number of objects observed.

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Figure 4: Gaia will improve the accuracy of astrometry measurements by several orders of magnitude compared with previous systems and observations (image credit: ESA)

 


 

Spacecraft:

Gaia is an exceptionally complex space observatory. ESA awarded Airbus Defence and Space (former Astrium SAS,Toulouse, France) the prime contract in May 2006 to develop and build the spacecraft. Together with the German and British branches of Astrium, more than 50 industrial subcontractor companies from across Europe are involved in building this discovery machine. The Gaia DPAC (Data Processing and Analysis Consortium) will process the raw data to be published in the largest stellar catalog ever made. 13) 14) 15) 16) 17) 18) 19) 20)

The Gaia spacecraft is composed of two sections: the Payload Module and the Service Module. The Payload Module is housed inside a protective dome and contains the two telescopes and the three science instruments. They are all mounted on a torus made of a ceramic material (silicon carbide). The extraordinary measurement accuracy required from Gaia calls for an extremely stable Payload Module that will barely move or deform once in space; this is achieved thanks to the extensive use of this material. 21)

Underneath the Payload Module, the Service Module contains electronic units to run the instruments, as well as the propulsion system, communications units and other essential components. These components are mounted on CFRP (CarbonFiber Reinforced Plastic) panels in a conical framework.

Finally, beneath the Service Module, a large sunshield keeps the spacecraft in shadow, maintaining the Payload Module at an almost constant temperature of around -110ºC, to allow the instruments to take their precise and sensitive readings. The sunshield measures about 10 m across, too large for the launch vehicle fairing, so it comprises a dozen folding panels that will be deployed after launch. Some of the solar array panels that are needed to generate power are fixed on the sunshield, with the rest on the bottom of the spacecraft.

The Gaia spacecraft configuration is driven by the required very high thermo-mechanical stability of the entire spacecraft. A low disturbance cold gas micro-propulsion is used for fine attitude control. The astrometric instrument is used for precise rate sensing in fine pointing operating mode.

Spacecraft launch mass

~2034 kg (dry mass = 1392 kg)

Spacecraft power

1.91 kW (EOL)

Spacecraft dimensions

Height = 3.5 m, deployed diameter = 10 m

Mission duration

5 years

Table 1: Parameters of the Gaia spacecraft

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Figure 5: Artist's rendition of the deployed GAIA spacecraft (image credit: ESA)

 

SVM (Service Module):

The SVM is generally referred to as 'the platform'. The SVM in turn is comprised of MSM (Mechanical Service Module) and an ESM (Electrical Service Module). 22) 23) 24)

MSM (Mechanical Service Module): The spacecraft main structure is of hexagonal conical shape. It is a sandwich panel structure with CFRP (Carbon Fiber Reinforced Plastic) face sheets, and a central cone supporting the propellant tanks. The MSM houses instruments needed for the basic control and operation of the satellite; this includes all mechanical, structural and thermal elements that support the instrument payload and spacecraft electronics. It also includes the chemical & micro propulsion systems, the deployable sunshield with solar arrays, the payload thermal tent and harness. The module consists of a central tube that is about 1.17 m long and hosts six radial panels to create a hexagonal spacecraft shape.

The service module also houses the communication subsystem, central computer and data handling subsystem, the high rate data telemetry, attitude control and star trackers. For telemetry and telecommand, low gain antenna uplink and downlink with a few kbit/s capacity are employed. The high gain antenna used for the science telemetry downlink will be used during each ground station visibility period of an average of about 8 hours per day.

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Figure 6: Photo of the SVM integration (image credit: EADS Astrium)

ESM (Electrical Service Module): The ESM design is driven by the science performance (attitude control laws with the hybridization of star tracker and payload measurements, high rate data telemetry, and regulated power bus for thermal stability). It houses the AOCS units, the communication subsystem, central computer and data handling subsystem, and the power subsystem.

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Figure 7: Diagram of the ESM (image credit: EADS Astrium)

AOCS (Attitude and Orbit Control Subsystem). The AOCS subsystem is characterized by:

- High precision 3-axis control

- The ASTRO (Astrometric) instrument is used for precise rate sensing during the fine pointing operational mode

- A high precision gyroscope is used for quick and efficient transitions during the fine pointing operational mode. Three FOGs (Fiber Optics Gyroscopes) use the interference of light to detect mechanical rotation. Each unit contains four closed-loop gyroscope channels to provide built-in redundancy.

- Rugged flight-proven initial acquisition and safe modes

- Three sun acquisition sensors plus one gyroscope provide spin-axis stabilization during the L2 transfer phase of the mission

- One large field of view star sensor plus use of the main instrument SM (Sky Mapper) for the 3-axis controlled operational phase.

