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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.

Small satellites have been used for remote sensing with a spatial resolution of several meters from LEO in recent years. However, it is difficult to increase temporal resolution for LEO remote sensing due to the short orbital period. Therefore, GEO remote sensing which enables observation of high temporal resolution from GEO or its nearby orbit is getting important. In order to obtain enough spatial resolution in GEO remote sensing, an optical system having a diameter of several meters is required because of the diffraction limit. It takes huge cost to realize such a large diameter primary mirror due to manufacturability and required accuracy. To address this problem, we propose a synthetic aperture telescope by small satellites formation flying. The synthetic aperture telescope is composed of several mirror satellites constituting a primary mirror of the telescope and an imaging satellite having a focal plane assembly. By optically synthesizing the light collected by each mirror satellite with the imaging satellite, a virtual large aperture telescope is constructed. In this paper, we assume the observation at near infrared to short wavelength infrared and show the specifications of the system. The apperture telescope and the image processing method used to extract high spatial frequency information from the observed images are also described.

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

Although astronomers have confirmed the existence of 4,000 exoplanets to date, it is still difficult to directly compare exoplanets with the planets in our solar system because most of the known transiting exoplanets have an orbital period shorter than 1 year. Recent analyses of the 4-year data from the Kepler spacecraft revealed dozens of long-period transiting exoplanets and showed that their abundance is of order unity around Sun-like stars. However, the stars targeted by Kepler are too faint to conduct follow-up observations. The on-going all-sky survey mission TESS, with four 10.5 cm cameras with a field of view of 24 deg x 24 deg, is finding nearby transiting planets; however, the nominal observation period (1 month{1 year) is too short to find long-period planets with au-scale orbits. Herein, we propose using the LOng-period Transiting exoplanet sUrvey Satellite (LOTUS) mission, which employs a 7.5 cm wide-field (33 deg x 33 deg) camera placed on a nanosatellite, to continuously monitor the same sky region and find long-period planets transiting nearby bright stars. We present a conceptual design for the optics and bus system of LOTUS. Our optical system has a uniform point spread function over the entire field of view and a wide wavelength range (0.5{1.0 um). The bus system is designed to ensure that the pointing precision is sufficient to achieve the sub-percent photometry required for the detection of transiting exoplanets.

