E-ST@R (Educational SaTellite @ politecnico di toRino)
E-ST@R (commonly written as e-st@r and pronounced as e-star) is a CubeSat project designed and developed by students under faculty guidance at the Politecnico di Torino, Turin, Italy. Next to the educational experience gained in such a project, the primary objective is the demonstration of an active 3-axis ADCS (Attitude Determination and Control Subsystem) technology, including an inertial measurement unit. The complexity of this task in the confinement of a CubeSat is indeed a great challenge. The secondary objective is the testing of commercial components and materials in space. 1) 2) 3)
The capability of an active attitude control mechanism in a picosatellite has an enabling potential for future CubeSat missions. However, in the basic operative mode, the e-st@r satellite does not need a high pointing accuracy to communicate with the ground stations, so a potential failure of the ADCS should not affect the mission success.
Figure 1: Overview of the e-st@r project guidelines (image credit: Politecnico di Torino)
In 2008, the est@r CubeSat project was accepted by ESA's Educational Office for a free launch opportunity on the maiden flight of the Vega launch vehicle.
Figure 2: Photo of the e-st@r flight unit (image credit: Politecnico di Torino)
The spacecraft structure is based on the CubeSat standards with a size of 10 cm x 10 cm x 10 cm and a mass of ≤ 1kg. Use of the CubeSat Kit of Pumpkin Inc. All components (panels) of the structure use the aluminum alloy 5005 H16 and were anodized. The CubeSat structure underwent vibration tests. Furthermore, the center of mass location and the inertia parameters are within their specified range. 4)
ADCS (Attitude Determination and Control Subsystem): The ADCS represents the main payload of the e-st@r mission. The CubeSat is 3-axis stabilized using a MTQ (Magnetic Torquer) assembly in all axis. The MTQs are actuated by a PWM (Power Width Modulation) driver and controlled by the microcontroller ARM9.
The MTQs have been built by students in the CubeSat laboratory, then integrated and tested on the satellite. The ADCS electronics are self-contained on a dedicated board, also developed at the CubeSat lab.
Figure 3: Overview of the ADCS architecture (image credit: Politecnico di Torino)
The attitude of the CubeSat is sensed by sun sensors, a magnetometer and an IRU (Inertial Measurement Unit). Initial attitude acquisition requires to determine the attitude on the basis of unknown angles. A dedicated algorithm is used to do this, a stabilization maneuver can only be started once the initial attitude is properly estimated. The scheme is based on a simple, inaccurate but efficient PID (Proportional Integrative Derivative) logic which allows to speed down the rotational motion of the satellite until a desired angular velocity is reached.
At the end of this phase, the satellite assumes its nominal mission and the attitude control is achieved by a LQR (Linear-Quadratic Regulator), which allows to better control the attitude with respect to the PID method even if it is more difficult to be implemented. A Kalman filter and other algorithms have also been implemented to improve the attitude determination and control performance.
EPS (Electric Power Subsystem): The EPS is a COTS product provided by Clyde Space Ltd. of Glasgow, UK. The power distribution uses a bus architecture. The solar cell assembly is supplied by CESI S.p.A. Of Milan, while the Li-ion batteries have been bought from NRG srl., Italy.
Figure 4: Overview of the EPS architecture (image credit: Politecnico di Torino)
There are three main areas in Figure 4, the solar array assembly (SP), the PCDU (Power Control and Distribution Unit), and the D-PCDU (Daughter–Power Control and Distribution Unit). The solar array assembly includes 5 solar panels, each of which is constituted by two TJ (Triple Junction) GaAs solar cells connected in series and one temperature sensor. Each solar panel gives more than 2 W when fully exposed to the sun.
The PCDU is devoted to control and to distribute the electrical power as needed. The D-PCDU hosts the energy storage part of the system (Li-ion battery packs and other devices); it has been designed and built at the CubeSat lab.
OBC (On-Board Computer): The OBC is based on an off-the-shelf processing unit developed by Pumpkin Inc. It consists of a microprocessor of Texas Instruments (MSP430, 16 bit), and works on the SALVO real-time operating system. The unit has been programmed by the students and researchers to accomplish a number of scheduled tasks such as: time set, time store, watchdog, activation sequence, ADCS interface, ADCS data handling, EPS data handling, diagnostics, uplink validation, uplink decoding, command execution, packet definition, downlink, and min-max telemetry. - The scheduler acts as the program manager, organizing the tasks as requested by the user.
