SwissCube is a joint CubeSat project of various laboratories at EPFL (Ecole Polytechnique Fédérale de Lausanne), Lausanne, and universities in Switzerland. The partner institutions are: 1) 2) 3) 4) 5)
• IMT (Institut de microtechnique) of the University of Neuchâtel
• HEIG-VD (Haute Ecole D'Ingenierie et de Geston du Canton de Vaud)
• HES-SO Valais - Domaine Sciences de L'Ingenieur, Sion
• Haute Ecole ARC Ingenierie, Neuchatel Berne Jura
• Ecole D'Ingenieurs et D'Architectes de Fribourg
• FHNW (Fachhochschule Nordwestschweiz), Solothurn
The primary objective of developing this satellite is to provide a dynamic and realistic learning environment for undergraduates, graduates and staff in the development of small satellite technology.
The overall mission objective is to focus on the observation of the airglow phenomena (Figure 1). The airglow is a photo-luminescence of the atmosphere occurring at approximately 100 km altitude. It is principally due to photodissociation, photo-excitation and excitation by fast electrons or ion recombination. The origin of nightglow is oxygen atom recombination, where the oxygen atoms are created in the daytime by solar photodissociation of O2.
Figure 1: Illustration of airglow phenomena (green line just below the Space Shuttle), image credit: NASA
The scientific objectives are thus to observe oxygen emission at 762 nm to characterize the airglow intensity as a function of the observation angle (zenith or limb measurements), the altitude, the latitude and the local time. The minimum science duration is 3 months, with an extended science mission of duration up to 1 year.
The spacecraft follows the CubeSat standard (≤ 1kg cube with a 1 liter volume) providing a fast and affordable access to space. Although the CubeSat is small, it contains all the critical subsystems and functions present in larger satellites.
Figure 2: Illustration of the SwissCube picosatellite (image credit: EPFL)
The outer mechanical interfaces and design of SwissCube are defined by the CubeSat design specifications. The internal layout of the SwissCube is limited by two principal restrictions: the payload and the arrangement of printed circuit boards (PCBs). The ideal configuration is one optimizing both constraints at the same time.
The satellite's primary structure is manufactured from a single block of aluminum (Figure 3). This "monoblock" approach offers the best relationship between mass and rigidity but has the disadvantage of significantly increasing the complexity of the satellite's assembly procedures. Secondary structures are directly attached to the external or internal sides of the monoblock (Ref. 2).
The electronic boards inside the satellite are arranged into two PCB stacks placed on each side of the optical payload. These stacks contain the ADCS (Attitude Control and Determination Subsystem), the COM and BEACON (Communication subsystem), the CDMS (Command and Data Management Subsystem) and the EPS (Electrical Power Subsystem). Electrical and data interfaces are routed through a connection and a power distribution board (motherboard) placed perpendicular to the PCB stacks. The electronic boards are separated using aluminum spacers. Their role is to mechanically connect the different PCBs together as well as to attach the PCB’s stack and battery subassembly to the primary structure.
Additionally, the spacers serve as a thermal path between the PCBs and the aluminum frame. Both stacks are fixed on the internal +Y and -Y faces of the satellite, allowing a large amount of free space for the payload subsystem and keeping an increased accessibility to components placed in the center of the satellite.
Figure 3: Exploded view of the SwissCube elements (image credit: EPFL)
Six external multifunctional panels provide the enclosure of the cube. They are directly screwed to the satellite's primary structure. These panels are made of PCB substrate and are subassemblies including solar cells, sun sensors, readout electronics and a radiation shield. An Omnetics space-graded miniature electrical connector is used to link the external panel with internal electronic boards. To protect the internal PCBs from space radiation, shielding plates are located just behind external the panels, more precisely on +Z, -Z, +Y and -Y panels.
Three magnetorquers constitute the active actuators of the SwissCube. They consist of copper coils which interact with the Earth’s magnetic field and are glued on the interior faces -X, +Y and -Z sides of the cube. The coils were built by the AEM (Atelier d'ElectroMécanique) at EPFL.
Figure 4: Photo of the magnetorquers and a PCB (image credit: EPFL)
For the attitude determination the sensors are:
• A 3-axis magnetometer to measure the Earth’s magnetic field intensity and direction. These magnetometers are located on the ADCS board. The HMC1043 from Honeywell was chosen.
