PRISM (Picosatellite for Remote¿sensing and Innovative Space Missions)
PRISM is an innovative next¿generation small¿satellite technology pathfinder mission of ISSL (Intelligent Space Systems Laboratory) at the University of Tokyo (UT), Japan. There are project cooperations with the Nagasuka Laboratory of UT, the Noguchi Laboratory of the University of Keio, and with industry. The main mission objective is to demonstrate high/medium¿resolution imaging (in the order of 30 m from an orbital altitude of 800 km) to be done on a low¿cost mission with a total spacecraft mass of ~8 kg (i.e., a nanosatellite). Along with the imaging mission, ISSL plans to demonstrate several new technologies applicable to future small satellite missions. 1) 2) 3) 4) 5) 6) 7) 8)
Figure 1: Artist's view of the PRISM nanosatellite in orbit (image credit: ISSL)
From the laws of optics, a long focal length and a sufficiently large aperture diameter are both necessary elements of an imager (i.e., the telescope) to acquire high-resolution imagery. Since these fairly large dimensional instrument requirements cannot be realized in a “tiny camera design,” and are fundamentally in conflict with a marginal satellite volume and mass of a nanosatellite - a new technical approach is being introduced to circumvent these launch constraints - by demonstrating a combined satellite/instrument compact design (i.e., a telephotographic digital camera system) based on deployable concepts (Figure ).
• An extensible boom concept is being used in the optics system to provide sufficient focal length for the imaging task. The extensible boom is deployed (after separation with the launch vehicle) with a lens attached at its tip. Deployment of the boom lengthens the distance between a lens and the imaging detector, which then realizes a long focal length. It implies also that the boom structure must be aligned accurately and remain stable (thermally) under the various orbital lighting conditions.
• Use of an improved spacecraft ejection mechanism design, i.e. T-POD (Tokyo Picosatellite Orbital Deployer) system, as was being used for the CubeSat XI-IV (X¿factor Investigator-IV) mission of UT (a multiple CubeSat launch of XI took place on June 30, 2003). The improved T-POD system is called PHS (PRISM Hosyutsu Souchi). 9)
• Solar array paddle (another deployment structure) and active attitude control system using magnetic torquers.
• Development of reinforced satellite bus technologies
• High¿performance OBC (Onboard Computer) with a real¿time operating system and a CAN bus
• A downlink system with a data rate (9600 bit/s). An onboard data compression technique is being employed (JPEG) to make image transmission feasible.
• Power management system using PPT (Peak Power Tracking) techniques.
Figure 2: The PRISM S/C in unextended boom configuration (image credit: ISSL)
Figure 3: The PRISM S/C in extended boom configuration (image credit: ISSL)
The PRISM spacecraft bus structure is a cuboid of size: 19.2 cm x 19.2 cm x 40 cm (i.e., larger than the triple CubeSat standard). The bus consists of four columns and ten body panels, made of duralumin. Two deploying panels with solar cells are also made of the same metal alloy.
Table 1: Parameters of the PRISM bus
PRISM is three-axis stabilized with ADCS (Attitude Determination and Control Subsystem) to satisfy the requirements for imaging observations. The extensible boom design provides in fact a double function - to stabilize the spacecraft (passively as a gravity gradient boom), and to function as a placeholder for the primary lens of the optics subsystem (telescope). Magnetic torquers (MTQ) are used as actuators. Attitude sensing is provided by magnetometers, gyros, and sun sensors. In addition, WAC (Wide Angle Camera) is being used as an Earth sensor (horizon sensing). A GPS receiver provides location and timing data. ADCS implements the following support functions: 10)
• Capability of pointing the boom¿mounted lens toward nadir - in a predefined direction (steering function by attitude control)
• Provision of spacecraft attitude motion damping; this allows imagery of improved resolution.
Table 2: Overview of ADCS elements
Once the boom is deployed, ADCS steers the boom-mounted lens at Earth's surface. In case the attitude of PRISM is stabilized in the opposite direction, magnetic torquers securely recover the situation. The pointing accuracy calls for a few tens of a degree with a stability of 0.5º/s.
