BRITE (BRIght-star Target Explorer) Constellation / BRITE Austria, UniBRITE
BRITE (BRIght-star Target Explorer) / CanX-3 (Canadian Advanced Nanosatellite eXperiment-3) is a low-cost Austrian/Canadian constellation of nanosatellites - a collaborative science demonstration mission of the University of Toronto, Institute for Aerospace Studies/Space Flight Laboratory (UTIAS/SFL), the Graz University of Technology (TU Graz), and the Institute of Astronomy at the University of Vienna, both of Austria, with Sinclair Interplanetary, and Ceravolo Optical Systems Ltd., Ontario, as subcontractors. 1) 2) 3) 4) 5) 6) 7) 8) 9)
The objective is to make photometric observations of some of the apparently brightest stars in the sky and to examine these stars for variability (low-level oscillations and temperature variations). The observations will have a precision at least 10 times better than achievable using ground-based observations; the paylaod will be packaged inside a CanX-class nanosatellite (CanX-3). The mission's science team includes collaborators from Canada and Austria: the University of British Columbia (UBC), l'Université de Montréal, the University of Toronto, and the University of Vienna (Universität Wien).
The BRITE constellation aims to be a modest sized instrument operating in low Earth orbit, above the effects of the atmosphere, capable of fulfilling the science objectives. It consists of a maximum of two nanosatellites, each equipped with a small-lens telescope, able to observe the brightest stars in the sky to visual magnitude 3.5, over a FOV of 24º, with a sampling time of up to 15 minutes once per satellite orbit (typically 100 minutes), and with a differential brightness measurement accurate to at least 0.1% per sample (all numbers are minimum requirements; actual performance may be better). A cluster of four satellites is needed to improve the duty cycle and would obtain color information (with two satellites having blue and two having red filters).
Table 1: Science requirements for a bright star photometry mission
Selection of a constellation: There are many reasons for using a constellation of nanosatellites over a single satellite. For one, each telescope will be optimized to work with only one color filter. By collecting color and intensity data from at least two different BRITE satellites, each with a different filter, the science capacity of BRITE is greatly enhanced. Color provides temperature information that helps astronomers to better identify modes of oscillation.
The BRITE telescope is being designed to have no moving parts (low cost amd risk mitigation). This implies that filter changes cannot be done by using one BRITE telescope in orbit. In having two nanosatellites in the same orbit, each with a different filter, the development costs can be minimized by keeping each BRITE satellite identical except for the telescope and by reducing nonrecurring engineering costs.
Another reason to have a constellation of nanosatellites is that multiple satellites observing the same region of interest can increase the overall duty cycle of observation beyond what a single nanosatellite can provide. Since the stars of interest are located in all parts of the sky, a continuous viewing zone for all targets of interest is not possible. - Having two pairs of BRITE satellites in two slightly different orbits that have different viewing times for the same region of interest doubles the duty cycle and significantly improves the spectral window.
A final reason for using a constellation of nanosatellites is the mitigation of risk. By having multiple, low cost satellites instead of a single large satellite, the associated launch risk is greatly reduced. Additionally, the failure of any single BRITE satellite would not end the mission and the other satellites in the BRITE-Constellation would still achieve all the primary science objectives (Ref 3).
The primary satellite design requirement calls for attitude control. The strict photometric error-tolerance places a high precision requirement on the ADCS system to ensure BRITE will always keep regions of interest surrounding each star within the same pixel areas on the imager.
Table 2: Minimum mission requirements for the BRITE-Constellation platform (Ref. 3)
The BRITE constellation will be the world‘s first nanosatellite constellation dedicated to an astronomy mission.
Table 3: Some background in the early phases of the BRITE Constellation 10)
Extension of the Nanosatellite Constellation:
1) The University of Vienna and FFG/ALR (Austria's space agency) are financing the development of two BRITE satellites and development is nearing completion.
2) SRC/PAS (Space Research Center/ Polish Academy of Sciences of Warsaw, Poland will be preparing two additional satellites. The first Polish satellite, BRITE -PL 1, will be a modified version of the original SFL design. The second Polish satellite, BRITE-PL 2, will include the significant changes to be implemented by SRC PAS.