Gaia AOCS architecture is based on a fully redundant set of equipment. Moving parts on board are strictly minimized (e.g. no reaction wheels, no mechanically steerable antenna). The data is downlinked through a novel electromagnetically steerable phased array antenna and attitude control is provided by a micro propulsion system that has its first flight use with Gaia. An atomic clock is used for precise time-stamping.

Two Autonomous Star Trackers are used in cold redundancy three FSS (Fine Sun Sensors) are used in hot redundancy through triple majority voting. Three Gyro packages provide coarse rate measurements where each gyro package comprises two fully independent co-aligned channels (i.e. a fiber optic gyroscope sensor plus associated electronics per channel), with the channels being used in cold redundancy. As mentioned above, the payload module provides very precise rate measurements when the spacecraft is operated in fine pointing science modes. 25)

For actuators, a bi-propellant Chemical Propulsion System (2 x 8 10 N thrusters used in cold redundancy) is used for orbit maintenance and attitude control in coarse AOCS modes (circa 350 kg MON + MMH).

The Micro-propulsion System (2 x 6 proportional micro-thrusters) can provide a range of 0 - 1000 µN at a resolution of 0.1 µN. The individual thrusters are driven by the micro-propulsion electronic, which is internally redundant and used in cold redundancy (circa 57 kg of GN2). The nominal science AOCS mode uses the cold gas micro-propulsion system.

C&DMS (Command & Data Management Subsystem). The C&DMS is characterized by:

- An ERC-32 based central computer and distinct input/output units for efficient software development

- Two segregated MIL-STD-1553 B data buses: one for the payload module and one for the service module

- SpaceWire data links for high-speed payload data

- FDIR architecture aiming at preserving payload integrity, with built-in autonomy for increased availability.

The PDHU (Payload Data Handling Unit) is, among other things, the 'hard-disk' of Gaia, responsible for temporary storage of science data received from the telescope before transmission back to Earth. It will receive thousands of compressed images per second from the observing system; this data will be sorted and stored. The individual star data objects will be prioritized based on the magnitude of the star. A complex file management system allows deletion of low-priority data in the event of data rates or volumes that exceed the capacity of the storage or transmission systems.

The solid-state storage subsystem of the PDHU has a capacity of 960 GB which, while not impressive by terrestrial standards, is extremely large for a space system. It uses a total of 240 SDRAM modules, each with a capacity of 4 GB, which populate six memory boards. The PDHU controller board is responsible for communication with the other spacecraft subsystems, file system management and the management of telemetry and telecommands. 26) 27)

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Figure 8: The PDHU (Payload Data Handling Unit), image credit: ESA

The PDHU communicates with the gigapixel focal plane over seven redundant 40 Mbit/s SpaceWire channels to acquire the scientific data coming from the seven VPUs (Video Processing Units) of the camera. The unit's controller sorts the incoming data according to star magnitude and manages deletion of low priority data should this become necessary. It sends data for transmission to Earth under the control of the CDMU (Command and Data Management Unit). The PDHU communicates with the CDMU via a MIL-STD-1553 data bus and delivers the science data over two 10 Mbit/s PacketWire channels.- The PDHU consumes only 26 W, has a mass of 14 kg, and occupies a volume of 2.3 liter.

EPS (Electrical Power Subsystem): The spacecraft is equipped with a 12.8 m2 high-efficiency triple-junction GaAs (Gallium-Arsenide) cell solar array, of which 7.3 m2 is in the form of a fixed solar array and 5.5 m2 is covered by 6 panels mechanically linked to deployable sunshield assembly.

For the launch, the deployable sunshield is folded against the payload module. After separation from the launch vehicle, it is deployed around the fixed solar array, in the same plane. During LEOP (Launch and Early Operations Phase), power is supplied by a 60 Ah mass-efficient Lithium-ion battery.

Optimum power supply during all phases of the mission is ensured by a PCDU (Power Control and Distribution Unit) with maximum power point tracking. The PCDU performs power management by generating a 28 V primary power bus that supplies power to all spacecraft subsystems. It also controls the battery state of charge and generates pyrotechnic commands as well as heater actuation as commanded by the C&DMS (Command & Data Management Subsystem).

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Figure 9: Photo of the battery (image credit: ABSL)

Propulsion: After injection into the L2 transfer orbit by the Soyuz-Fregat launcher, a chemical bi-propellant propulsion system (8 x 10 N) is used for the transfer phase. It will cover attitude acquisition, spin control, mid-course corrections, L2 orbit injection, and safe mode.

