ShindaiSat (Shinshu University Satellite) / GINREI
ShindaiSat is a VLC (Visible Light Communication) experimental microsatellite of Shinshu University (Wakasato, Nagano Prefecture, Japan) for an on-orbit technology demonstration over a long distance (> 400 km), by using LED (Light Emitting Diode) light as an optical communications link. The project is educational in nature with student participation on all levels. GINREI is the nickname of ShindaiSat. 1) 2) 3)
VLC is an emerging wireless communication technique using visible light in the spectral range from 360 - 830 nm, defined by JIS Z8120 (Japanese Industrial Standard Z8120), and different from laser communications [actually, visible light is defined in the wavelength range between ~400 nm (750 THz) and ~700 nm (428 THz)]. In the ShindaiSat project, the LED scheme is being used as a lighting source with low power consumption. By modulating the light signal of the LED with a high frequency, it can be used as a data transmission and communications device.
Background: The use of solid-state sources offers the possibility of high data-rate communication, in addition to provision of illumination. Sources can be modulated at high-speed, providing a data channel in addition to the illumination, which is provided by the average signal level. Research on VLC systems originated in Japan, where the VLCC (Visible Light Communications Consortium) was founded in 2003. Interest is now growing rapidly, both in Asia and Europe, where the WWRF (Wireless World Research Forum) has worked in this area. 4) 5) 6)
The following list provides some features/properties of potential VLC technology applications: 7)
1) Eye safety: Home/office high power lighting can be used directly as a communication means. Visible light is not injurious to vision.
2) VLC systems use modulated light wavelengths emitted (and received) by a variety of suitably adapted standard sources, such as indoor and outdoor lighting, displays, illuminated signs, televisions, computer screens, digital cameras and digital cameras on mobile phones for communication purposes, primarily through the use of LEDs (Light Emitting Diodes).
3) Visibility of the communication. Prevention of a communication leakage can be implemented by controlling the emitting area.
4) At present, no legal regulation exists. The incorporation of VLC components into everyday technology is being investigated by a number of universities, corporations and organizations worldwide, and has already resulted in the creation of the JEITA (Japan Electronics and Information Technology Industries Association) standards (2007) for a “visible light ID system”, and a Specification Standard in 2008 by the VLCC - as a result of its joint cooperative agreement with the IrDA (Infrared Data Association). 8)
5) The RF spectrum is rather crowded, providing low-rate data transfers; the spectrum is regulated by the ITU. The optical spectrum has the capability to provide many GHz of information transfer on optical carriers.
6) VLC has the potential of new application fields such as underwater communication and space. VLC systems are recognized as creating a possible valuable addition to future generations of technology, which have the potential to utilize light for the purposes of advanced technological communication at ultra high speed surpassing that of current wireless systems.
Table 1: VLC versus RF characteristics 9)
The service area of the VLC technique covers all our daily life environments under visible lighting conditions. A few hundred Mbit/s of data transmission rate can be obtained over short distances. The laser technique is suitable for long distance communications. This is not the case for VLC transmissions, whose data rates are limited and strongly depend on the distance and on the atmospheric conditions. The distance dependence is mainly caused by the irradiation misalignment angle of the visible light (incorrect pointing). Without precise pointing between the transmitter and the receiver, only low data transmission rates can be obtained by a VLC system.
Table 2: Success criteria of the ShindaiSat mission
Figure 1: Illustration of the ShindaiSat microsatellite (image credit: Shinshu University, Ref. 1)
ShindaiSat features a near cubic structure of 40 cm side length and a mass of 35 kg. The main structure uses an aluminum frame and CFRP panels. Four aluminum double cross panels are fixed inside the structure, on which components are assembled. The optical lens system for receiving modulated LED light from the ground station is fixed at the center of the satellite (Ref. 3).
Figure 2: Layout and elements of the microsatellite (image credit: Shinshu University)
The satellite is comprised of the ACS (Attitude Control Subsystem), the Data Management Subsystem, the EPS (Electrical Power Subsystem), the Communication Subsystem, and the VLC sensor complement. The satellite operation is managed by the central OBC (OnBoard Computer), attitude control and VLC payload are managed by the SH-4 microprocessor and by the mission control microprocessor using FPGA board, respectively.