Figure 4: Block diagram of the ADCS (image credit: ISSL)
The C&DH (Command and Data Handling) subsystem employs a CAN (Controller Area Network) bus for all onboard data transport. The OBC [a 32 bit SH7145F (Super H family of Renesas Technology) microprocessor] is in charge of all monitoring and control functions including data storage. The functions of C&DH, ADCS, and the optics subsystem are each being implemented into one chip. The management of the various tasks is being performed using the ISSL inhouse task management system. The main CPU is reprogrammable.
Figure 5: Block diagram of the C&DH subsystem (image credit: ISSL)
The EPS (Electric Power Subsystem) has its own CPU to control power generated by solar cells, based on peak power tracking technology, and to monitor the behavior of the main CPU. The EPS uses GaAs type solar cells (25% efficiency) for power generation. Two deployable solar array paddles are being used to optimize power collection. Rechargeable lithium-ion batteries are being used for eclipse operations support. The power is distributed to each subsystem. Furthermore, power subsystem includes the CW beacon transmitter.
Figure 6: Equipment layout of the PRISM engineering model (image credit: ISSL)
RF communications: The RF communications are based on amateur radio standards. The downlink frequency is in UHF (437 MHz band) and the uplink is in VHF (145 MHz). The AX.25 protocol is implemented for all communications. Use of three transmitters and two receivers (custom made); the output power is up to 3 W. Imagery is downlinked at a rate of 9.6 kbit/s using GMSK modulation. The JPEG2000 image compression method is implemented onboard to reduce the amount of downlink transmission.
• A CW (Continuous Wave) beacon downlink transmits the S/C heath data in Morse code at 50 wpm (words per minute)
• A downlink at 1.2 kbit/s (UHF, 8 bit characters, AFSK modulation) is being used for S/C status information or for imagery transmission
• A 9.6 kbit/s downlink is the main downlink for imagery using GMSK (Gaussian Minimum Shift Keying) modulation.
• The uplink transmission is at 1.2 kbit/s (VHF) using AFSK modulation. Uplinks are being by the TNC (Terminal Node Controller) receiver (PIC16c716) of the S/C and sent to OBC.
Table 3: Specification of the onboard transceivers
A ground station at ISSL is being used for all satellite communications - and a control center is being used for all S/C operations.
Launch: The PRISM spacecraft was launched as a secondary payload to GOSAT (primary payload) on January 23, 2009 from TNSC (Tanegashima Space Center),Japan. The launch vehicle was the H-IIA launcher of JAXA. 11) 12)
The seven secondary payloads on this flight were:
- SDS-1 (Small Demonstration Satellite-1) of JAXA (~100 kg)
- SOHLA-1 (Space Oriented Higashiosaka Leading Association-1), Japan (50 kg)
- SpriteSat (Tohoku University), Japan (microsatellite of ~50 kg)
- PRISM (Picosatellite for Remote¿sensing and Innovative Space Missions) of ISSL of the University of Tokyo, 8.5 kg
- Kagakaki (SORUNSat-1), Japan, 20 kg
- KKS-1 (Kouku Kosen Satellite-1) of Tokyo Metropolitan College of Industrial Engineering), nanosatellite of 3 kg
- STARS-1 (Space Tethered autonomous Robotic Satellite-1) of Kagawa University, Japan, ~ 10 kg.
Orbit: Sun-synchronous orbit, altitude of 660 km, inclination = 98º, LTAN (Local Time at Ascending Node) at 13:00 ± 0.15 hours.
Figure 7: Illustration of the deployed PRISM nanosatellite (image credit: ISSL)
• In January 2014, the PRISM nanosatellite is operating nominally. The project is in contact with the nanosatellite three to four times per day, mainly for acquiring images of Earth (Ref. 13).
• On Feb. 23, 2012, PRISM was 3 years on orbit. The spacecraft is operating nominally (Ref. 13).
Figure 8: Sample WAC (Wide Angle Camera) image of the northern Tohoku area (image credit: ISSL)
Legend to Figure 8: Tohoku was the damaged area in Japan's last earthquake and tsunami on March 11, 2011.