Feasibility of radio occultation measurement (RO) among small satellites is investigated by observing system simulation experiment (OSSE) of the Venus data assimilation system in two test cases introducing real orbits. One observes cold collar in polar region. The other observes thermal tides in equatorial region. The reproducibility of the cold collar or thermal tides is investigated by several types of orbits. Results suggests that the cold collar is successfully reproduced with two or three satellites. In addition, zonal wind can be improved by temperature assimilation of the thermal tides with two satellites. Therefore, RO among small satellites would be promising to reproduce global phenomena in the Venus atmosphere. 1. Introduction Venus atmosphere has a global, thick cloud cover at about 48–70 km altitude. Therefore, it is very difficult to observe under the cloud by the images. One of the most useful methods to obtain vertical temperature profiles is RO. For Earth, daily observations have been performed by GNSS (Global Navigation Satellite System). However, Venus has been observed only between one satellite, e.g., Venus Express or Akatsuki [1], and Earth. For multiple satellites, by transmitting and receiving radio waves between them, more observations, multiple points and better time resolution, could be obtained. In the previous study, we have shown that RO among small satellites can reproduce the cold collar with fixed observational points [2]. In order to check feasibility, however, we have to consider realistic orbits of flying multiple satellites around Venus. So far, we have developed Venus atmospheric general circulation model (AGCM) named AFES-Venus (AGCM for the Earth Simulator for Venus) [3]. Recently, using the local ensemble transform Kalman filter (LETKF) [4], we have succeeded in developing the AFES LETKF data assimilation system for Venus (ALEDAS-V) [5]. In this study, we performed OSSE assuming RO among small satellites in two test cases with real orbits targeting the cold collar and thermal tides. 2. Experimental setting AFES-Venus solves dry 3-D Primitive equation on sphere. The physical parameters are based on Venus. The latitude-longitude grids (128 times 64) with 60 vertical layers are used. The simulation starts from idealized super rotation and spin up for 4 Earth years. Reproduced zonal wind is in good agreement with observations [6]. ALEDAS-V uses the LETKF [4] to produce an improved estimate (called analysis) by combining observations and short time ensemble forecasts of AFES-Venus. The number of ensemble members is 31. Assimilation cycle is 6 hourly interval. Observational errors and inflation are set to 3.0 K and 10%, respectively. Details are described in [2, 5]. Two experiments are conducted; one is evaluated by the cold collar (experiment 1), and the other is evaluated by the thermal tides (experiment 2). Idealized temperature observations at 40-90 km are prepared by the GCMs assuming RO among small satellites. Experiment 1 uses data from IPSL VGCM because the cold collar is more realistically reproduced [7]. Note that temperature bias between IPSL VGCM and AFES-Venus is corrected. Experiment 2 uses data from AFES-Venus itself but experimental settings are different. Observation points are based on real orbits; Experiment 1 covers polar region and experiment 2 does equatorial region. They are assimilated to AFES-Venus by ALEDAS-V and results are compared with frf (free run forecast). The reproducibility of the cold collar or thermal tides is investigated by several orbits. 3. Results Figure 1 shows the temperature distributions at 30-90°N at ~66 km in experiment 1. Four situations are considered: (a) frf and (b,c,d) has different orbits. p1, p2 and p3 indicate each pair of satellites. Cold collar is most clearly reproduced for (d) in which all 3 satellites proceed RO. Figure 1: Temperature distributions at ~66 km (at 30-90°N seen from the north pole): (a) frf, (b) 1 pair of satellites (p1), (c) 2 pairs (p1 and p3), and (d) 3 pairs (p1, p2 and p3). Figure 2 shows latitude-height cross sections of zonal wind (contour) and difference of zonal wind between frf and the Qt8 cases (color) in which the solar heating is reduced to 80% compared with that used in frf. The 4 types of orbits are considered. The zonal wind above the cloud top (~80 km) is significantly decelerated in (d). It is suggested that temperature observations of the thermal tides can improve zonal wind. Figure 2: Latitude-height cross sections of zonal wind (contour) and difference of zonal wind between frf and the Qt8 cases (color). 4 types of orbits are shown: a2 (a), b1 (b), b2 (c), and c2 (d). c2 has most observations, following b2, b1 and a2. Summary and Conclusions In the present study, we have checked feasibility of the Venus future mission of RO among small satellites by introducing real orbits. In experiment 1, the cold collar is reproduced with 2 or 3 satellites observing polar region. In experiment 2, zonal wind is improved by the temperature observations of the thermal tides in equatorial region. Results suggest that RO with two or three small satellites would be promising to reproduce global phenomena. Furthermore, it is also suggested that this kind of observing system simulation experiment would be very useful to design future missions. Acknowledgements This study and supported by the Japan Science and Technology Agency (JST). References [1] Ando, H. et al.: Thermal structure of Venus atmosphere from sub-cloud region to the mesosphere observed by radio occultation, Scientific Rep., 2020. [2] Sugimoto, N. et al.: Observing system simulation experiment for radio occultation measurements of the Venus atmosphere among small satellites, Journal of JSCE A2: Applied Mechanics, 2019. [3] Sugimoto, N. et al.: Baroclinic instability in the Venus atmosphere simulated by GCM, J. Geophys. Res., Planets, 2014. [4] Miyoshi, T. and S. Yamane: Local ensemble transform Kalman filtering with an AGCM at a T159/L48 resolution. Mon. Wea. Rev. 135, 2007. [5] Sugimoto, N. et al.: Development of an ensemble Kalman filter data assimilation system for the Venusian atmosphere, Scientific Rep., 2017. [6] Sugimoto, N. et al.: Waves in a Venus general circulation model, Geophys. Res. Lett., 2014. [7] Garate-Lopez, I and S. Lebonnois: Latitudinal variation of clouds’ structure responsible for Venus’ cold collar, Icarus, 2018.

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.

Recently, small satellites have been employed in various satellite missions such as astronomical observation and remote sensing. During these missions, the attitudes of small satellites should be stabilized to a higher accuracy to obtain accurate science data and images. To achieve precise attitude stabilization, these small satellites should estimate their attitude rate under the strict constraints of mass, space, and cost. This research presents a new method for small satellites to precisely estimate angular rate using star blurred images by employing a mission telescope to achieve precise attitude stabilization. In this method, the angular velocity is estimated by assessing the quality of a star image, based on how blurred it appears to be. Because the proposed method utilizes existing mission devices, a satellite does not require additional precise rate sensors, which makes it easier to achieve precise stabilization given the strict constraints possessed by small satellites. The research studied the relationship between estimation accuracy and parameters used to achieve an attitude rate estimation, which has a precision greater than 1 × 10−6 rad/s. The method can be applied to all attitude sensors, which use optics systems such as sun sensors and star trackers (STTs). Finally, the method is applied to the nano astrometry satellite Nano-JASMINE, and we investigate the problems that are expected to arise with real small satellites by performing numerical simulations.