In the VLC mission mode, the high gain LED panel(+ZB axis) points into the Earth direction by stabilizing attitude control system using three reaction wheels and magnetic torquers. The satellite attitude is determined by the compound navigation system using magnetic sensor, sun sensor, GPS receiver and FOG(Fiber Optical Gyro) or MEMS gyro. The GaAs solar cells generate about 24 W of electric power (max) which is stored in lithium-ion batteries. Amateur bands are used for the basic satellite communications; the VLC is considered an experimental communication demonstration.
Figure 3: Block diagram of the ShindaiSat microsatellite (image credit: Shinshu University)
ACS(Attitude Control Subsystem): The ACS is comprised of attitude sensors(sun sensor, magnetic sensor, fiber optical gyro, MEMS gyro), actuators(reaction wheel, magnetic torquer), GPSR (GPS Receiver) for orbit information and attitude control board (Figure 4). The satellite roll and pitch attitudes are controlled by reaction wheels and magnetic torquers within a few degrees pointing accuracy, on the other hand, the yaw axis is only controlled by magnetic torquer to dump the rotation. Yaw angle attitude accuracy is not necessary to control within a few degrees for the VLC mission. To reduce the satellite mass and power consumption, the yaw reaction wheel is unprepared.
After separation from the launcher, the attitude of ShindaiSat is roughly controlled by the magnetic torquers, the +ZB axis is aligned parallel to the geomagnetic field line, and the oscillations are damped by B-dot control. The attitude is controlled by the three reaction wheels and the magnetic torquers for the VLC mission (Earth pointing). In idle mode, or in electric power generation mode, the attitude is controlled for sun pointing, i.e. one solar panel is oriented to the sun. 10)
Figure 4: Block diagram of the ACS (image credit: Shinshu University)
EPS: The solar cells are face-mounted on the four CFRP panels orthogonal to the +ZB axis (4 series and 6 parallel cells are mounted on one panel, adding up to 96 GaAs cells). The cell size is ~40 mm x 80 mm and the efficiency is 27 % (max). Li- ion batteries, NCR-18650A, of Panasonic Corp. are used to store the generated electric power. The nominal capacity is 3,100 mAh at 3.6 V. A battery unit contains 12 cells, packaged in 2 (series) x 6 (parallel); three units are onboard for electric power supply.
Figure 5: Illustration of the battery unit (image credit: Shinshu University)
RF communication subsystem: ShindaiSat is controlled by using RF communication from the ground station, located at Shinshu University in Nagano. IARU(International Amateur Radio Union) coordinated four frequencies for ShindaiSat in February 2013. Two of them are for downlink with 437 MHz UHF bands and another two bands of 145 MHz (VHF) for uplink. Two rod antennas will be extended after the separation from the H-IIA launcher and transmit satellite data to the ground station and one longer rod antenna is used for receiving communication signals, which is open for public use. Two loop antennas are used for receiving the command signals from the ground operation center.
Table 3: Main parameters of the ShindaiSat microsatellite
Launch: The ShindaiSat microsatellite was launched as a secondary payload on Feb. 27, 2014 (18:37:00 UTC) on the primary GPM (Global Precipitation Measurement) mission of NASA and JAXA. JAXA sponsored the launch on the H-2A vehicle from the Tanegashima Space Center, Japan with MHI (Mitsubishi Heavy Industries, Ltd.) as the service provider. 11)
The secondary Japanese payloads manifested by JAXA on the GPM Core mission were: 12)
• ShindaiSat (Shinshu University Satellite), a microsatellite (35 kg) to demonstrate LED light as an optical communications link.
• The STARS-2 (Space Tethered Autonomous Robotic Satellite-2) nanosatellite technology mission of Kagawa University, Takamatsu, Kagawa, Japan
• TeikyoSat-3, a bioscience microsatellite (~20 kg) of Teikyo University
• ITF-1 (Imagine The Future-1), a 1U CubeSat of the University of Tsukuba, Tsukuba, Japan.