Figure 5: Photo of the 3-axis magnetometer (image credit: EPFL)
• Three 1-axis MEMS gyroscopes to measure the spin rate of each axis. The first one is directly mounted on the ADCS board. The two remaining ones are mounted on a bracket screwed onto the ADCS board.
• Six novel MEMS sun sensors to determine the sun vector. They are glued on each external panel and electrically connected by wire bonding.
RF communications: For the communication with the ground stations two antennas are placed on the -Y face of the satellite. The first one for downlink data is a 180 mm long UHF monopole antenna of 437 MHz frequency. The second one for uplink is a 610 mm long VHF monopole antenna using a frequency of 146 MHz. As the antennas are longer than the satellite, they are wrapped around plastic guides and released once the satellite is in orbit by melting a polymer wire.
- The uplink data rate (VHF) is 1200 bit/s with AFSK modulation
- The downlink data rate (UHF, 437.505 MHz) is 1200 bit/s with FSK modulation
- The downlink beacon data rate (UHF, 437.505 MHz) 14 bit/s with ASK/OOK CW modulation.
Figure 6: Block diagram of the communication subsystem (image credit: EPFL)
EPS: The triple junction Solar cell assemblies (SCA’s) used on SwissCube have a maximum efficiency of 25%. The average power generated by the satellite during the daylight period of its polar orbit is ~1.5 W. SwissCube also carries two rechargeable Lithium-ion polymer batteries (1.2 Ahr capacity) from VARTA. Under high vacuum, these batteries can undergo a physical expansion. To counteract this effect a milled aluminum box is used to enclosed them.
Figure 7: Block diagram of the EPS (image credit: EPFL)
Figure 8: Functional architecture of the CDMS (image credit: EPFL)
The I2C (Inter-Integrated Communication) has been chosen for the main bus due to its low power consumption and availability on most small microcontrollers. The 32-bit microcontroller ATMEL AT91M558800A used on CDMS is able to communicate with a SPI bus, but not directly with the I2C. Hence, an MSP430F1611 is used to do the SPI to I2C conversion. In this configuration, all the I2C are being managed by the same microcontroller for all subsystems.
Launch: SwissCube was launched as a secondary payload on Sept. 23, 2009 on a PSLV launcher from SDSC (Satish Dhawan Space Center), Sriharikota, on the east coast of India. The primary payload on the flight is OceanSat-2 of ISRO (Indian Space Research Organization). 6)
Further secondary payloads (CubeSats) on this flight are: BeeSat (Berlin Experimental Educational Satellite) mission of the TUB (Technical University of Berlin), Berlin, Germany, ITUpSat-1 (Istanbul Technical University PicoSatellite-1) of Istanbul Technical University, Turkey, and UWE-2 (University of Würzburg Experimentalsatellit-2), Würzburg, Germany.
All CubeSats are using the SPL (Single Picosatellite Launcher) system for on-orbit deployment. SPL is a development of Astro und Feinwerktechnik Adlershof GmbH, Berlin, Germany. 7)
Orbit: Sun-synchronous near circular orbit, altitude = 720 km, inclination = 98.4º, period = 99.31 min, the local equatorial crossing time is at 12:00 hours.
Figure 9: Artist's view of SwissCube observing airglow phenomena (image credit: EPFL)
Sensor complement: (Telescope)
The payload, a miniaturized telescope of Ø 30 mm x 45 mm, is placed in the center of the +X face of the satellite. The PCB containing the optical sensor is directly attached to the payload assembly. This orientation gives favorable values for inertial properties and allows placing two solar cells on the external panel containing the camera aperture. 8)
The objective is to perform spaceborne observations of the airglow occurring in the upper atmosphere at an altitude of about 100 km. The telescope consists of a PCB, including a detector and its control electronics, and an optical system, comprising a baffle, the bandpass filter which selects the desired wavelength of the oxygen emissions and focusing optics.
