Nobuo Sugimura

Ground test of attitude control system for micro satellite RISING-2

This paper summarizes the attitude control system and evaluation system of the 50kg class micro satellite RISING-2, which has developed by Hokkaido University and Tohoku University from 2010. In 2011, the flight models of each component were completed and now the software of RISING-2 is adjusted in order to put into an orbit in 2013. The missions of RISING-2 are observation of the earth surface, cumulonimbus clouds and planets with a telescope, measurement of the earth surface and ocean temperature distribution using a bolometer array and photography of sprite luminescence phenomenon using thunder observation camera and fish eye camera. To accomplish these scientific missions, the attitude control system needs to satisfy the requirements of observation equipments. This satellite attitude is controlled by the 3-axis reaction wheels in order to point to an arbitrary target direction. Especially, in the case of bolometer array observation, the target direction needs to be changed from deep space direction to the Earth-Center or the earth surface direction. We tried this observation sequence using the static closed loop test system which is constructed by the pre-flight model. Then, the test results illustrate that the attitude control system satisfy the requirements.

Satellite-to-ground optical communication system on Low Earth Orbit micro-satellite RISESAT

Within the scope of a Japanese FIRST (Funding Program for World-Leading Innovative R&D on Science and Technology) program led by Professor Nakasuka of University of Tokyo, Tohoku University is developing a 50kg-class international scientific microsatellite named RISESAT. In addition to various scientific instruments, RISESAT is also equipped with a laser communication terminal VSOTA, developed by Japanese National Institute of Information and Communications Technology (NICT). Tohoku University and NICT are now developing the engineering model of the satellite and undertaking its ground tests. VSOTA has two different wavelengths of laser outputs in 980 nm and 1540nm. The collimators for these are fixed with the satellite structure pointing toward the Earth direction. RISESAT aims to control the direction of the laser beams being precisely pointed toward the NICTs optical ground station with a pointing accuracy of better than 0.4 deg (3σ) during the fly-by. RISESAT can send actual scientific data obtained by payload instruments through this optical communication link. This will be the world first demonstration of microsatellite-to-ground optical downlink. This will bring innovation to misrosatellites system engineering, utilization, and communication network. This paper describes the detailed specification, system design strategy, and real-life implementation of laser communication system on the micro-satellite RISESAT.

Development of fast tracking algorithm using nearest neighbor star search approach

New tracking algorithm based on the nearest neighbor star search approach is proposed for fast star identification. In order to improve the performance of tracking, this algorithm is composed of two parts. One is a searching part of unknown stars in the field of view (FOV) using nearest neighbor star search approach. The feature of this method is that each star has connections with some adjacent stars. Unknown stars can be identified by tracing the nearest star from the previously recognized star with reference to the star catalog. Star neighborhood information is a list of neighbor stars in the order of closeness and included in the catalog. The other is a predicting part of position of stars on the current image frame. Star position can be predicted from satellite angular velocity and previous attitude information. In this technique, angular velocity is estimated by the last two captured images without gyroscope observation. Most of the stars in the FOV are tracked properly by matching star centroid position on the captured image with the predicted star position. Star trackers for micro-satellite developed by Tohoku University so far had only lost-in-space attitude determination algorithm, whose operation frequency was limited down to 1 Hz. It is expected that the above mentioned star identification method enables improvement both in reliability and operational frequency. This algorithm was evaluated in PC simulation. The results show that attitude determination can be carried out over twenty times faster compared to the conventional method. Hence, it is illustrated that the efficiency of star identification is improved by these approaches.

Attitude determination and control system for nadir pointing using magnetorquer and magnetometer