• OPUSat (Osaka Prefecture University Satellite), a 1U CubeSat
• INVADER (INteractiVe satellite for Art and Design Experimental Research) of Tama Art University, a 1U CubeSat
• KSat-2 (Kagoshima University Satellite-2), a CubeSat mission with a mass of ~ 1.5 kg. 13)
Orbit: Non-sun-synchronous circular orbit, altitude = 407 km, inclination = 65º.
After the release of the GPM Core Satellite , the second stage performed attitude maneuvers and slightly changed its orbit for the deployment of the seven secondary payloads that include small spacecraft and CubeSats dedicated to scientific missions, technical demonstrations and outreach projects. 14)
Table 4: Launch sequence of GPM mission and secondary payloads
Sensor complement: (VLC)
VLC (Visible Light Communication)
The VLC experimental mission uses a high-gain and a low-gain LED unit configuration (Figure 6). The LED units are products of NICHIA corporation; they provide an output efficiency of ~140 lumen/W.
• The high-gain LED unit is used for data transmissions with a bit rate of 1.2 kbit/s or 9.6 kbit/s. This unit is composed of 32 LEDs located in the focal plane of the 32 parabolic mirrors, respectively. The irradiation angle is about 6º. The total light flux is ~10,000 lumen at ~90 W power consumption. The minimum irradiation circular area on the ground is about 40 km from an orbital satellite altitude of 400 km. This spotlight circle defines the communication area of the high-gain LED on the ground. The high-gain LED is also modulated as CW mode, which can be easily observed and decoded from the ground without using any optical instruments. - The 9.6 kbit/s rate will only be sustained under precise attitude control stabilization conditions, i.e., when proper transmitter-receiver alignment prevails.
• Low-gain LED unit: One LED unit is mounted on each side panel of the satellite (total of 4 units). These LEDs emit about 1,920 lumen for CW (Continuous Wave) communications. The irradiation angle of the low-gain LED is ~110º, which implies that at least one low-gain LED can be detected from any direction. Each light is identified to its respective satellite panel to obtain the coarse attitude information. The low gain LED unit is used at the acquisition mode just after the separation from the launcher; it is also used in the emergency mode when the attitude information cannot be determined.
Figure 6: Schematic view of of high- and low-gain LED units (image credit: Shinshu University)
Table 5: Specification of the LED units
Figure 7 shows the VLC uplink and downlink components. One downlink LED unit is composed of one modulation board and driver and 4 LED elements/parabolic mirrors. A candidate ground station optical receiver is a 1 m class parabolic mirror to concentrate the photons from the satellite. An APD (Avalanche Photodiode) detector array is located in the focal plane of the mirror. The onboard optical receiver is also a photodiode array to detect the modulated light and also to sense the light direction for closed-loop attitude control.
Figure 7: VLC up/down-link components (image credit: Shinshu University)
Figure 8 shows the brightness and irradiation area for high-gain and low-gain LED lighting. The irradiation angle is about 6º and the irradiation footprint on the ground is about 40 km in diameter from an orbital altitude of 400 km. The maximum brightness is estimated to be equivalent of a star of magnitude -2; the light of which passes through the receiver footprint in about 5 seconds for satellite nadir pointing. For VLC communications, the satellite points continuously to the ground tracking station, the maximum communication time of a pass is about 5 minutes, depending on the aspect of satellite orbit relative to the ground station. The brightness of the low gain light is estimated to be equivalent of a star of magnitude 8. Some optical device is necessary to find the light. This low gain LED with a broad irradiation angle is used for the satellite acquisition just after the separation from launcher, as well as for an unexpected attitude signal loss.
The main RF ground station for the operation of ShindaiSat is located at Shinshu University in Nagano. The RF backup station is located at Matsumoto . For the VLC mission, fixed and/or mobile optical stations are under consideration as shown in Figure 9. Shinshu university is also designing a mobile station to move to an adequate atmospheric condition area, prior to the arrival of the satellite AOS (Acquisition of Signal), in wait for the VLC experiment.