The telescope system has a size of 30 mm x 30 mm x 65 mm for the optical system and a volume of 80 mm (length) x 35 mm (width) x 15 mm (depth)] for the payload board. The total mass of the payload is 50 g. A CMOS detector is being used for photon detection. The payload consumes 450 mW (peak power) during 30 s for each image capturing. The optical system provides a FOV of 18.8º x 25º and a resolution of 0.16º/pixel for the chosen CMOS detector. In order to avoid straylight, the payload is mounted to one of the sides of the satellite via a baffle as shown in Figure 12.
Figure 10: Cross-section of the telescope its elements and mass budget (image credit: EPFL)
Figure 11: Detailed configuration of the telescope (image credit: EPFL)
In a first phase airglow emissions shall be observed at different regions and under different angles of observation. These measurements will provide a first idea of expected minimum, maximum and mean intensities of airglow emissions during both day and night. Furthermore, it will allow analyzing background radiation due to scattered sun- or moonlight. The first observation phase shall last 3 months. During this period, 20 images of the airglow shall be taken.
In a second phase only observations of airglow emissions at limb between 50 and 120 km shall be carried out. Since they constitute the basis for a new low-cost Earth sensor, their variation in intensity has to be studied more carefully. The duration of this second phase will be determined by the lifetime of the satellite.
Figure 12: Photo of the payload aperture with the baffle (image credit: EPFL)
Introduction of WEDM (Wire Electrical Discharge Machining) techniques
The manufacture of the satellite primary structure employs a novel technique referred to as WEDM. Most conventional machining techniques used in the space field are CNC (Computer Numerical Control) milling and CNC turning, both schemes are chip cutting operations. Wire EDM cuts metal by producing a rapid series of repetitive electrical discharges between a very thin wire and the piece of metal being machined. The advantages of this technology are the capability to machine complex and thin shapes where conventional methods would fail due to excess cutting tool pressure (Ref. 2).
This technique has been used for many parts of SwissCube, especially the satellite structure, a complex lightweight monoblock frame that serves as primary structure and provides part of the secondary structure. With 95 grams, the SwissCube structure is one of the lightest in the CubeSat community. Finite elements analyses and modal tests have been performed to ensure that its rigidity satisfies the requirements.
Figure 13: Illustration of the SwissCube monoblock frame (image credit: EPFL)
Thanks to the WEDM, a complex satellite structure containing a lot of various attach points has been machined in one part. This machining technology provides the means to obtain very thin structural components that would not be possible to machine by traditional process due to excess cutting tool pressure.
The second mechanical part of the SwissCube machined by WEDM is the battery box. A common problem with the Lithium-ion polymer battery cell is that they may rapidly loose performance and eventually entirely cease function when subjected to a high vacuum. Typically this effect is related to a physical expansion of the battery block. To counteract this effect, the batteries are being enclosed in a milled aluminum box. The gap between this box and both Lithium-ion polymer cells is filled with epoxy resin. This is also a solution to provide mechanical interface between the cell and the satellite structure. With WEDM technology, thin walls of 0.8 mm and inner right angles are possible. The inner dimensions of the box are 11.7 mm x 39 mm x 66 mm, which gives enough space for two batteries. The box is attached to the structure by the PCB-screws and spacers (Figures 3 and 14).
Figure 14: Attachment of the battery box (purple) to the monoblock frame (image credit: EPFL)
• On Sept. 23, 2013, SwissCube was on orbit for 4 years and is still in good health. Operations are now being shared with the Swiss Radio Amateurs association. 9)
The SSO orbit of SwissCube (altitude of 720 km, inclination of 98.4º) is crossing the debris field from the Cosmos/Iridium collision in 2009. The project typically receives 3 to 5 warnings a year regarding a possible collision. That is currently its highest probability of failure.
Furthermore, right after ejection from the PSLV launch vehicle, SwissCube has experienced a large rotation rate (tumbling). In early 2011, a detumbling procedure was successfully implemented and SwissCube’s rotation has been low and stable since. However, the on-board attitude controller is not active by default, and over the last 2 years, several sudden and sharp increases in the rotation rates have been observed. Actions were taken and SwissCube was re-stabilized. Figure 15 shows the observed peaks in rotation. Once non-operational, it is to be expected that SwissCube may experience similar peaks. Thus CleanSpace One shall accommodate tumbling rates of SwissCube up to 50º/s. 10)
Figure 15: Measured angular speed magnitude of SwissCube as a function of mission days since January 1, 2011. When the angular speed goes above 2º/s, an on-board controller is used to reduce rotations. The controller is “off” by default (image credit: Swiss Space Center)
• In early 2012, the EPFL Space Center changed its name officially to Swiss Space Center. On February 15, 2012, the Swiss Space Center announced the birth of a new NanoSat project called CleanSpace One. 11)
• The SwissCube mission is operational in the spring of 2011 — after experiencing a most remarkable recovery effort by the project team (Ref. 14). Final stabilization of the spacecraft was achieved on Feb. 15, 2011 ( Figure 18). Congratulations to all involved!