After arriving at L2, one redundant set of micro-propulsion thrusters will control the spin and precession motion of the spacecraft. Regular orbit maintenance will be performed by using the chemical propulsion thrusters. - The spacecraft uses a cold gas micropropulsion system for fine attitude control.

CPS (Chemical Propulsion Subsystem): CPS is a bi-propellant system using two tanks of Herschel/Planck heritage filled with with a total of ~400 kg of propellant featuring a blowdown ratio of 4:3. Use of monomethylhydrazine as fuel and nitrogen tetroxide as oxidizer. The 10 N thrusters are manufactured by Astrium consisting of a platinum alloy combustion chamber and nozzle that tolerates the operational temperature of 1,500°C. The thruster can be operated in a thrust range of 6 to 12.5 N with a nominal thrust of 10 N which generates a specific impulse of 291 seconds.

MPS (Micro Propulsion Subsystem): The MPS is being used for fine attitude pointing and spin rate management. A total of 12 cold gas thrusters are installed on the spacecraft being grouped in three clusters each featuring four cold gas thrusters. The thruster system uses high-pressure nitrogen propellant to provide very small impulses with a thrust range of 1 - 500 µN. The system uses two nitrogen tanks, each containing 28.5 kg of N2, stored at a pressure of 310 bar (Ref. 19). The CG-MPS (Cold Gas-Micro Propulsion Subsystem) was developed by TAS-I and Selex ES S.p.A., Italy. 28)

RF communications: All communication with the Gaia spacecraft is done using the X-band. For TT&C (Tracking Telemetry and Command), a low gain antenna uplink and downlink with a few kbit/s capacity and an omnidirectional coverage are employed. The science telemetry X-band downlink is based on a set of electronically-scanned phased array antennae accommodated on the service module bottom panel. This high gain antenna is used during each ground station visibility period of about 8 hours per day.

The X-band payload downlink rate is 10 Mbit/s from L2. To achieve this, Gaia uses a specially designed on-board phased array antenna to beam the payload data to Earth (a conventional steerable antenna would disturbed the very precise measurements).

Gaia is equipped with a total of three communication antennas – two LGAs (Low Gain Antennas) and a single X-band Medium Gain Phased Array Antenna. One LGA is located pointing in the +X direction while the other points to –X being located on the Thermal Tent and the base of the spacecraft, respectively. The two LGAs build an omni-direction communications system for housekeeping telemetry downlink and command uplink with data rates of a few kbit/s.

The MGA (Medium Gain Antenna) is located on the base of the Payload Module, protruding the DSA(Deployable Sunshield Assembly) . This directional antenna can achieve data rates of up to 10 Mbit/s for science data and telemetry downlink and telecommand reception.

Demodulation of the uplink signal is completed by the transponder units before the data flow is passed on to the CDMU (Command & Data Management Unit). The downlink data is encoded by the CDMU and modulated in X-band within the transponders before being amplified by the SSPA (Solid State Power Amplifier). The signal is combined in the phased array of the active antenna in order to orient the beam towards the Earth.

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Figure 10: Photo of the phased array antenna (image credit: EADS Astrium)

CCSDS Image Data Compression ASIC: In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers', such as bright stars and planets, and which are marred by the momentary ‘hot pixels' due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution, enabling all Gaia mission data to reach their home planet. 29)

CWICOM (CCSDS Wavelet Image COMpression ASIC) is a very high-performance image compression ASIC that implements the CCSDS 122.0 wavelet-based image compression standard, to output compressed data according to the CCSDS output source packet protocol standard. This integrated circuit was developed by Airbus DS through an ESA contract.

CWICOM offers dynamic, large compressed-rate range and high-speed image compression potentially relevant for compression of any 2D image with bi-dimensional data correlation (such as a hyperspectral data cube). Its highly optimized internal architecture allows lossless and lossy image compression at very high data rates (up to 60 Mpixels/s) without any external memory by taking advantage of its on-chip memory – almost 5 Mbit of embedded internal memory).

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Figure 11: The CWICOM ASIC is a customized microchip for imaging data compression (image credit: Airbus DS, ESA)

CWICOM is implemented using the largest matrix of the Atmel ATC18RHA ASIC family, and is provided within a standard surface mount package (CQFP 256). CWICOM offers a low-power, cost-effective and highly integrated solution for any image compression application, performing CCSDS image compression treatments without requiring any external memory. The simplicity of such a standalone implementation is achieved thanks to a very efficient internal embedded memory organization that removes any need for extra memory chip procurement and the potential obsolescence threatened by being bound to a specific external memory interface.