The necessary aperture of the optical parabolic mirror or telescope is calculated to be ~1 m. Public optical observatories and organizations, as shown in Figure 9, have the facilities of satellite tracking capability and large aperture telescopes. The VLC link between the satellite and ground station is dependent on the atmospheric conditions.
On the other hand,
Figure 9: Optical ground station and optical telescopes with the capability for ShindaiSat tracking (image credit: Shinshu University)
VLC has the potential applications for future light communications both in space and on the ground. Figure 10 shows the Shinshu University proposal for future LED VLC application in the field of water, air and space environments. As the very long distance VLC experiment will be demonstrated in 2014 FY and ISL (Intersatellite Link) VLC applications will be proposed and demonstrated as the post ShindaiSat project. Long distance VLC applications in the air for aircraft communications will be another future application candidate.
Figure 10: Proposed future LED VLC applications (image credit: Shinshu University)
1) Atsushi Nakajima, Yusuke Shimizu, Arata Fukuzawa, Hironori Sugiyama, Nobutada Sako, Hidekazu Hashimoto, “R&D on Visible Light Communication Micro-Satellite, ShindaiSat,” Proceedings of the 29th ISTS (International Symposium on Space Technology and Science), Nagoya-Aichi, Japan, June 2-8, 2013, paper: 2013-f-06
2) Atsushi Nakajima, Nobutada Sako, Hidekazu Hashimoto, Arata Fukuzawa, Yusuke Shimizu, ““ShindaiSat, a Visible Light Communication Micro-satellite,” Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013
3) Atsushi Nakajima, Nobutada Sako, Masato Kamemura, Yuuki Wakayama, Arata Fukuzawa, Hironori Sugiyama, Naoki Okada, “ShindaiSat : A Visible Light Communication Experimental Microsatellite,” Proceedings of the ICSOS (International Conference on Space Optical Systems and Application) 2012, Ajaccio, Corsica, France, October 9-12, 2012, URL: http://icsos2012.nict.go.jp/pdf/1569603077.pdf
4) Dominic C. O'Brien, Lubin Zeng, Hoa Le-Minh, Grahame Faulkner, Joachim W. Walewski, Sebastian Randel, “Visible Light Communications: challenges and possibilities,” 2008, URL: http://126.96.36.199/proc/pimrc2008/content/papers/1569135393.pdf
5) Shinichiro Haruyama, “Visible Light Communications: Recent Activities in Japan,” Feb. 8, 2011, URL: http://smartlighting.rpi.edu/resources/PDFs/smartspaces2011/Smart-Lighting_ERC_Haruyama_2011_02_08.pdf
8) “Visible Light Communications Consortium (VLCC) Released Visible Light Communication Standard Based on IrDA Core Specification,” March 12, 2009, URL: http://www.reuters.com/article/2009/03/12/idUS260804+12-Mar-2009+BW20090312
10) Yusuke Shimizu, “Attitude Control System for Micro-Satellite ShindaiSat,” Proceedings of the 29th ISTS (International Symposium on Space Technology and Science), Nagoya-Aichi, Japan, June 2-8, 2013, paper: 2013-s-63d
11) “Launch Result of H-IIA Launch Vehicle No. 23 with GPM Core Observatory onboard,” JAXA, MHI, Feb. 28, 2014, URL: http://www.jaxa.jp/press/2014/02/20140228_h2af23_e.html
12) Toshinori Kuwahara, Kazaya Yoshida, Yuji Sakamoto, Yoshihiro Tomioka, Kazifumi Fukuda, Nobuo Sugimura, Junichi Kurihara, Yukihoro Takahashi, “Space Plug and Play Compatible Earth Observation Payload Instruments,” Proceedings of the 9th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 8-12, 2013, Paper: IAA-B9-1502
14) Patrick Blau, “GPM Core - Mission Updates,” Spaceflight 101, Feb. 27, 2014, URL: http://www.spaceflight101.com/gpm-core-mission-updates.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.