Figure 5: Overview of the OBC architecture (image credit: Politecnico di Torino)
ComSys (Communications Subsystem): ComSys consists of a radio modem unit connected to a dipole deployable antenna mounted on one of the CubeSat external faces. A commercial transceiver (Radiometrix) is employed and integrated on the onboard shelf contained electronics, equipped also with a PIC16 that accomplishes the modem function. The CubeSat transmits and receives in the UHF band (437.445 MHz) assigned by the IARU (International Amateur Radio Union).
Figure 6: Overview of the ComSys architecture (image credit: Politecnico di Torino)
The multiple payload launch encompasses a primary payload of 400 kg called LARES (LAser RElativity Satellite), and CubeSats (educational payloads) as secondary payloads, whose launch is sponsored by ESA. The free launch of CubeSats was offered by the ESA Education Office in Oct. 2007 (Announcement Opportunity) in cooperation with the Vega program. 7)
CubeSat passenger payloads: Although ESA's Education Office is providing 9 CubeSat positions on the maiden flight of Vega, only 7 CubeSats are confirmed as of December 2011 (Ref. 8). Not all universities that were preselected for the launch opportunity in June 2008, were able to deliver their CubeSat and the requested documentation. Other CubeSat projects, like SwissCube and HiNCube, decided to be launched on commercial flights.
ALMASat, a microsatellite of the University of Bologna, is another secondary payload of the flight.
Use of P-POD (Poly Picosat Orbital Deployer) for the deployment of all CubeSats.
Orbit of secondary payloads: Elliptical orbit, altitude of 354 km x 1450 km, inclination = 69.5º, orbital period = 103 minutes (14 revolutions/day), eccentricity = 0.075. About 75% of the orbit is in sunlight.
Status of e-st@r mission:
• March 2012: e-st@r’s signal has been received at the team’s ground station in Italy, and by radio amateurs around the world. However, operations have been affected by unexpected tumbling of the CubeSat. Until it achieves attitude stabilization, the team has placed e-st@r in power saving mode. 10)
The ground segment consists of one main ground station, one mobile backup station, and the global community of radio amateurs. The main ground station is located in Bra, 60 km south of Turin, Italy.
Figure 7: Architecture of the ground control station (image credit: Politecnico di Torino)
2) Sabrina Corpino, Sergio Chiesa, Fabrizio Stesina, Nicole Viola, “Cubesats development at Politecnico di Torino: the e-st@r program,” Proceedings of the 61st IAC (International Astronautical Congress), Prague, Czech Republic, Sept. 27-Oct. 1, 2010, IAC-10.B4.6B.11, URL of presentation: http://areeweb.polito.it/ricerca/E-STAR/Documents/Cubesats_development_at_Politecnico_di_Torino.pdf
3) “Satellites at Polito: programs overview,” International Workshop on Aerospace Technology (IWAT), Torino, Italy, Oct. 26-27, 2009, URL: http://areeweb.polito.it/ricerca/E-STAR/Documents/ESTAR_iWAT.pdf
4) “Pumpkin, Inc. CubeSat Kits™ Aboard Upcoming ESA Vega Maiden Flight,” URL: http://www.leodium.ulg.ac.be/cmsms/uploads/Pumpkin_PR-5.pdf
5) “ESA’s new Vega launcher scores success on maiden flight,” ESA, Feb. 13, 2012, URL: http://www.esa.int/SPECIALS/Vega/SEMJ8LYXHYG_0.html
6) “Vega VV01 launch campaign,” ESA, URL: http://www.esa.int/SPECIALS/Vega/SEMY64BX9WG_mg_1.html
7) Jakob Fromm Pedersen, “CubeSat Educational Payload on the Vega Maiden Flight, Interface Control Document,” ESA/ESTEC, Feb. 13, 2009, URL: http://www.ies.univ-montp2.fr/robusta/satellite/IMG/pdf/SP_GN_2009.02.13_ICD.pdf
8) “ESA’s CubeSats ready for flight,” ESA, Dec. 16, 2011, URL: http://www.esa.int/SPECIALS/Education/SEMG1C8XZVG_0.html
9) “ESA Cubs delivered for first Vega flight,” ESA, Nov. 14, 2011, URL: http://www.esa.int/esaMI/Education/SEM3L0WWVUG_0.html
10) “CubeSats satellite operations update,” ESA, March 28, 2012, URL: http://www.esa.int/SPECIALS/Education/SEM2KRGY50H_0.html
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.