ESTCube-1 (Estonian Student Satellite-1)
ESTCube-1 is an Estonian student CubeSat project of the University of Tartu, which started in the summer of 2008. The objective is to get students involved in space projects. Another goal is to foster the development of Estonian space and high-tech industry by training experts and disseminating knowledge about space technologies. 1) 2) 3)
In the meantime, the project has grown into a full-scale international collaboration with participating students from the University of Tartu, Tallinn University of Technology, and the Estonian Flight Academy. By now students from University of Surrey, UK and ISU (International Space University), of Strasbourg, France, are also taking part in the development of the project. The payload is a joint cooperation with the Finnish Meteorological Institute (FMI, which leads payload development) and the German Aerospace Center (DLR).
The main goal of the ESTCube-1 mission is to perform the first on-orbit test of the E-Sail (Electric Solar Wind Sail) concept, invented by Pekka Janhunen of FMI (Finnish Meteorological Institute). This is a novel propellantless propulsion technology utilizing electrostatic interaction between the fast moving ions in the solar wind and the electric field generated by a spacecraft that allows to partially transfer the momentum of the ions to the spacecraft, effectively producing thrust. This electric field can be generated by a set of electrically charged tethers, effectively forming an electric sail. The test mission involves deployment of a thin electrically conductive tether in LEO (Low Earth Orbit) and charging it to a high voltage to measure the force of interaction between the semi-stationary plasma in Earth’s magnetosphere and the satellite itself. 4) 5) 6) 7)
Designed, built and operated by the students of several Estonian universities (Estonian Aviation Academy, Tallinn University of Technology and University of Life Sciences) and led by the University of Tartu with Tartu Observatory, ESTCube-1 is part of ESA's Plan for Cooperating States agreement with Estonia, a one-year program of activity taking place as a prelude to the country joining ESA as a Member State.
The main objectives of the ESTCube-1 mission are:
1) Test the deployment of a 10 m tether, which is an integral part of the electric solar wind sail
2) Measure the force being exerted to the sail
3) To take a picture of the tether.
4) As an added mission to take a picture of the Earth and, if possible of Estonia.
Figure 1: Artist's rendition of the deployed ESTCube-1 (image credit: University of Tartu)
ESTCube-1 conforms to the standard 1U CubeSat specifications with a side length of 10 cm and a mass of 1.33 kg. COTS (Commercial-off-the-Shelf) components are used for the spacecraft bus, designed by students.
ESTCube-1 aims to perform a single axis spin-up using only electromagnetic coils. The spin-up is required to deploy an E-Sail tether by centrifugal force. 8)
Figure 2: Illustration of the CubeSat and its components (image credit: University of Tartu)
ADCS (Attitude Determination and Control Subsystem): ADCS is responsible for measuring and adjusting satellite’s attitude in orbit, including adjustments of rotation speed and rotation axis. This is done using magnetic torquers. The satellite is equipped with sun sensors, magnetometers and gyroscopes. By integrating the sensor reading, the satellite's attitude with respect to Earth and Earth's magnetic field and its speed of rotation can be determined.
EPS (Electric Power Subsystem): The CubeSat is powered by 12 GaAs triple junction solar cells from Azur Space Solar Power with rated efficiencies of 30 %, which will generate between 2.4 to 3.4 W at BOL (Beginning of Life). The solar cells are surface mounted. The EPS consists of 4 major blocks (Figure 3): energy harvesting, energy distribution, energy storage and control module. All of these systems are connected together using the MPB (Main Power Bus), which transfers energy within the EPS and has a variable voltage between 3.7 and 4.2 V during normal operation dependent on the battery charge state (Ref. 4). 9)
The 12 solar cells are arranged into 3 groups, each containing the solar cells from the opposite sides of the satellite. This allows to efficiently track the MPP (Maximum Power Point) of the solar cells using three independent controllers. Energy is stored in two Panasonic P-CGR 18650C Li-ion cylindrical battery cells (9 Wh), both of which have their own independent protection circuitry which connect them to the MPB. The energy is distributed to the subsystems through 3 voltage lines (3.3 V, 5 V and 12 V), each of which is powered by two parallel redundant switching regulators. The outputs of the regulators are summed by Schottky diodes to make the system immune to single converter failure.
The EPS is controlled through an ATMega1280 8 bit AVR microcontroller from Atmel. The processor has been tested before and deemed suitable for conditions similar to the current mission.