• The SwissCube project declared the end of its CubeSat mission in December 2011 for the EPFL Space Center (no more funding for mission operations). However, since the spacecraft is still working quite well in 2012, the operations are being continued by the Radio Amateur community, the SwissCube project is staying in touch with them. 12)
Figure 16: Back to regular communications of SwissCube (image credit: EPFL) 13)
• Detumbling of the spacecraft: After the reset was accomplished on Jan. 20, 2011, the next step was to stabilize SwissCube’s rotation. After a few verifications of the power system (the batteries had an equivalent of 3000 cycles at that point), the project turned the ACS “On” (January 31, 2011), and downloaded the gyroscopes, sun sensor and magnetometer data. The data of these sensors confirmed a rotation rate of ~ 80º/s around the Y-axis of the satellite.
- The project was then able to implement the detumbling strategy that a talented student from TU Delft, Arthur Overlack, came up with during his Master's thesis. He is now working for ISIS, along with Herve Peter-Conte sse, the former lead attitude control engineer on SwissCube. With both their help, the project could implement the detumbling procedure as shown in Figure 17.
Figure 17: B-dot controller stability diagram (image credit: EPFL)
Legend to Figure 10: This diagram shows the region of stability of the b-dot controller as a function of its sign and lambda parameter. The detumbling strategy followed the green box path.
This procedure consisted in:
1) Uploading new software parameters of the b-dot controller (to actively detumble this time with the magnetotorquers).
2) Verifying correct function of the b-dot controller.
3) Uploading new parameters when the rotation had reached its minimum value with the previous parameters.
With a thorough verification of the ACS and power system at each step in the process, the first b-dot parameters (top right in Figure 17) were sent to the satellite on February 7, 2011. A significant reduction of the rotation rate was observed. - The project then uploaded a new set of parameters to the controller on February 14, and got final stabilization on February 15. 2011.
Figure 18: This diagram shows the attitude evolution of SwissCube (image credit: EPFL)
• SwissCube reset: After almost one year of letting SwissCube detumble “alone”, the project got to a slow enough rotation that the gyroscopes onboard were not saturated anymore (about 80º/s). That happened around November 2010. That said, the onboard systems had degraded and there is still a big internal communication problem (I2C bus inside the satellite). 14)
- The project decided to try to reset the satellite. Of course the satellite was not designed for it 'as a safety guard: a failure of a reset system could fully compromise the mission' but hey! that’s what engineers are for (do impossible things!). A way around this shortfall was found: drain the batteries by turning on the communication’s power amplifier (PA) continuously.
- Florian George, the former lead software student working on the project, who knew the satellite in all its details and could implement the right commands. Good software design. These commands activated the power amplifier, which consumes about 3.5 W, and forced it to stay turned on. The average power measured on SwissCube coming from its solar panels while under sunlight is 2.8 W. Thus by letting the power amplifier “ON”, the batteries would be drained to a level where the power system resets and the spacecraft reboots in “safe” mode.
- The project tested this option a few times on the EQM (qualification model, the twin brother of the flight model in space), and saw various behaviors of the power system, but eventually all behaviors ended with a total reset of the satellite. So, this command was implemented to turn on the power amplifier on January 20, 2011 and it worked beautifully! The discharge started at 13h06 local and the reset was executed about 2h41 minutes after.
The main consequence was that the I2C bus, which had been stuck and prevented internal communication, did reset and the satellite was back to normal (Figure 19). In addition, the ACS (Attitude Control System) and the telescope were fully usable again.