The Polish technical participation in the BRITE project is supported by the Ministry of Science and High Education. The participation in the BRITE consortium gives Poland the possibility to launch its first Polish scientific satellite into space. The Polish participation in the BRITE consortium was established in October 2009 by SRC/PAS and NCAC/PAS (Nicolaus Copernicus Astronomical Center/ Polish Academy of Sciences). 13)
3) The CSA (Canadian Space Agency) is also funding two satellites in the constellation. In January 2011, CSA signed an agreement to support the BRITE constellation by contributing two satellites targeting a launch in 2012. The two Canadian satellites will join the four other satellites funded by the Austrian and Polish governments. 14)
The operation of three pairs of BRITE nanosats will significantly improve the coverage of the parameter space addressed in this proposal (compared to only one BRITE), and in particular the statistical significance of the science conclusions to be drawn. The best configuration would be to have two launches, each with a pair of BRITE nanosats with each kind of filter, and each orbit pair separated in the sky as much as possible. This is the rationale behind the BRITE constellation of four individual satellites.
Each BRITE satellite utilizes a number of innovative technologies including miniature reaction wheels, star tracker and optical telescope, all sized and designed around the generic CanX (Canadian Advanced Nanosatellite Experiment) bus of UTIAS/SFL (University of Toronto, Institute for Aerospace Studies/Space Flight Laboratory).
For a better overview of the BRITE constellation, the eoPortal will create a separate file for the spacecraft of each consortium participant when the information becomes available — with this file being the description of the BRITE Austria constellation.
The initial BRITE constellation is based on pioneering Canadian space technology, built in partnership (joint venture) with Austrian institutions. The Austrian share of the project (BRITE-Austria and UniBRITE) is funded by the Austrian Space Program - representing the first Austrian satellites.
• The University of Vienna (Austria) has funded UTIAS/SFL to build one satellite, called UniBRITE.
• The Graz University of Technology (TU Graz) is also cooperating with UTIAS/SFL to produce a further nanosatellite of the constellation, called BRITE-Austria, funded by FFG/ALR.
Each of these two BRITE satellites will have a different filter, one will be equipped with a 390-460 nm (blue-spectrum) bandpass filter and the other will be equipped with a 550-700 nm (red spectrum) bandpass filter.
Table 4: BRITE constellation consisting of 6 nanosatellites operating in pairs, all are based on GNB of UTIAS/SFL
All spacecraft in the constellation use the GNB (Generic Nanosatellite Bus) platform, also referred to as CanX-3, developed at UTIAS/SFL (of CanX-2 heritage). The first BRITE satellite, UniBRITE, is being built by SFL for the University of Vienna. The second BRITE satellite, BRITE-Austria, is being developed by the Graz University of Technology with assistance (components) from SFL. The BRITE team has asked the Canadian Space Agency (CSA) to complete the BRITE constellation with funding for two Canadian BRITE nanosatellites.
Figure 1: Artist's rendition of the BRITE-Austria nanosatellite in orbit (image credit: TU Graz) 17)
The GNB is a modular spacecraft bus designed around a 20 cm x 20 cm x 20 cm cube form factor that provides all basic functional capabilities for a wide range of nanosatellite missions (up to 12 kg and potentially bigger), and provides a platform for state-of-the-art, high-performance applications not previously achievable with nanosatellites. A typical GNB consists of one ARM7 housekeeping computer, two ARM7 computers for attitude/propulsion control and payload operations, CMOS imagers, a power system with triple-junction solar cells and Lithium-ion batteries, passive thermal control, UHF uplink, a 32-256 kbit/s S-band downlink, and a 1 arcmin three-axis attitude control system consisting of tiny reaction wheels, sun sensors, magnetometer and star tracker. 18) 19)
EPS (Electrical Power Subsystem): The GNB design incorporates a direct energy-transfer power system utilizing between four and eight solar cells on each face and battery storage to supply 10 W peak and 5.6 W nominal for the satellite.
Each BRITE nanosatellite contains three processor boards, the OBC board handles the housekeeping and communications, a second computer is used for all ADCS support functions, while the third board is used for the science payload and its data handling. Communications between the boards occurs through serial peripheral interfaces. Each processor board is based around an ARM7TDMI processor with a code memory of 256 kB and triple-vote 2 MB of hardware EDAC (Error Detection And Correction) protected SRAM memory to store program variables and data. Longer term payload data storage is provided in 256 MB of flash memory. The on-board computer uses a multi-threaded operating system developed at SFL.