• In February 2011, PRISM is still in good health; the project is operating the spacecraft for capturing and donwlinking Earth images and testing of the attitude control system. Images of 30 m ground resolution have already been acquired at several locations on the Earth. The SNR (Signal-to-Noise Ratio) is not perfect, but rivers of 30 m width can be recognized, which confirm the success of the mission. 13) 14)
• On January 23, 2010 the PRISM nanosatellite was 1 year in orbit. The spacecraft is operating nominally.
• An important aspect of research in the PRISM mission is to estimate and compensate for the magnetic disturbances - needed for accurate attitude control to keep the direction of the telescope nadir pointing (this is difficult to achieve using only the torque of the gravity gradient boom). In this mission, the magnetic dipole moment is estimated by the telemetry data of the magnetometer and the gyro sensor using the EKF (Extended Kalman Filter) function, which achieve 0.001 Am2 accuracy estimation and compensation. 15)
Figures 9 and 10 illustrate the in-orbit performance of the PRISM mission in the early mission phase. The objective of this satellite operation is to stabilize the attitude with the magnetic compensation.
- Figure 9 shows the telemetry data without compensation of the magnetic disturbance. In this case, the attitude is stabilized to less than 0.7º/s, which is the attitude requirement; however, the attitude is becoming instable with time because of the magnetic disturbance.
- Figure 10 shows the telemetry data with compensation of the magnetic disturbance. In this figure, the satellite attitude is more stable achieving an accuracy of 0.1º/s.
Figure 9: Telemetry data of attitude stabilization without magnetic disturbance compensation (image credit: ISSL)
Figure 10: Telemetry data of attitude stabilization with magnetic disturbance compensation (image credit: ISSL)
• The satellite was actively stabilized to < 0.12º/s using MTQ. Methods to calibrate the sensor were also tested to achieve more precise attitude control. From these results it was concluded that the ADCS on PRISM is useful for the attitude control of a nanosatellite (Ref. 10).
• The boom was successfully extended on Feb. 27, 2009. This was confirmed by telemetry and by images of the WAC (Figure 11, showing the extended flexible boom against the background of clear Earth image).
Figure 11: WAC image after extension of the flexible boom (image credit: ISSL)
• On the same day after launch, the CW beacon was received at the Kiruna station. Then the AFSK communication system was tested. It was confirmed that the radio system worked properly and power and temperature balance in orbit was checked from telemetry data. The housekeeping data showed that PRISM worked nominally in the initial phase of flight.
Optics subsystem: A refractive optical system design is adopted. The reason is that a simple refractive optical system requires a less stringent collimation accuracy with regard to alignment issues of the optical components - as compared to even the simplest reflective optical system. With this background in mind, there is a good chance that the extensible boom concept as `telescope tube framework' is feasible for observation imagery. A reduction of chromatic aberration of the collector optics is achieved by using a fluorite apochromatic lens.
Table 4: Specification of the optics subsystem
The overall objective is to demonstrate/validate the acquisition of high¿resolution imagery (in the visible range) through a large¿diameter apochromatic lens with a long focal length. The COTS (Commercial¿Off¿The¿Shelf) CMOS area array detector has a size of 1.3 million pixels (1280 x 1024). The CMOS image detector is the IBIS-5A product of FillFactory NV (Mechelen, Belgium).
Figure 12: Block diagram of the optics subsystem (image credit: ISSL)
The extensible boom concept comes with the following features:
• The boom is hollow tubular structure. There are no obstructions in the optical path as in Figure 13, when the deployment is completed.
• The boom is deployed using the concept of restoring force of elastic materials. This passive deploying mechanism makes the satellite's bus system compact and simple.
• The extended boom is provided with several baffles and shade curtains to reduce the effect of stray light collection.
• The optics subsystem has an onboard one dimensional focusing mechanism to optimize the focal length compensation (a micro stepping motor is used to avoid any degradation of the imagery).
The flexible extensible boom is the key component for the mission (Figure 13). The GFRP (Glass Fiber Reinforced Polymer) frame plays a major role of the coiled spring, which pushes the lens and baffle plates out when the structure is unfolded and extended. Some threads are connecting the baffle plates to each other constraining each distance between them; they determine the total length of the boom. They also determine the relative position as well as the tilt angle of the lens toward the image sensor.
