Toshinori Kuwahara

Attitude control system of micro satellite RISING-2

This paper summarizes the attitude control system of the 50-kg micro satellite RISING-2, which is now under development by the Tohoku University and Hokkaido University. The main mission of the RISING-2 is Earth surface observations with 5-m resolution using a Cassegrain telescope with 10-cm diameter and 1-m focal length. Accurate attitude control capability with less than 0.1 deg direction errors and less than 0.02 deg/s angular velocity errors is required to realize this observation. In addition, because of the larger power consumption of the science units than expected, actuators must be operated with sufficiently low power. The attitude control system realizes 3-axis stabilization for the observation by means of star sensors, gyro sensors, sun attitude sensors and reaction wheels. In this paper the attitude control law of the RISING-2 is analyzed to keep the power of reaction wheels under the limit. This simulation is based on component specifications and also includes noise data of the components which are under development. The simulation results show that the pointing error is less than 0.1 deg in most time with the RISING-2 attitude control system.

Satellite system integration based on Space Plug and Play Avionics

Professor Shinichi Nakasuka of University of Tokyo is now leading a small satellite development activity within the scope of a Japanese FIRST (Funding Program for World-Leading Innovative R&D on Science and Technology) program. In this program several 50 kg class microsatellites are going to be developed and launched by the end of Japanese fiscal year of 2013, including one scientific microsatellite under international cooperation. Tohoku University has been assigned as the project leader of this international scientific microsatellite named as RISESAT (Rapid International Scientific Experiment Satellite), and is developing the bus system as well as organizing scientific payload instruments from all over the world. This satellite shall demonstrate the performance of its bus system which is supposed to be offered as a common bus system for international scientific missions in the future. In order to accommodate a wide variety of payload instruments and to provide them flexible and comfortable integration environment, Space Plug and Play Avionics (SPA) standard is applied for the electrical interface between the payload instruments and the satellite bus system. The development of RISESAT flight model will be completed by the March 2013. This paper summarizes the system design of the satellite based on the SPA technology.

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.

Static closed loop test system for attitude control system of micro satellite RISING-2

50-kg class micro satellite RISING-2 is now under development by Tohoku University and Hokkaido University. The development is at Flight Model phase. The main mission of the RISING-2 is Earth surface observations with 5-m resolution using a Cassegrain telescope with 10-cm diameter and 1-m focal length. Accurate attitude control capability with less than 0.1 deg direction errors and less than 0.02 deg/s angular velocity errors is required to realize this observation. The attitude control system realizes 3-axis stabilization for the observation by means of star sensors, gyro sensors, sun attitude sensors and reaction wheels. In this paper the static closed loop test system for the attitude control system of the RISING-2 is described. This test system is the simulation including the hardware of the attitude control system of the RISING-2. The results of the tests show that the pointing error is very larger than the results of software simulation.

HPT: A High Spatial Resolution Multispectral Sensor for Microsatellite Remote Sensing

Although nano/microsatellites have great potential as remote sensing platforms, the spatial and spectral resolutions of an optical payload instrument are limited. In this study, a high spatial resolution multispectral sensor, the High-Precision Telescope (HPT), was developed for the RISING-2 microsatellite. The HPT has four image sensors: three in the visible region of the spectrum used for the composition of true color images, and a fourth in the near-infrared region, which employs liquid crystal tunable filter (LCTF) technology for wavelength scanning. Band-to-band image registration methods have also been developed for the HPT and implemented in the image processing procedure. The processed images were compared with other satellite images, and proven to be useful in various remote sensing applications. Thus, LCTF technology can be considered an innovative tool that is suitable for future multi/hyperspectral remote sensing by nano/microsatellites.

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.

The balloon-borne telescope system for optical observation of planets

Our team is carrying out the project of Planet observation with high precision using balloon-borne telescope. The first model, BBT-1 was equipped with a three stage pointing system and an optical system to observe the detailed structure of the atmospheric motion of Venus. The first flight test was conducted in 2009, and the performance of the system was verified. However, because of a trouble of onboard computer, the flight operation could not been finished as planned. The second model “BBT-2” is now being developed and the next flight test is planned in 2011. The BBT-2 has a bus system including FPGAs and CPU, and it is expected to be more stable than the BBT-1.

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.

Development of small optical transmitter for microsatellites

In this paper the small optical transmitter for microsatellite which is now under development is described. In recent years, the amount of downlink data is increasing and the faster speed communication system has been required. Therefore the optical communication which can realize the high data rate transmission by far than conventional radio waves has been attracting attention. However the high precision pointing is required in optical communication. Although conventional optical communicators of large satellites have been achieving high pointing accuracy by mechanical gimbal and movable pointing mirror, this type optical communicator would occupy a high proportion of mass and power resources of microsatellites. Then the optical transmitter without mechanical gimbal which points to the target by the Attitude Control System (ACS) of the satellite was proposed. Optical communicator of this method can be very compact and can be mounted on microsatellites. However initial discovery by ground station communicator is difficult in this method. For this reason the adjustable beam spread angle type was adopted in desired optical transmitter. The beam divergence from this transmitter can be expanded and initial discovery will be easy. The status of development is test model. The behavior of received light was analyzed in simulation and receive gain of more than - 40 dBm.