Figure 2: Illustration of the CanX-3/BRITE spacecraft (image credit: UTIAS/SFL)
ADCS (Attitude Determination and Control Subsystem). The spacecraft is 3-axis stabilized with 1 arcmin stability. The altitude control software implements an Extended Kalman Filter that uses the various attitude sensors to predict and correct the attitude of the spacecraft.
The attitude sensor suite in each GNB spacecraft comprises of a three-axis magnetometer, six sun sensors (each consists of a phototransistor and digital pixel arrays for coarse and fine attitude determination), and a star tracker. Three magnetorquers are employed to provide coarse attitude control and momentum dumping capability. Three orthogonal reaction wheels perform fine attitude controls.
A reaction wheel for nanosatellites was developed by Sinclair Interplanetary of Toronto in collaboration with SFL. The wheel fits within a box of 5 cm x 5 cm x 4 cm, has a mass of 185 g, and consumes only 100 mW of power at nominal speed. A max torque of 2 mNm is provided (30 mNms momentum capacity). No pressurized enclosure is required, and the motor is custom made in one piece with the flywheel. The reaction wheel technology is scalable. CanX-3/BRITE is the first mission to use this actuator (3 wheels). 20) 21)
UniBRITE and BRITE-Austria use the ComTech/AeroAstro MST (Miniature Star Tracker) as their primary attitude sensor for attitude determination. The update rate of the MST limits the cadence of the attitude determination and control cycle to 0.5 Hz. This slow cadence has a significant impact on the overall attitude performance, but this can be mitigated through the use of high bandwidth attitude filter and controller. 22)
The attitude subsystem of the BRITE satellites is among the most critical spacecraft systems in ensuring mission success. For massive stars, the period of light variations are on the scale of hours to months, therefore the satellites will perform 15 minute observations of multiple target star fields each orbit. Upon returning to a previously imaged target, the attitude system is required to hold the point-spread-function of the imaged stars to within 3 pixels of the original point locations, in order to trim out the pixel-to-pixel variations of the telescope detector. This stringent requirement implies a one arc-minute pointing control with long duration attitude stability and rapid reacquisition. Until recent advances in the miniaturization of attitude hardware, these requirements were insurmountable on a nanosatellite scale. Despite these advancements, a major challenge for the BRITE satellites was to characterize and tune these attitude components (reaction wheels and star trackers, in particular), as they operate at the edge of the required performance envelope. Further, novel attitude estimation and control techniques were applied, which were essential, given both the hardware compliment and their limitations (Ref. 22).
Figure 3: Photo of the MST Star Tracker (image credit: TU Graz)
Reaction wheels: The BRITE satellites make use of the Sinclair-SFL 30 mNms reaction wheels. These highly capable reaction wheels have over five years of flight heritage onboard the CanX-2 satellite, and an additional three year of heritage aboard the AISSat-1 satellite and continue to operate without incident. The reaction wheels are capable of storing more than 30 mNms of angular momentum and delivering torques up to 2 mNm.
Figure 4: Photo of three GNB miniature reaction wheels (image credit: Sinclair Interplanetary)
Figure 5: Photo of the magnetometer (image credit: TU Graz)
RF communications: Each CanX-3/BRITE nanosatellite is capable of full duplex communications with the ground. The uplink uses UHF while an S-band transmitter with BPSK modulation is used in the downlink. Each satellite can also support a VHF beacon. The expected data volume is 2-8 MB/day.