Figure 3: Schematic view of the EPS architecture (image credit: University of Tartu)
CDHS (Command and Data Handling Subsystem): The system contains two STM32F103 ARM processors, one of which is turned on. The two processors give the possibility to activate the second one if the first is defective, to make sure that the satellite remains operational.
Figure 4: Illustration of the CDHS (image credit: University of Tartu)
RF communications: Use of a UHF/VHF system for uplink and downlink data transmissions. In addition, a CW beacon is used.
Launch: The ESTCube-1 Cube Sat was launched as a secondary payload on May 7, 2013. The primary payload on this flight was PROBA-V of ESA. The launch vehicle was Vega (with Arianespace as launch provider); the launch site was the Guiana Space Center, Kourou. 10) 11)
The secondary payloads on this flight were:
• VNREDSat-1A (Vietnam Natural Resources, Environment and Disaster-monitoring Satellite) of STI-VAST (Space Technology Institute-Vietnam Academy of Science and Technology). The microsatellite VNREDSat-1A (120 kg) has been built by EADS Astrium, Toulouse, France.12) 13)
• ESTCube-1 (Estonian Student Satellite-1), a 1U CubeSat technology demonstration mission of the University of Tartu.
PROBA-V will ride in the upper position of the Vespa adapter, while VNREDSat-1A and ESTCube-1 will sit in the lower position in the structure. The upper stage of the Vega vehicle is a liquid propulsion module with multiple re-ignition capability. The secondary payloads will be deployed last after re-ignition of the Vega upper stage.
Orbit of PROBA-V: Sun-synchronous orbit, altitude = 820 km, inclination = 98.8º, LTDN (Local Time on Descending Node) = 10:30 hours (with a drift limited between 10:30 and 11:30 AM during the mission lifetime). Note: In contrast to the SPOT-4 and SPOT-5 missions, PROBA-V will not have the capability to maintain its orbit.
Orbit of VNREDSat-1 and ESTCube-1: Sun-synchronous orbit, altitude =670 km, inclination = 98.5º. VNREDSat-1A was released 1 hour 57 minutes into flight. ESTCube-1 was ejected from its dispenser three minutes later. A last burn will now place the spent upper stage on a trajectory that ensures a safe reentry that complies with new debris mitigation regulations (Ref. 10).
Figure 5: Photo of the ESTCube-1 (image credit: University of Tartu)
Figure 6: Illustration of the various mission phases (image credit: University of Tartu) 14)
Status of mission:
• The ESTCube-1 satellite is operational as of fall 2013. The project is monitoring and operating the satellite at least 6 times per day during good passes. Most system tests are still ongoing to check long-term performance and stability.
• The EPS and the COM have been tested to be able to perform regular operations and the satellite is able to produce and store enough power to perform the E-sail experiment. The CDHS and the ADCS have been verified to be functional. Images of the Earth have been acquired by the CAM and downloaded to the ground continuously since the start of the operations. The EPS, the CDHS and the CAM have been tested to be able to update firmware and software sent from the ground station. 16)
The satellite is in the process of being updated and tested for the experiment. While the subsystems can be thoroughly tested before the experiment, payloads will be verified just before the experiment to minimize the impact of wearing out of critical components. After successfully completing the verification of all subsystems in the near future, the spacecraft will be prepared for the experiment by means of fully charging the batteries and switching the satellite to the experiment mode. During the experiment phase a specific set of telemetry parameters will be logged and EPS will take different measures to ensure availability of energy for the experiment and to account for added risk of using high voltage devices for charging the tether and using electron guns.
• On the basic level, the project verified most systems during the first days.
• After orbit insertion, ESTCube-1 was deployed by the ISIPOD. After separation the satellite was in autonomous boot-up mode for 30 minutes to allow it to clear the launcher. Antennas were then deployed and a radio beacon containing basic telemetry was turned on. The CubeSat transmitted a beacon for 2 days after which two-way communication was established as planned. Since then the subsystems have been verified during the early orbit phase.
