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This study presents a command-centric architecture (C2A), which is a satellite software architecture exhibiting a flexible reconfiguration capability. C2A was developed through the software development period of the Hodoyoshi satellites. C2A is a software architecture that describes all the actions of a spacecraft by commands to ensure that C2A can change the function of a spacecraft without rewriting the memory. By applying C2A to the software of PROCYON, the world's first 50 kg-class interplanetary spacecraft, it has become possible to reduce the development period and reuse some parts of the previous satellites' software. PROCYON's software was successfully reconfigured several times after its launch using the C2A characteristics. Because of this software architecture, it has become possible to develop reliable onboard software (OBS) for satellites in the short term.

EQUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft) will be the world's smallest spacecraft to explore the Earth-Moon Lagrange point. It is being jointly developed by JAXA (Japan Aerospace Exploration Agency) and the University of Tokyo, and will be launched by NASA's Space Launch System Exploration Mission-1. The spacecraft will fly to a libration orbit around the Earth-Moon L2 point (EML2) and will demonstrate low-energy trajectory-control techniques within the Sun-Earth-Moon region for the first time by a nano-class spacecraft. EQUULEUS also carries three scientific observation missions: imaging of Earth's plasmasphere by extreme ultraviolet wavelength, lunar impact flash observation on the far side of the moon, and micrometeoroid flux measurements in the cis-lunar region. While all these missions have their own scientific objectives, they will also contribute to future human activity and/or infrastructure development in the cis-lunar region. Most parts of the spacecraft system use commercial off-the-shelf components, or are designed based on the experiences of various past space missions, with the exception of the newly developed water resistojet propulsion system. EQUULEUS uses X-band frequency for deep space telecommunication. Japanese deep space antennas (64-m and 34-m) will be nominally used for spacecraft operation, and support from the deep space network of JPL (Jet Propulsion Laboratory) is also being planned, especially for the initial phase of operation. The spacecraft will fly to EML2 in less than one year, and will remain there for scientific observations until shortly before the depletion of the onboard propellant, when the spacecraft will leave the orbit for space-debris compliance.

Earth remote sensing from geostationary orbit (GEO) realizes high time resolution that is essential for disaster monitoring; however, the spatial resolution is commonly worse than observation from low Earth orbit (LEO). In order to achieve high-resolution and high-frequency GEO remote sensing, we have proposed a “Formation Flying Synthetic Aperture Telescope (FFSAT)” with multiple micro-satellites. The FFSAT can improve the spatial resolution by using the technique of a synthetic aperture, and therefore the relative positions and attitudes between the optical units of each satellite must be controlled with an accuracy better than 1/10 of the observation wavelength. In order to verify feasibility of such highly accurate control, μ-class control experiments were conducted by using COTS components, and numerical models of the components were constructed. Results of the experiments were integrated into a software simulator, and the μ-class formation flying control of the entire FFSAT system was numerically evaluated. In this simulation, highly accurate control was achieved with dual-stage actuators, which consist of piezo actuators and thrusters. The simulation results show that the formation can be controlled in μ-class accuracy under some assumptions.

Recently, a variety of small spacecraft have been launched and used for interplanetary missions. Conventionally, reaction wheels (RWs) and thrusters are used for these attitude control systems in almost all interplanetary spacecraft. While these actuators are promising for attitude control, the lifetime of the spacecraft mission is limited by the extra fuel needed for the thrusters. Moreover, it is difficult to install thrusters in all small spacecraft due to low reliability and strict limitations on mass and power consumption. To obtain both fuel-free and available attitude control for small spacecraft, this study proposes an interplanetary magnetic attitude control system including attitude stabilization and angular momentum unloading based on an interplanetary magnetic field (IMF) Kalman filter. In the proposed method, an electromagnetic coil interacting with the IMF is used for an attitude control system. To achieve the proposed method, the faint magnetic field must be detected. However, the IMF is too weak to sense using only on-board magnetic sensors. To deal with the technical issue, this study proposes a magnetic attitude control system with an unscented Kalman filter using gyro measurements and generated magnetic moment by the electromagnetic coil to estimate the weak magnetic field. With the estimated magnetic field vector, the spacecraft can achieve fuel-free attitude stabilization and RW unloading under the constraints. This proposed system does not require fuel for attitude control. Furthermore, the simple structure and electronic circuits of the electromagnetic coil allow the spacecraft to achieve a simple and reliable attitude control system. Numerical simulations demonstrate the effectiveness of the proposed attitude stabilization and RW unloading methods.

Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy., Comment: 10 pages, 3 figures

Accepted: 2018-08-05, 資料番号: SA1180110000

Accepted: 2016-12-30, 資料番号: SA1160369000

Conventionally, spacecraft control their total angular momentum with magnetic torquers or thrusters. However, for a micro interplanetary spacecraft such as PROCYON (Funase et al. 2014), magnetic torquers cannot be used when it is far from the earth and thrusters also cannot be used frequently because of limited fuel storage. In that case, the attitude should be controlled using reaction wheels and the total angular momentum is fixed in the inertial frame. The problem is that when the total angular momentum is large, the wheels are required to store large angular momenta, which, in some attitude conditions, exceed the capacity of the reaction wheels. In order to perform three-axis attitude maneuver using sun sensors and gyroscopes as sensors and reaction wheels as actuators even with a large total angular momentum, the proposed control method applies pointing control of the angular momentum and that of the sun direction in sequence. The results show that the success rate of a three-axis maneuver from a certain attitude to another within one hour using the proposed method is 99.9% on average, even when the spacecraft has a large total angular momentum, while that of the conventional method is 48.3% on average.