Table 5: Summary of the CanX-3 / BRITE-Austria / TUGSat-1 spacecraft bus specifications
Figure 6: Block diagram of the BRITE/CanX-3 nanosatellite (image credit: UTIAS/SFL, TU Graz)
Figure 7: Exploded view of the CanX-3 nanosatellite showing the structural elements (image credit: UTIAS/SFL, TU Graz)
Figure 8: Photo of the TUGSat-1 / BRITE-Austria nanosatellite (image credit: TU Graz)
Launch: The first two nanosatellites of the BRITE/CanX-3 constellation [Austrian mission: BRITE-Austria (CanX-3B) and UniBRITE (CanX-3A) were launched as secondary payloads on Feb. 25, 2013 on the PSLV-C20 vehicle of ISRO/Antrix from Shriharikota/India. The primary payload on this mission was SARAL (Satellite with Argos and AltiKa), a collaborative mission of ISRO and CNES. 23) 24) 25)
To facilitate rapid launches, SFL has adopted an approach to build customizable separation systems for any nanosatellite. These separation systems can be integrated with the satellites prior to launch site delivery and hence, make launch coordination easier. The SFL XPOD (Experimental Push Out Deployer) separation system interfaces the GNB-based spacecraft to practically any launch vehicle. Spacecraft up to 12 kg may be accommodated in existing XPOD designs.
UTIAS/SFL refers to the XPOD launch of the nanosatellites as NLS-8 (Nanosatellite Launch Service-8). UniBRITE (NLS-8.1), BRITE-Austria (NLS-8.2) and AAUSAT3 (NLS-8.3).
Figure 9: Illustration of the nanosatellite (GNB) separation system (XPOD Duo), image credit: UTIAS/SFL
The six secondary payloads manifested on this flight were:
• Sapphire (Space Surveillance Mission of Canada), a minisatellite with a mass of 148 kg.
• NEOSSat (Near-Earth Object Surveillance Satellite), a microsatellite of Canada with a mass of 74 kg.
• BRITE-Austria/TUGSat-1 (Graz University of Technology), Austria, a nanosatellite with a mass of ~6.5 kg.
• UniBRITE (Technical University of Vienna), Austria, a nanosatellite with a mass of ~ 6.5 kg.
• AAUSat-3 (Aalborg University CubeSat-3), a student-developed nanosatellite (1U CubeSat) of AAU, Aalborg, Denmark. The project is sponsored by DaMSA (Danish Maritime Safety Organization).
• STRaND-1 (Surrey Training, Research and Nanosatellite Demonstrator), a 3U CubeSat (nanosatellite) of SSTL (Surrey Satellite Technology Limited) and the USSC (University of Surrey Space Centre), Guildford, UK. STRaND-1 has a mass of ~ 4.3 kg.
Orbit: Sun-synchronous near-circular orbit, mean altitude = ~775 km, inclination = 98.6295º, orbital period of 100.32 minutes, LTAN (Local Time on Ascending Node) = 6:00 hours.
Note: SARAL (and also the BRITE constellation) will fly on the same orbit as Envisat, to ensure a continuity of altimetry observations in the long term. On the other hand, the local time of passage over the equator will be different due to specific cover requirements for the instruments constellation of the Argos system.
Figure 10: This image shows the various spacecraft on the PSLV-C20 upper stage (image credit: UTIAS/SFL, Ref. 25)
Mission status of the BRITE constellation:
• May 2016: The BRITE-Constellation has been successfully observing the night sky's most luminous stars since late 2013. Over 300 stars have been studied during the course of 15 completed campaigns, with each star assigned to a PI and tied to a unique observation proposal. To date, data sets corresponding to the first 10 of these campaigns have been released to BRITE PIs. Within only a short time since the first data sets were released, notable scientific outputs have already been produced. Three papers have been published in refereed journals with many more in the pipeline. With a huge volume of data already being studied on the ground and with the capabilities of the constellation expanding all the time, the future looks bright for further discovery. 26)
- BRITE is a unique and stand-out mission in many ways. It is the first and only mission to conduct two-color spaceborne photometry, being critical for stellar pulsation mode identification (as observed pulsation frequencies and amplitudes will vary across measurement bands), which has already paid dividends in revealing first-time observed insights of stellar properties (Ref. 29). The fact that BRITE is a constellation enables uninterrupted and overlapping coverage of star fields, allowing the mission to resolve stellar pulsation modes with periods from scales of hours to months. The BRITE satellites are believed to be the first nanosatellites dedicated to astronomy, the first orbiting astronomy constellation of any size ever launched, and the first spacecraft at this scale to achieve arc-second level pointing.