Change in data compression implementation:

In order to transmit all the data generated on board, a particularly challenging compression factor averaging 2.8 was necessary. Unfortunately the standard suite of algorithms [which includes CCSDS standards, including the 122.0 implemented by CWICOM] was not able to reach this target, because of the peculiarities of Gaia imagery, which include ‘outliers', such as bright stars and planets, and which are marred by the momentary ‘hot pixels' due to cosmic rays in deep space. Instead, with the support of ESA compression experts, industry developed an ad hoc solution,enabling all Gaia mission data to reach their home planet.

Instead of CWICOM, Gaia applies a tailored data compression algorithm (using a heavily tailored pre-processing stage followed by a variant of the Rice coder), using a software implementation running on VPUs (Video Processing Units). The GOCA (GAIA Optimum Compression Algorithm) project was entrusted by ESA to GTD System & Software Engineering (project prime) and IEEC (Institut d'Estudis Espacials de Catalunya), scientific partner), aimed in providing a deep understanding of the GAIA compression problem and offering a complete data compression system, both at an algorithm and implementation level. The main objectives of GOCA not only encompassed the review and evaluation of the already proposed compression scheme but the design of new algorithms for the mission. 30) 31) 32)

The CCDs in the focal plane are commanded by video-processing units (VPUs). Gaia has seven identical VPUs, each one dealing with a dedicated row of CCDs. Each CCD row, contains in order, two SM CCDs (one for each telescope), 9 AF CCDs, 1 BP CCD, 1 RP CCD, and 3 RVS CCDs (the latter only for four of the seven CCD rows). The VPUs run seven identical instances of the video-processing algorithms (VPAs), not necessarily with exactly the same parameter settings though. This (mix of some hardware and mostly) software is responsible for object detection (after local background subtraction), object windowing (see below), window conflict resolution, data binning, data prioritization, science-packet generation, data compression, etc.) 33)

TCS (Thermal Control Subsystem): A deployable sunshield with optimal thermoelastic behavior, made of multi-layer insulation sheets, is attached to the service module and folded against the payload module for the launch. After separation of the Gaia spacecraft from the launch vehicle, the Sun shield is deployed around the fixed solar array, in the same plane. - A thermal tent covers the payload, offering extra protection against micrometeoroids and radiation.

The very high stability thermal control is mostly passive and is achieved through optical surface reflector material, multilayer insulation sheets on the outer faces of the service module, and a black painted cavity, supplemented by heaters where required. Thermal stability is guaranteed by a constant solar aspect angle and the avoidance (as far as possible) of any equipment switch-ON/OFF cycles during nominal operation.

 

DSA (Deployable Sunshield Assembly): The bottom floor of the SVM is a dodecagonal-shaped panel to comply with the 12 frames of the DSA The main structure consists of carbon-fiber reinforced plastic face sheets.

DSA is folded up during launch and is deployed early in the flight. It is required to shade the payload unit and protect it from direct sunlight that could compromise instrument accuracy. Keeping the instrument at a constant temperature prevents expansion and contraction during temperature variations which would alter the instrument geometry ever so slightly with a large effect on data quality. The DSA is 10 m in diameter.

The DSA is an umbrella-type structure that consists of MLI (Multilayer Insulation) as the primary shield material and six rigid deployment booms as well as six secondary stiffeners. These booms have a single articulation on the base of the Service Module for easy deployment in the radial direction by a spring system. Spacing cables link the booms to the others to ensure a synchronized deployment sequence. The booms and strings are located on the cold side of the cover to limit thermoelastic flexing.

Attached to the DSA are six rectangular solar panels (with triple-junction solar GaAs cells) that are constantly facing the sun once the shield is deployed. They provide 1910 W of EOL (End of Life) power.

 

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Figure 12: Photo of Gaia's DSA deployment (image credit: Astrium SAS)

Legend to Figure 12: The DSA during deployment testing at Astrium Toulouse. Since the DSA will operate in microgravity, it is not designed to support its own weight in the one-g environment at Earth's surface. During deployment testing, the DSA panels are attached to a system of support cables and counterweights that bears their weight, preventing damage and providing a realistic test environment. The flight model thermal tent is visible inside the deploying sunshield and the mechanically representative dummy payload can be seen through the aperture in the tent.