Tether Experiment (payload):
The main mission is testing the electric solar wind sail. For this a small conductive Hoytether (10 m in length) must be deployed from the CubeSat. Subsequently, the tether will be electrically charged to 500 V, using electron guns. Everything needed for that is housed in the payload module. 17)
An electric solar sail, or ‘e-sail’ bears little resemblance to the more usual sail concepts, shaped like a web-like net. But when electricity is applied through the e-sail, the resulting electrostatic forces repel charged plasmas found in space – including the Sun’s solar wind – to generate momentum. The e-sail technology is being developed through the EU’s Seventh Framework Program by a partnership of nine institutes across five countries.
E-sail principles: The electric sail is a new space propulsion concept which uses the solar wind momentum for producing thrust. A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 µm) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watt) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of meters into the surrounding solar wind plasma. Therefore the solar wind ions "see" the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or "flying" the resulting spacecraft electrically. 18) 19)
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20 km long electric sail wire weighs only a few hundred grams and fits on a small reel, but when opened in space and connected to the spacecraft's electron gun, it can produce several km2 effective solar wind sail area which is capable of extracting about 10 mN (milliNewton) force from the solar wind. For example, by equipping a 1000 kg spacecraft with 100 such wires, one may produce an acceleration of about 1 mm/s2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example. Alternatively, one might choose to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to be launched from Earth is small of the electric sail.
Figure 7: Conceptual view of the e-sail (image credit: University of Tartu)
ESTCube-1 will unfurl a 10 m long single-strand e-sail to demonstrate its potential as a compact and economical deorbiting method. It will measure the resulting force acting on the e-sail as it comes into contact with space plasma.
Figure 8: Schematic view of the tether deployment (image credit: University of Tartu)
The payload module contains:
• A high voltage circuit for charging the tether to ±500 V.
• A reel, on which the tether has been reeled. An aluminum end mass has been attached to the end of the tether and it simplifies unreeling of the tether.
• A motor for unreeling the tether.
Figure 9: Photo of the payload module showing the electronics board to electrically charge the tether and to drive the electron guns. The camera module is physically on the same circuit board (image credit: University of Tartu)
The camera on the payload module is used to verify the tether deployment and to take an image of Earth.
Figure 10: Photo of the electronics board with the tether reel and motor (image credit: University of Tartu)
ESTCube-1 will unfurl a 10 m long single-strand e-sail to show its potential as a compact and economical deorbiting method, measuring the resulting force acting on the e-sail as it comes into contact with space plasma.
A 100 m E-sail tether will fly on the Finnish student CubeSat Aalto-1, planned for launch in 2014.
The ground segment for ESTCube-1 consists of a radio station designed for 2 m (VHF) and 70 cm (UHF) amateur radio bands and of an open-source ground segment software based on Hummingbird, which is configured to ESTCube-1 specific mission needs.
The satellite communication ground station is located in Tartu, Estonia (WGS84 coordinates 58.37345 N, 26.72656 E). It consists of four Wimo WX7036 70 cm band cross-yagi antennas yielding amplification of 22 dBi and two Wimo WX214 2 m band cross-yagi antennas, all mounted on fiberglass poles. Signals from all the same band antennas are added by using custom-made power dividers. The antennas are pointed using a Yaesu 5500 antenna rotator which is controlled by a custom made rotator controller.
For satellite communication, signals are preamplified on mast next to antennas to compensate for transmission line losses. UHF (Ultra-high Frequency) signals are filtered to remove interference generated by nearby telecommunication stations and amplified to further improve signal-to-noise ratio. A transceiver Icom IC-910H is used to receive the satellite signal, the AX.25 data packets are decoded by a Kantronics 9612+ Terminal Node Controller. A custom-made FSK transmitter and Tokyo Highpower HL300 power amplifier are used to send commands to the satellite. Typical output power of the ground station is 200 W. SDR (Software Defined Radio) FUNcube Dongle Pro+ and USRP N210 are used for real-time radio spectrum monitoring. SDR and a GNU Radio (software toolkit) signal processing chain can be used for satellite reception as well.