Figure 13: Components of the boom (image credit: ISSL)
The compensation of structural errors of the boom after deployment represents the most distinctive technology introduction of the project. The following steps were thoroughly investigated to arrive at a workable solution:
1) Derivation of an analytical model for a dynamically stable structure. This involved the calculation of boom stability against tiny errors (i.e. inequality of thread lengths or that of elastic coefficients of GFRP). Eventually, a dynamically stable point of the design was found.
2) Static error compensation: This work involved the compensation of any inequalities of thread lengths - by using thin spacers between the baffle plate and the thread stopper. Such spacers may also be inserted between the root of the boom and the bottom plate. This scheme permits to provide an offset for the total length of boom. The accurate length of the boom or of each thread is measured by laser technique; it may also be derived/confirmed from the experimental imagery obtained by the whole system.
3) Extension shock absorber: The boom structure and the main cubic body of PRISM are jointed by only 4 springs. These springs absorb the extension shock due to the inertia of the lens. This shock may cause damage or errors of the boom structure without such a shock absorber.
4) Design of the focusing mechanism: The image sensor board may be pushed by a stepping motor while it is being pulled by springs (Figure 14, bottom detail). The image board is guided by a linear rail permitting a parallel motion in the viewing direction. This feature enables the dynamic relative position control of the image sensor toward the lens. In turn, it permits also to compensate for any remaining structural errors of the boom (the focal length compensation acts as an “autofocussing” mechanism).
Figure 14: Illustration of the focusing mechanism with a detail view at the bottom (image credit: ISSL)
Verification by experiment: A special microgravity experiment was conducted with the PRISM engineering model of the boom assembly in a Sept. 2007 campaign by using a Gulfstream-II aircraft parabolic flight to demonstrate and validate the instrument behavior regarding the compensation of structural boom errors with the designed focusing mechanism (see Ref. 1).
In a 1 g environment, the experiment yielded the expected results of the static boom offset. It turned out that the measured standard deviation of the boom length obtained was much smaller than the available compensation range of the focusing mechanism.
Figure 15: Illustration of the flexible and extensible boom system (image credit: ISSL)
After the launch of PRISM, the predicted deployment parameters should be compared with the data of real system. For this reason, PRISM has many sensors onboard including three wide angle cameras (WACs) to check the behavior of the boom on orbit. The sensor data will be analyzed and compared with predicted parameters from actual operation.
Ejection System for PRISM:
The PRISM mission uses a new and improved design of T-POD (Tokyo - Picosatellite Orbital Deployer) system heritage, called PHS (PRISM Hosyutsu Souchi) where Housyutu means 'ejection' and Souchi means 'system' in Japanese (Ref. 9).
PHS introduces an original mechanism and structure (above that of T-POD) by providing a) a flexible boom-defending mechanism, and b) packaging of the turnstile antenna structure into a suitable launch configuration.
Figure 16: Photo of the PHS ejection mechanism (image credit: ISSL)
1) Y. Sato, S. K. Kim, Y. Kusakawa, K. Shimizu, T. Tanaka, M. Komatsu, Yoo Il-Yun, C. Lambert , S. Nakasuka, “Extensible Flexible Optical System for Nano-Scale Remote Sensing Satellite “PRISM”,” Proceedings of the 26th ISTS (International Symposium on Space Technology and Science) , Hamamatsu City, Japan, June 1-8, 2008, paper: 2008-m-13