A low-cost attitude determination and control system (ADCS) is proposed for nadir-pointing control. This system comprises three-axis magnetorquers and magnetometers. The aim for developing this system is to establish a nadir-pointing control method using only low-cost spacecraft components for active control. Recently, low-cost and reliable development has become a required for spacecraft development. Generally, star trackers, reaction wheels, and thrusters are used for accurate attitude determination and spacecraft control. They have high reliability but their cost becomes a barrier for low-cost spacecraft realization. Contrarily, ADCS, having only magnetorquers and magnetometers, can be low-cost due to their simple composition. Although the magnetic torque generated by magnetorquers is low, nadir-pointing control with magnetorquers can be performed using optimal control algorithm. A Kalman filter for a gyroless spacecraft is applied for attitude determination with a magnetometer. These systems are combined and can realize the pointing accuracy against nadir direction as well as gravity gradient stabilization. Theoretically, spacecraft attitude control with magnetic torque is a well-known singularity problem. Herein, PD control based on an attitude control algorithm, which includes a Singularity Robust (SR) inverse matrix, is proposed as a solution. A PD controller calculates the control torque against attitude error and then an SR inverse matrix is used for computing output magnetic moment. The role of SR inverse matrix is to avoid singularity of the pseudoinverse matrix. Optimal magnetic moment is given by measured magnetic-field value and reference control torque. This study considered two different types of magnetorquers: one with fixed output current and another with variable current. The maximum output is defined with an assumption that this system is used for microsatellites. A fixed output magnetorquer is controlled by a method that is similar to pulse-width modulation to generate desired torque. In the variable model, magnetorquer output is limited by maximum magnetic moment. This paper describes results of nadir-pointing control using both models. Meanwhile, magnetometer-only attitude estimation theory is used for attitude determination. This method is based on extended Kalman-filter estimation. Attitude quaternion and angular velocity are continuously estimated. Although magnetometer-only attitude estimation requires a long conversion time, this method is advantageous for estimating attitude without star trackers, regardless of day and night. Simulations are conducted on various initial attitude and orbital conditions to denote the effectiveness of this method for various Earth-observation satellites such as the Sun-synchronous and International Space Station orbits. Simulation results illustrate that attitude control error can be below 5 deg with both fixed and variable output magnetorquers. It is considered that this control system could be used for active control alternative to gravity gradient stability and backup-control system in high precision attitude control system using star trackers, reaction wheels, and thrusters.

Model-Based Environment for Verification and Integration of Micro-Satellites

Abstract The Space Robotics Laboratory of Tohoku University has been developing multiples of micro-satellites for years and has gathered experiences in their development, verification, integration, and operation. SRL has recently started development of model-based simulation, verification and integration environment to realize rapid and cost-effective development of reliable micro-satellites. The conceptual design and its functionality have been verified through the real-life micro-satellite project RISESAT, which is a 50kg class international scientific micro-satellite. The developed environment can be utilized in different configurations depending on requirements in each satellite development phase. This environment is designed to be modular and very flexible and can be utilized for other micro-satellites and possibly even much smaller space system projects in the future.

Lessons learned on structural design of 50kg micro-satellites based on three real-life micro-satellite projects

The Space Robotics Laboratory (SRL) of Tohoku University has developed three 50kg Micro-satellites. The first satellite “SPRITE-SAT” has been successfully launched into Earth orbit, and also been operated. The flight model of the second satellite “RISING-2” has been assembled and its software development is now finalized, being ready for the launch planned in next year. The third satellite “RISESAT” project is during the EM (Engineering Model) development phase at the time of writing. The launch is planned in the later half of 2013. The structural design of all these satellites is based on central pillar configuration, while continuous improvements have been made through the projects. The paper will discuss these aspects of structural design and evaluation summarizing the results of numerical analyses and mechanical tests conducted by the SRL during the past 5 years of micro-satellite development activities.

Radiation effect mitigation methods for electronic systems

Effects of space radiation on space systems have been considered as the main challenge in designing sustainable space systems, and investigations have been done about the mitigation methods against various types of radiation effects. On March 11, 2011, a massive earthquake and tsunami hit the eastern Japan, particularly Tohoku area. Since then, the Fukushima Daiichi Nuclear Power Station has been facing a crisis. To respond to this situation, Tohoku University conducted studies to redesign mobile robots for disaster response missions, and has realized the fact that there was no practical information available about the radiation effects on electronic devices to be installed on those mobile robots and the mitigation methods against them, which revealed the importance of establishing a knowledge-base about the way of designing radiation-tolerant or radiation-hard electronics systems even for terrestrial applications. Space Robotics Laboratory of Tohoku University has been conducting researches on space systems engineering and has gathered knowledge about the radiation effects on space systems electronics devices through its real-life microsatellite development and operation activities. Based on this background, this paper summarizes the general radiation effects on electronics devices and the cost-effective way of their mitigation methods, together with the application example of microsatellite systems developed by the Space Robotics Laboratory. This paper aims to contribute to establish such kind of knowledge-base together with a variety of aerospace and terrestrial engineering communities.