Figure 11: Setup of ESTCube-1 mission control system (image credit: University of Tartu, Ref. 16)
1) “Estonian Student Satellite,” URL: https://sites.google.com/a/estcube.eu/estonian-student-satellite-program/
2) Jouni Envall, “ESTCube Mission— Testing the Electric Sail with the First Estonian Satellite,” URL: http://www.kaukokartoituskerho.fi/sites/default/files/Envall.pdf
3) Urmas Kvell and the EstCube team, “Estonian Student Satellite Program ESTCube-1,” Proceedings of the 2010 CubeSat Developers' Workshop, Cal Poly, San Luis Obispo, CA, USA, April 21-23, 2010, URL: http://mstl.atl.calpoly.edu/~jfoley/Spring2010/Spring010%20Edent%20Satellite%20Program%20ESTCube-1.pdf
4) Mihkel Pajusalu, Erik Ilbis, Jaanus Kalde, Henri Lillmaa, Risto Reinumägi, Ramon Rantsus, Martynas Pelakauskas, Ahto Leitu, Viljo Allik, Mart Noorma, Silver Lätt, Jouni Envall, “Electrical power system for ESTCube-1: a fault-tolerant COTS solution,” Proceedings of the 63rd IAC (International Astronautical Congress), Naples, Italy, Oct. 1-5, 2012, paper: IAC-12-C3.4.5
6) P. Janhunen, A. Sandroos, ”Simulation study of solar wind push on a charged wire: basis of solar wind electric sail propulsion,” Annales Geophysicae, Vol. 25, pp. 755–767, March 2007, URL: http://www.electric-sailing.fi/paper2.pdf
7) Mart Noorma, “ESTCube-1: Stepping Stone for Fast Interplanetary Travel,” First Interplanetary CubeSat Workshop, MIT, Cambride, MA, USA, May 30, 2012, URL: http://icubesat.files.wordpress.com/2012/06/icubesat-org-2012-c-1-3-_presentation_noorma_201205301151.pdf
8) Andris Slavinskis, Urmas Kvell, Erik Kulu, Tobias Scheffler, Silver Lätt, Mart Noorma, “Magnetic attitude control algorithms for ESTCube-1,” Proceedings of the 63rd IAC (International Astronautical Congress), Naples, Italy, Oct. 1-5, 2012, paper: IAC-12-B4.5.12
9) M. Pajusalu, R. Rantsus, M. Pelakauskas, A. Leitu, J. Kalde, E. Ilbis, H. Lillmaa, R. Reinumägi, V. Allik, M. Noorma, S. Lätt, “Design of the Electrical Power System for the ESTCube-1 Satellite,”Latvian Journal of Physics and Technical Sciences, Vol. 49, issue 3, 2012, pp: 16-24, URL: http://venta.lv/wp-content/uploads/M-Pajusalu_Designing-the-electrical-power-system-for-estcube-1.pdf
10) “ESA's Vega launcher scores new success with PROBA-V,” ESA press release No 12-2013, May 7, 2013, URL: http://www.esa.int/For_Media/Press_Releases/ESA_s_Vega_launcher_scores_new_success_with_Proba-V
11) “Estonia’s student cubesat satellite is ready for the next Vega launch,” Arianespace, March 20, 2013, URL: http://www.arianespace.com/news-mission-update/2013/1026.asp
12) Stephen Clark, “Vietnamese satellite booked for second Vega launch,” Spaceflight Now, January 4, 2013, URL: http://www.spaceflightnow.com/news/n1301/04vnredsat1a/#.UOwGWpGQk9Y
13) “Arianespace to launch VNREDSat-1A built by Astrium for Vietnam,” Space Travel, January 08,2013, URL: http://www.space-travel.com/reports/Arianespace_to_launch_VNREDSat_1A_built_by_Astrium_for_Vietnam_999.html
15) Information provided by Erik Kulu of the University of Tartu, Tartu, Estonia.
16) Erik Kulu, Andris Slavinskis, Urmas Kvell, Mihkel Pajusalu, Henri Kuuste, Indrek Sünter, Erik Ilbis, Tõnis Eenmäe, Kaspars Laizäns, Andres Vahter, Elo Eilonen, Jaanus Kalde, Paul Liias, Andreas Sisask, Lauri Kimmel, Viljo Allik, Silver Lätt, Mart Noorma, “ESTCube-1 nanosatellite for electric solar wind sail technology demonstration in low Earth orbit,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B4.2.10
18) Petri Toivanen, Pekka Janhunen, Jouni Envall, Sini Merikallio, “Electric solar wind sail control and navigation,” First IAA Conference on Dynamics and Control of Space, Porto, Portugal, March 19-21, 2012,Vol. 145, Advances in the Astronautical Sciences, paper: IAA-AAS-DyCoSS1 -03-10, URL: http://openconf.astrodynamics.org.pt/modules/request.php?module=oc_program&action=view.php&a=&id=192&type=3
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.