- As a result of its many accomplishments, the mission has been a large success. The constellation produces a quality of output which meets and exceeds both requirements and expectations. Although BRITE-Constellation has already swollen to a size not dreamed of when it was first conceived, the mission continues to garner interest in participation from other nations. Attracted by the enticing nature of the science, the success of the transfer programs with Austria and Poland and the relatively low-entry cost, to date, six other nations have expressed interest in contributing additional satellites to the constellation. In anticipation of potential future expansion, discussion had taken place on how those satellites can best be used to further enhance the science as the addition of BRITE-Austria did years before. At the top of the list is a BRITE satellite capable of observing in the ultraviolet, an addition which would push this already highly valuable mission into even new territory.
- Observation targets are selected by the BRITE Executive Science Team (BEST), which is comprised of twelve voting members (four per member country) and six non-voting members. 27) Once per year BEST convenes to generate observation schedules and strategies (based on the evaluation of scientific merit of submitted target proposals), review mission status, and provide overall management and control of the mission. Spacecraft operations, which are heavily automated, are managed part-time by small engineering teams residing at each of the respective mission control centers.
- Spanning the two and a half years since the BRITE constellation began nominal science operations, fifteen observation campaigns have been executed. A Lambert projection of the night sky,illustrating the star fields observed (as well as those planned in the near future) by the BRITE constellation is shown in Figure 11, where the ovals represent the field of view of the imager at each field. Within the figure, it is evident that the majority of the observed fields are clustered in a single plane. This is not a coincidence, as the greatest density of stars (which include BRITE's target luminous stars) lie within the Milky Way's galactic plane.
• Feb. 5, 2016: The BRITE Constellation of five nanosatellites is operational in 2016. — The constellation is revealing new information about a well-studied star, Alpha Circini. A data analysis of the BRITE Constellation Team shows a behavior in this star that has not been observed before according to Werner Weiss from the University of Vienna, Austrian Principal Investigator for BRITE and lead author of the paper. The "BRITE Constellation shows complex behavior in Alpha Cir due to both rotation and pulsation. Moreover, that behavior is different when observed in different colors. This result clearly demonstrates the power of BRITE-Constellation and the unique science that is possible using these tiny two-color precision instruments in space." 28) 29)
- The BRITE-Constellation is a coordinated mission of five nanosatellites in LEO, each hosting an optical telescope of 3 cm aperture feeding an uncooled CCD, and observing selected targets in a 24º FOV. Each nanosatellite is equipped with a single filter; three have a red filter (~ 620 nm) and two have a blue filter (central wavelength (~ 420 nm). The satellites have overlapping coverage of the target fields to provide two-color, time-resolved photometry.
- The five nanosatellites, each of ~7 kg, are designated as: BRITE-Austria and UniBRITE (Austria), BRITE-Lem and BRITE-Heweliusz (Poland) and BRITE-Toronto (Canada). A sixth nanosat, BRITE-Montreal, did not detach from its launch vehicle.
Figure 12: Location of α Circini in the southern constellation Circinus (image credit: BRITE collaboration)
- The BRITE data are so valuable to astronomers due to their multi-colored observation. For stars, color and temperature go hand-in-hand. Having the ability to examine stars in different colors with data taken every few minutes for up to six months is providing new insights into their inner workings.
- Using these precision instruments, the BRITE-Constellation's mission is to perform a survey of the most luminous stars in the Earth's sky via a branch of astronomy called asteroseismology – literally, the study of "starquakes". Typically, massive and short-lived, these stars dominate the ecology of the Universe and are responsible for seeding the interstellar medium with the "heavy" elements critical for the formation of planetary systems and organic life. In short, BRITE studies classes of stars that, billions of years ago, made life on Earth possible.
- With an apparent magnitude of 3.19, Alpha Cir is the brightest star in the southern constellation Circinus and belongs to the class of stars known as rapidly oscillating Ap stars. The star in question was observed by four of the BRITE satellites from March to August 2014 and will be observed again in 2016. It is hoped, that these new observations will provide both a better understanding of its complex behavior and a chance to confirm a new oddity about this already peculiar star; that its speed of rotation is decelerating.
• On Feb. 25, 2015 the BRITE-Austria and UniBRITE nanosatellites were 2 years on orbit. BRITE-Austria is operated from Graz while UniBRITE is operated from Toronto. 30)
- BRITE-Constellation is the world‘s first nanosatellite constellation dedicated to astronomy. Five components of BRITE-Constellation are operational.