Figure 19: The I2C reset recovered the normal communications of SwissCube (image credit: EPFL)
• Right after launch in Sept. 2009, SwissCube experienced a high rotation rate around one of its axis (~ 200º/s), which basically prevented the team to take meaningful measurement of the Airglow (its scientific objective). After trying a few things, which did not necessarily improve the situation, the project decided to let SwissCube detumble naturally. The planned design lifetime of SwissCube was 4 months (1 month of commissioning and 3 months of science observations).
2) Guillaume Roethlisberger, Fabien Jordan, Anthony Servonet, Maurice Borgeaud, Renato Krpoun, Herbert R. Shea, “Advanced Methods for Structural Machining and Solar Cell Bonding Allowing High System Integration and their Demonstration on a Pico-satellite,” Proceedings of the 22nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 11-14, 2008, SSC08-XI-4, URL: http://ctsgepc7.epfl.ch/12%20-%20SwissCube%20papers/G.Roethlisberger_SSC08-XI-4.pdf
3) Maurice Borgeaud, Noémy Scheidegger, Muriel Noca, Guillaume Roethlisberger, Fabien Jordan, Ted Choueiri, Nicolas Steiner, “SwissCube : The first entirely-built Swiss student satellite with an Earth observation payload,” Proceedings of the 7th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, May 4-7, 2009, IAA-B7-0205P
4) Muriel Noca, Fabien Jordan, Nicolas Steiner, Ted Choueiri, Florian George, Guillaume Roethlisberger, Noémy Scheidegger, Hervé Peter-Contesse, Maurice Borgeaud, “Lessons Learned from the First Swiss Pico-Satellite: SwissCube,” Proceedings of the 23nd Annual AIAA/USU Conference on Small Satellites, Logan, UT, USA, Aug. 10-13, 2009, SSC-09-XII-9, URL: http://ctsgepc7.epfl.ch/12%20-%20SwissCube%20papers/S3-D-SET-1-5-Small%20Sat%2009%20paper%20SSC09-XII-9.pdf
5) Gavrilo Bozovic, Omar Scaglione, Christian Koechli, Muriel Noca, Yves Perriard, “SwissCube: development of an ultra-light and efficient Inertia Wheel for the attitude control and stabilization of CubeSat class satellites,” Proceedings of the 59th IAC (International Astronautical Congress), Glasgow, Scotland, UK, Sept. 29 to Oct. 3, 2008, IAC-08 B4.6, URL: http://ctsgepc7.epfl.ch/12%20-%20SwissCube%20papers/IAF_08_SwissCube_IWA_Bozovic.pdf
6) “India launches Switzerland's first satellite,” Sept. 23, 2009, URL: http://www.swissinfo.ch/eng/front.html?siteSect=109&ty=st&sid=11253287&front=br
8) N. Scheidegger, Science Payload,” March 8, 2007, URL: http://obswww.unige.ch/~wildif/eivd/diplomes/2007/S3-B-Optics-Requirements.pps
9) Stefano Rossi, Anton Ivanov, “Thermal Model for Cubesats: a simple and easy model from the SwissCube’s thermal flight data,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-E2.1.1
10) M. Richard, L. Kronig, F. Belloni, S. Rossi, V. Gass, S. Araomi, I. Gavrilovich, H. Shea, C. Paccolat, J.P. Thiran, “Uncooperative Rendezvous and Docking for MicroSats,” Proceedings of the 6th International Conference on Recent Advances in Space Technologies (RAST), Istanbul, Turkey, June 12-14, 2013, URL: http://space.epfl.ch/files/content/sites/space/files/CleanSpaceOne%20RAST%20paper,%20June%202013.pdf
11) Cleaning up Earth's orbit: A Swiss satellite tackles space debris,” EPFL, Feb. 15, 2012, URL: http://actu.epfl.ch/news/cleaning-up-earth-s-orbit-a-swiss-satellite-tack-2/
12) Information provided by Muriel Richard, EPFL, Lausanne, Switzerland.
13) Muriel Richard, Florian George, Anton Ivanov, “SwissCube ......2 years in Space,” Space Center EPFL, Annual Meeting 2011, Dec. 1, 2011, URL: http://swisscube.epfl.ch/pdf/S3-E-1-0-Swisscube_Dec_1_Closing_Event.pdf
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.