[[Series 6: Introduction of the Current Status of Small Japanese Satellites]]
Introduction of SPRITE-SAT, Tohoku University

Six small piggyback satellites are planned to be launched in FY2008 together with JAXA's Greenhouse Gases Observing Satellite (GOSAT)

Report by Dr. Yukihiro Takahashi, principal investigator of SPRITE-SAT project, Dept. Geophysics, Tohoku University

Purpose and background

SPRITE-SAT is a scientific satellite dedicated to observing two cutting-edge subjects: one of the transient luminous events (TLEs) in the middle and upper atmosphere, called "sprites", and terrestrial gamma-ray flashes (TGFs). Sprites were reported for the first time in 1990 by U.S. scientists as a luminous event appearing well above thunderclouds (altitude of about 40 to 90 km) at almost the same time as a cloud-to-ground discharge. Since then, extensive studies have been made in observations, using ground-based instruments, aircraft, balloons and spacecraft. Extensive theories have also been postulated. Fundamental ideas for the mechanism by which sprites are generated have been proposed. And these ideas have been at least partly confirmed by observations. However, a number of characteristics of sprites have not been fully explained yet. One of the most important but unidentified properties of sprites to establish their mechanism is the horizontal distribution of "columns" in a sprite event. Thick thundercloud always prevents us from seeing sprites from the ground. On the other hand, spacecraft have taken images of many sprites but only by viewing them from the side in the limb direction. SPRITE-SAT will try to take pictures of sprite columns from directly above.
The discovery of TGF by a gamma-ray astronomical satellite was reported in 1994. But because of the limited number of TGF that have been observed, no drastic progress was made until the next satellite observation started. In 2004, a large amount of data on TGF obtained by RHESSI was released to scientists around the world. After one month's examination, it was found that the parent lightning of TGF is much smaller than expected. That is a mystery because many scientists had expected that such high-energy phenomena must be related to large-scale lightning, the same as with sprites. Now the scientists are trying to solve this problem based on the TGF data obtained by astronomical satellites. However, unfortunately, such satellites do not carry lightning imagers nor do they have the function to add time stamps with the accuracy required for comparing data with ground-based radio-wave measurements. As discussed above, it is obvious that taking images of sprites from above and detecting TGF with visible images and radio waves from parent lightning are considered to be the next steps to take in the research field of TLE and TGF phenomena. Actually, the French space agency CNES plans to launch a 150-kg-class small satellite called TARANIS for these purposes as one of the Myriad series of satellites. The ESA team led by Denmark is considering an ISS project, called ASIM, to observe TLE, lightning and TGF in almost the same way.

Developing and manufacturing satellites

The SPRITE-SAT project was started based on collaboration between the graduate school of science, responsible for the science equipment, and the graduate school of engineering, responsible for the satellite bus, in Tohoku University in 2003. However, it was in the summer of 2008 that we were awarded the grant we needed to realize a full-spec satellite. And we began FM fabrication in September 2008. But our abundant experience in making instruments for use in space and the fruitful cooperation by these two groups and other collaborators including colleagues at JAXA/ISAS and University of Tokyo, and several manufacturing companies, made it possible to finish building the satellite in about a year. This quick performance enabled us to precede other projects by other countries by three years or more. There are significant limitations to SPRITE-SAT's specifications such as its telemetry rate and weight/power budgets coming from its size; still we expect that this ambitious satellite will break new ground in science and related work as a precursor of following space missions, providing them with very informative data. Our policy in designing and developing this satellite was to take full advantage of our experience gained in previous missions, including those related to small satellites such as WOES and REIMEI, which allows us to use time and money to tackle the more advanced science shown above and technological experiments. In other words, SPRITE-SAT is a suggestion of a new methodology in collaboration between the science and engineering communities, that is, the application of a small satellite to world-beating science at the shortest distance. Another difference with past science satellite missions is that the financial resources for satellite development and fabrication mostly came from the grant-in-aid for scientific research (No. 119002002, amounting to about 3.5 million US dollars and covering not only the satellite fabrication but also ground-based scientific measurements of TLE/TGF), and not from a space agency. However, the launching chance and the final and significant environmental testing support were provided by JAXA. SPRITE-SAT is a highly functional satellite, which carries five scientific instruments: two CMOS cameras with different color interference filters, a CCD camera with fish-eye lens, a gamma-ray detector, and a VLF radio wave receiver. The data obtained by these instruments are stored in its memory and transmitted to the ground only when a lightning flash (or sprite) or TGF event is detected. This event triggering is performed by FPGA, and the advanced judgment based on the sprite occurrence is processed by CPU logic. Development and evaluation of these algorithms have been carried out by the science team of SPRITE-SAT. And this team has also developed some individual pieces of equipment with some manufacturing companies.
SPRITE-SAT also plans to try advanced engineering experiments with Tohoku-AAC MEMS Unit, called TAMU, and a CCD star attitude sensor. TAMU is composed of four types of devices manufactured using leading-edge technologies: a 3-axis geomagnetic sensor, an MPU chip fabricated by 3D-System-in-Package technology, a 4-Mbit MRAM chip and an IMU chip for inertia measurement. Especially the geomagnetic sensor and IMU will significantly contribute to making the scientific data more valuable. One of the distinctive aspects of SPRITE-SAT is the international collaborative work that has gone into making it. The VLF receiver is manufactured and provided by the Department of Electrical Engineering, Stanford University in the US. The informative suggestions made by a group from the University of California, Santa Cruz were absolutely necessary both in planning this satellite project and in making the gamma-ray sensor. Also, TAMU was designed and fabricated in cooperation between Tohoku University and Angstrom Aerospace Corporation in Uppsala Innovation Centre (UIC), Uppsala University.

Development and preparation for operations

Now the fabrication of the satellite is almost done. The satellite body is undergoing environmental testing in the Tsukuba and Higashi-Osaka facilities, JAXA. We have already received a preliminary license for a radio station both for satellite and ground stations. The installation of a 2.4-m S-band dish antenna, which will be used for downloading data, at the rooftop of a 12-story building of Tohoku University has been completed. And construction of another 2.4-m dish in Kiruna, Sweden, which will increase the observation data amount by 5 to 10 times, is also in preparation to be in time for the scientific operation of the satellite.

Scope of future applications

In the course of developing SPRITE-SAT, we established several new and alterative types of devices, instruments and algorithms including: 1) a very simple deployable mast consisting of Be-Cu ribbon and aluminum multi-sleeves, which is dedicated to the gravity-gradient stabilization and radio antenna, 2) a high-sensitivity CCD camera without an image-intensifier, as the world's cheapest and maybe smallest scientific/star-sensing imager, and 3) a scientific data handling unit (SHU) using FPGA logics, used for autonomous event triggering. The VLF micro-receiver, the high radiation-tolerant compact CMOS camera and all other bus instruments are also applicable for future space missions. Actually, a replica of the CMOS camera, VLF receiver and SHU will be used as the main part of the lightning/TLEs observation unit on Kibou/ISS, named GLIMS, which will be launched in a couple of years. Also the experience gained in developing FPGA logics will contribute to the design of the TARANIS mission.
We are very positive about the possibility of promoting new work with colleagues from around the world who are interested in planning future collaborative projects, using our experience gained with SPRITE-SAT both in aspects of engineering and science.