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Figure 13: Photo of the Gaia SVM in the EMC chamber at Intespace, Toulouse, during launcher EMC compatibility testing (image credit: Astrium SAS)

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Figure 14: Exploded view of the Gaia spacecraft (image credit: EADS Astrium)

Propulsion systems

- 8 x 10 N bipropellant thrusters for coarse attitude control, orbit injection and trajectory corrections
- Low disturbance cold gas micro-propulsion for fine attitude control

Command and data management system

- ERC 32 based central computer with embedded solid state mass memory, complemented by a distinct input / output management unit
- Two segregated MIL-STD-1553B data bus, one for Payload Module and one for Service Module
- SpaceWire data links for high speed payload data

3-axes AOCS (Attitude and Orbit Control Subsystem)

- Payload instrument used for precise rate sensing in science mode
- High precision gyroscope to support transition to fine science pointing with instrument in the loop
- 3-axes autonomous star sensor for inertial attitude measurement
- Fine sun sensor for independent attitude monitoring

TT&C (Telemetry, Tracking and Command Subsystem)

- All X-band system
- Low Gain Antenna for spacecraft commanding, used also for spacecraft telemetry during critical phases
- Electrically steerable high gain antenna with built in solid state amplifiers for science downlink up to 8.7 Mbit/s from L2 orbit without mechanical disturbances

EPS (Electrical Power Subsystem)

- 60 Ah Li-ion battery
- 13 m2 triple junction cell solar array
- Power control and distribution with maximum power point tracking system

Payload data handling

- 7 Video Processing Units in parallel, each provided with 1 GIPS (Giga Instruction/s) processing capability
- 1 Terabit mass memory
- Atomic rubidium clock for accurate sequencing of the focal plane and star measurement dating

TCS (Thermal Control Subsystem)

- Fully passive thermal control
- Thermal tent for payload instrument, providing extra protection against space environment
- 100 m2 sunshield featuring a 0.1º flatness

Structure

- All Silicium Carbide instrument: structure & mirrors
- CFRP (Carbon Fiber Reinforced Plastic) SVM structure

Table 2: Summary of spacecraft subsystems (Ref. 6)

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Figure 15: Alternate exploded view of the Gaia spacecraft elements (image credit: EADS Astrium)

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Figure 16: The Gaia flight model spacecraft undergoing final electrical tests at Astrium Toulouse in June 2013 (image credit: EADS Astrium)

 

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Figure 17: Photo of the Gaia spacecraft in Nov. 2013 with an Astrium AIT engineer installing the transponders at the launch site (image credit: ESA)

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Figure 18: Photo of the Gaia spacecraft, tucked up inside the Soyuz fairing, ready to be mated with the Soyuz lower stages (image credit: ESA, M. Pedoussaut) 34)

 

Launch: The GAIA spacecraft was launched on December 19, 2013 (09:12:19 UTC) from Kourou by Arianespace, Europe's Spaceport in French Guiana. The launch vehicle was a Soyuz-STB with a Fregat-MT upper stage The launch is designated as Soyuz flight VS06. 35) 36) 37)

- About ten minutes later, after separation of the first three stages, the Fregat upper stage ignited, delivering Gaia into a temporary parking orbit at an altitude of 175 km.

- A second firing of the Fregat 11 minutes later took Gaia into its transfer orbit, followed by separation from the upper stage 42 minutes after liftoff. Ground telemetry and attitude control were established by controllers at ESOC (European Space Operation Centre) in Darmstadt, Germany, and the spacecraft began activating its systems.

- The sunshield, which keeps Gaia at its working temperature and carries solar cells to power the satellite, was deployed in a 10 minute automatic sequence, completed around 88 minutes after launch. Gaia is now en route towards L2 (Ref. 35).

Orbit: Large Lissajous orbits around L2 (Lagrangian Point 2), about 1.5 million km from Earth. L2 offers a stable thermal environment because the sunshield will protect Gaia from the Sun, Earth and Moon simultaneously, allowing the satellite to keep cool and enjoy a clear view of the Universe from the other side. In addition, L2 provides a moderate radiation environment, which benefits the longevity of the instrument detectors.

• The critical LEOP (Launch and Early Orbit Phase) will last approximately four days. In this phase, Gaia will perform the first activations – transmitter switch ON, priming of the chemical thrusters, first attitude control and finding of the sun position – followed by the sun shield deployment. Engineers on ground will perform orbit determination, then prepare and execute the critical 'Day 2' maneuver to inject Gaia into its final transfer trajectory toward the L2 Lagrange point (Ref. 128).

• LEOP will be followed by the transfer cruise phase, lasting up to 30 days, an L2 orbit injection maneuver, then the in-orbit commissioning phase, during which all operations to prepare for the routine operational phase are performed. In particular, the scientific FPA (Focal Plane Assembly) and related avionics will be thoroughly tested and calibrated. The commissioning phase is expected to last four months.

• The insertion into the final 300,000 x 200,000 km Lissajous orbit around L2 was performed one month after launch.

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Figure 19: Gaia mission scenario, from launch to in-orbit operations (image credit: ESA)