• Oct. 2014: Of the five operational spacecraft, UniBRITE and BRITE-Toronto are operated by UTIAS/SFL, from the Toronto Earth station. BRITE-Austria is operated by the Technical University of Graz and the Polish BRITEs are operated by the Polish Space Research Center and the Copernicus Astronomical Center. 31)
- Due to the fact that UniBRITE and BRITE-Toronto are based on SFL's GNB (Generic Nanosatellite Bus), with which SFL has considerable on-orbit experience, commissioning of the core hardware complement was conducted at an accelerated pace. Within two weeks UniBRITE had commissioned everything but the payload and fine pointing hardware/algorithms. As one of the first BRITE spacecraft in orbit, a few notable hurdles were encountered and overcome before regular science operations were achieved in Sept 2013. — By the launch of BRITE-Toronto in June 2014, operations were running very smoothly and, tellingly, within eight days of the launch of BRITE-Toronto it had been fully commissioned and was collecting science data (Ref. 31).
- Fine Pointing Stability and Accuracy: The performance of fine-pointing on BRITE is typically assessed using high cadence star-field images taken by the payload. Shown in Figure 13 is the motion of the centroid of a stellar PSF, over a single 15-minute observation, for both UniBRITE and BRITE-Toronto. In each plot the red circle represents the 78" requirement and the green circle the RMS error. Clearly, even for UniBRITE with the AA-MST, performance (45") is well within requirements, and quite consistent with the simulation results. BRITE-Toronto, with the ST-16 startracker, over a similar time frame is almost four times better than UniBRITE, having RMS performance of 12".
- BRITE not only needs to point stably during an observation, but also over the span of several months as the target is lost and reacquired. To assess pointing performance over a longer period, BRITE instrument scientist Rainer Kuschnig of the University of Vienna, compiled several weeks of UniBRITE and BRITE-Toronto data and found the long-term RSS values of the standard deviations in each axis to be 52" for UniBRITE and 15" for BRITE-Toronto. Hence, in both cases, from field acquisition to reacquisition, the spacecraft is able to return the stars to the same set of pixels, and the observed pointing stability is significantly better than the requirement.
• UniBRITE commenced routine science observations of the Orion star-field in October 2013. Starting in December, BRITE-Austria joined UniBRITE on the campaign providing the mission's first two-color photometric data. Conveniently, in late 2013, MOST (Microvariability and Oscillations of STars) was also observing Eta-Orionis (a quadruple system with an eclipsing binary pair) and, for a time, all three spacecraft were performing simultaneous observations. Through the simultaneous observations with MOST, the team was able to confirm that, to first order, the quality of the BRITE data was as good as expected. A minimally post-processed light-curve of Eta-Orionis, taken by UniBRITE and with a binary transit clearly evident, is shown in Figure 14 (Ref. 31).
Figure 14: Light curve from Eta-Orionis, from UniBRITE data (image courtesy of Rainer Kuschnig, University of Vienna). The large black squares are the means of the vertical groups of small red squares. The light curve is entirely consistent with simultaneous observations from MOST on the same star (image credit: University of Vienna, UTIAS/SFL)
• Summer 2014: The commissioning of the Austrian BRITEs, being the first to launch, was complete by October 2013, and since then they have been routinely collecting science data, each orbit. The first field investigated, from October 2013 to March 2014, was Orion. Once Orion had set, the spacecraft moved on to observing the field Centaurus, while also observing secondary target field Sagittarius in tandem (Ref. 10).
- While commissioning of such a cutting-edge mission met with some hurdles, all now have been cleared, resulting in a quality of science generation which meets and exceeds both requirements and expectations. Although BRITE-Constellation has already swollen to a size not dreamed of when it was first conceived, the mission continues to garner interest in participation from other nations. Attracted by the enticing nature of the science, the success of the transfer programs with Austria and Poland and the relatively low-entry cost, to date, at least six other nations have expressed interest in contributing additional satellites to the constellation.
• Feb. 2014: The BRITE constellation is the world‘s first nanosatellite constellation dedicated to an astronomy mission. 32)
- The first 3 members of the BRITE constellation are on orbit, operating nominally
- The planned constellation will be completed this year
- Scientific & mission requirements fully met
- Scientific data collection under way.
• Activities in the period summer – fall 2013 (Ref. 32).