2) Mitsuhito Komatsu, “University of Tokyo Nano Satellite Project “PRISM”,” Proceedings of the 26th ISTS (International Symposium on Space Technology and Science) , Hamamatsu City, Japan, June 1-8, 2008, paper: 2008-s-08
3) A. Enokuchi, Y. Nakamura, R. Funase, M. Nagai, Y. Hatsutori, N. Miyamura, M. Komatsu, Y. I. Yun, S. Nakasuka, “Technology demonstration of a new extensible-boom-based telescope by 5 kg class student satellite `PRISM',” Proceedings of the 4S Symposium: `Small Satellite Systems and Services,' Chia Laguna Sardinia, Italy, Sept. 25-29, 2006, ESA SP-618
4) M. Nagai, T. Eishima, Y. Nakamura, R. Funase, A. Enokuchi, T. Funane, Y. Nojiri, F. Sasaki, S. Nakasuka, “University of Tokyo's Picosatellite Project `PRISM',” Proceedings of the 56th IAC 2005, Fukuoda, Japan, Oct. 17-21, 2005, IAC-05-B5.4.09
5) Y. Nakamura, T. Eishima, M. Nagai, R. Funase, A. Enokuchi, K. Nakada, Y. Cheng, E. Toshiyuki Takei, T. Funane, F. Sasaki, Y, Nojiri, T. Yamamoto, E. Nagayama, S. Nakasuka, “University of Tokyo's Ongoing Student¿Lead Picosatellite Projects - CubeSat XI and PRISM,” Proceedings of IAC 2004, Vancouver, Canada, Oct. 4¿8, 2004, IAC¿04¿IAA¿4.11.4.06
6) Akito Enokuchi, Masaki Nagai, Ryu Funase, Yuya Nakamura, Shinichi Nakasuka, “Remote Sensing by University of Tokyo's Pico-Satellite Project PRISM,” Proceedings of the 5th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 4¿8, 2005, URL: http://www.dlr.de/iaa.symp/Portaldata/49/Resources/dokumente/archiv5/0605_Enockuchi.pdf
8) C. Lambert, Y. Sato, T. Inamori, S. Nakasuka, “Vibration Analysis of Nanosatellite PRISM Extendable Boom,” Proceedings of the 26th ISTS (International Symposium on Space Technology and Science) , Hamamatsu City, Japan, June 1-8, 2008, paper: 2008-c-01
9) Y. Kusakawa, S. Chikura, K. Shimizu, M. Komatsu, S. Nakasuka, “The Development of a Satellite Ejection System of PRISM,” Proceedings of the 26th ISTS (International Symposium on Space Technology and Science) , Hamamatsu City, Japan, June 1-8, 2008, paper: 2008-c-10
10) Takaya Inamori,.Yoo Ilyun,Yuta Suzaki, Yuki Sato, Toshiki Tanaka, Mitsuhito Komatsu, Shinichi Naksuka, “Attitude Determination and Control System in Pico-satellite for Remote-sensing and Innovative Space Missions (PRISM),” Proceedings of the 27th ISTS (International Symposium on Space Technology and Science) , Tsukuba, Japan, July 5-12, 2009, paper: 2009-d-05,
12) Yosuke Nakamura, Naomi Murakami, Yuta Horikawa, Hiroshi Tachihara, Hiroshi Horiguchi, Keiichi Hirako, Hidekazu Hashimoto, “Vitalization of Japanese Small Satellite Community through Publicity-offered Piggy-back Launch,” Proceedings of the Symposium on Small Satellite Systems and Services (4S), Funchal, Madeira, Portugal, May 31-June 4, 2010
13) Information provided by Shinichi Nakasuka of ISSL (Intelligent Space Systems Laboratory) at the University of Tokyo, Japan.
15) Takaya Inamori, Sako Nobutada, Shinichi Nakasuka, “Magnetic dipole moment estimation and compensation for accurate attitude control in nano-satellite missions,” Proceedings of the 60th IAC (International Astronautical Congress), Daejeon, Korea, Oct. 12-16, 2009, IAC-09.E2.1.2.
16) Toshiki Tanaka, Yuki Sato, Yasuhiro Kusakawa, Kensuke Shimizu, Takashi Tanaka, Sang Kyun Kim, Mitsuhito Komatsu, Yoo Il-Yun, Casey Lambert, Shinichi Nakasuka, “The Operation Results of Earth Image Acquisition using Extensible Flexible Optical Telescope of PRISM,” Proceedings of the 27th ISTS (International Symposium on Space Technology and Science) , Tsukuba, Japan, July 5-12, 2009, paper: 2009-n-15
17) Kensuke Shimizu, “University of Tokyo Nano Satellite Project PRISM,” Proceedings of the 27th ISTS (International Symposium on Space Technology and Science) , Tsukuba, Japan, July 5-12, 2009, paper: 2009-s-03u,
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.