- Payload characterization (TUGSAT-1): Verification of PSF (Point Spread Function), CCD sensitivity, identifications of hot pixels on CCD (removal by software).
- Attitude control system optimization (UniBRITE)
- Reduction of commissioning time
- Fine pointing achieved in November 2013, performance better than specification
- Science data collection since November.
• October 1, 2013: Calibration of attitude sensors of BRITE-Austria is ongoing to increase the fine pointing performance. In addition payload characterization and hot pixel variations of the instrument's CCD sensor are investigated (Ref. 36).
Table 6: On-orbit attitude performance of a nanosatellite telescope (Ref. 22)
• In Sept. 2013, UniBRITE and BRITE-Austria have started to observe stars in the Orion constellation. While testing and optimization efforts are still ongoing science data are also collected regularly. In Figure 15, the exposure time was set to 1 second. The outer circle marks the unvignetted field of view of the instrument which has has a diameter of about 24º.
• In September 2013, the commissioning of BRITE-Austria is not yet completed, as the fine tuning and in-orbit calibration of sensor parameters, as well as performance characterization of the instrument is still ongoing. 33)
- Nevertheless, the ground station network and distributed software concept has already been used and validated. On the one hand, contacts to UniBRITE were successfully established from Graz, on the other hand during a breakdown of the ground station in Graz, the communication with BRITE-Austria was commanded through the ground station in Warsaw/Poland.
- Furthermore, during commissioning it was successfully shown, that the station in Graz is able to run autonomously and the remote access enables operators to monitor and control the ground station's and satellite's behavior although off-site.
Figure 16: Ground coverage of BRITE-Austria for the ground station network (image credit: TU Graz)
• July 15, 2013: BRITE-Austria is currently performing the transition to fine pointing. The overall satellite is in a good health state and is looking forward to start observations of bright stars (Ref. 36).
• On March 28, 2013, the UniBRITE nanosatellite of the University of Vienna experienced a very close encounter with OSCAR-15 [aka UoSat-4, a microsatellite of SSTL which was launched on January 22, 1990 as a secondary payload to SPOT-2 of CNES from Kourou. OSCAR-15 experienced an on-board electronics failure shortly after launch, and is not operational anymore]. The UniBRITE project team experienced some anxious moments during the predicted close flyby of OSCAR-15. 34)
• March 23, 2013: After successful detumbling the TUGSat-1 spacecraft was put into coarse pointing mode. During the 404th orbit the first star image was taken by the scientific instrument (telescope) in coarse 3-axis pointing. The first star image showing Delta Corvus B9V (magnitude 2.95) was downloaded and analyzed by the experts from the Institute of Astrophysics in Vienna. Initial assessment of the payload performance indicates that the specifications are met. 35)
Figure 17: Image of Delta Corvus B9V mag=2.95 (left); point spread function of the brightness distribution (right), image credit: TU Graz, University of Vienna (Ref. 32)
• March 5, 2013: TUGSAT-1 completed its 100th orbit. All subsystems tested so far show excellent health status. At present the attitude control system is checked out. Detumbling of the spacecraft is planned for the next days.
• A successful contact with the TUGSat-1 (BRITE-Austria) nanosatellite was established during the first orbital pass over Graz (3 hours after launch). This constituted the start of the commissioning phase of the satellite which is expected to last for 3 months. 36)
The objective is to examine the apparently brightest stars in the sky for variability using the technique of precise differential photometry in time scales of hours and more. The constellation of four nanosatellites is divided into two pairs, with each member of a pair having a different optical filter. The requirements call for observation of a region of interest by each nanosatellite in the constellation for up to 100 days or longer. 37)
The science payload of each nanosatellite consists of a five-lens telescope with an aperture of 30 mm and the interline transfer progressive scan CCD detector KAI 11002-M from Kodak with 11 M pixels, along with a baffle to reduce stray light. The optical elements are housed inside the optical cell and are held in place by spacers. The photometer has a resolution of 26.52 arcsec/pixel and a field-of-view of 24º. The mechanical design for the blue and for the red instrument is nearly identical; only the dimensions of the lenses are different (Ref. 6) and Ref. 7).
Figure 18: Illustration of the BRITE telescope and baffle (image credit: UTIAS/SFL, TU Graz)
Figure 19: The optics design of the photometer (image credit: UTIAS/SFL, Ceravolo)
The effective wavelength range of the instrument is limited in the red by the sensitivity of the detector and in the blue by the transmission properties of the glass used for the lenses. The filters were designed such that for a star of 10,000 K (average temperature for all BRITE target stars) both filters would generate the same amount of signal on the detector. The blue filter covers a wavelength range of 390-460 nm and the red filter 550-700 nm; both are assumed to have a maximum transmission of 95%.
Table 7: Characteristics of the Kodak KAI 11002-M CCD detector
The photometer instrument has a mass of ≤ 0.9 kg and a power consumption of ≤ 3.5 W. The instrument uses a custom set of electronics to operate the imager. The electronics include four A/D converters (14 bit) to convert the analog pixel values, and 32 MB of memory to temporarily hold a full frame image. The imager and memory timing and signals are being controlled using a CPLD (Complex Programmable Logic Device).
Figure 20: Schematic layout of the CMOS detector array (image credit: University of Vienna)
Figure 21: The BRITE photometer and star tracker (image credit: TU Graz)
Figure 22: Photo of the CCD detector (image credit: TU Graz)
Ground segment of the BRITE constellation:
All participants in the BRITE constellation will have their own ground station. 38)
• Graz, Austria, TUG (Mission Control for BRITE-Austria and UniBRITE)
• TUV (Technical University Vienna), a second station is located at the ITC of TUV
• Toronto, Canada, UTIAS/SFL (Mission Control for BRITE/CanX-3). A ground station at UTIAS-SFL has already been established since early 2003. It has served as a technical template design and concept for two other ground stations (one in Vienna and one in Vancouver) which communicate with the MOST satellite. Furthermore, this station has been used now regularly for broadcasting with three other satellites in orbit. This well-proven, equipement is also the baseline for BRITE-Constellation ground-space communications. At UTIAS/SFL , an additional ground station will be installed in 2011 which will also support BRITE-Constellation operations.
• Warsaw, Poland (Mission Control for BRITE-PL). The ground station is located at the CAC (Nicolaus Copernicus Astrophysical Center) in Warsaw.
All stations will track and collect data from all BRITE nanosatellites.
• Distributed automatic ground station operations
• Science teams can retrieve verified raw data from servers.
Ground Station Network: For operating the BRITE constellation, a ground station network is used. The advantages of the network are the increased availability and redundancy (Ref. 39).
The general operations concept foresees that each master ground station is in charge of its own satellite(s) and tracks other satellites only in case of unavailability of the correspondent master station or in case of emergency.
The master station is responsible for its own satellite(s) and is the only one actually controlling the spacecraft. If other stations attempt contacting a satellite, they normally act as relay stations, up-linking incoming commands from the master station and forwarding downlinked data to the master station. In case of failure of a master station, its duties can be temporarily taken over by another station in the network.
An example of ground station network operations and data flow for BRITE-Austria is shown in Figure 23. While all stations can establish contact with the satellite, the entire data flow is handled by the Graz ground station as the satellite's master station. The data flow is handled by a distributed ground software concept.
BRITE-Austria ground station: A major driver for the ground station design is the amount of science data to be downloaded. For BRITE-Austria, the data amount to be downloaded is up to 10 MByte/day. The contact times with the satellite are limited to about 10-12 minutes/pass and a total contact time of roughly 1 hour/ day. A downlink data rate of 32 kbit/s allows to download the daily data in about 42 minutes, providing sufficient margins. 39)
In addition, the ground station in Graz serves as mission control for BRITE-Austria. It is responsible for the spacecraft and shall guarantee data integrity and storage of raw satellite data.
The ground station antennas are a 3 m parabolic meshed antenna for S-band (35 dBi gain) and an 18 element, circular polarized cross-Yagi antenna for UHF (16 dBi gain). Both antenna are mounted on the same tower and are controlled by the same azimuth and elevation rotators, allowing to achieve the same tracking performance for uplink and downlink.
Figure 24: Block diagram of the BRITE-Austria ground (image credit:TU Graz)
Figure 25: The orbits and ground station network of the BRITE-constellation (image credit: UTIAS/SFL)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).
The BRITE Constellation: