高尾 勇輝

This paper describes autonomous optical navigation to estimate relative positions of a spacecraft with a target body for distant small body exploration. The small body explorations have received attention around the world in recent years. In these missions, high-accuracy optical navigation is important for the landing or rendezvous. Therefore, Terrain Relative Navigation (TRN) to estimate deviations by comparing nominal terrain information with actual terrain information is often used. Enough observation of a target body to make a shape model is possible after arrival, especially in the small body explorations. Accordingly, the shape model of the target body is utilized for the generation of the nominal terrain information. In the case of near-Earth asteroid exploration spacecraft Hayabusa2, ground-based navigation using communication with the Earth is used. On the other hand, in the case of explorations to small bodies farther than the asteroid main belt, communication delay with the Earth is unacceptably large for feedback guidance. This situation becomes worse for larger bodies because the time constant of the dynamics becomes faster. Therefore, the importance of high-accuracy real-time autonomous navigation is highlighted for explorations to the distant small bodies. In this study, an autonomous optical navigation method based on TRN is proposed. Firstly, the reference image from a nominal position is generated by rendering. Three-dimensional positions on the shape model relative to each pixel of the reference image are also memorized in addition to luminance. Secondly, multiple small images extracted from the reference image are compared with a captured image by template matching. Therefore, the relationships between the three-dimensional positions on the shape model and the multiple small images in the captured image can be determined. Finally, the actual position of the spacecraft can be determined by estimation of perspective projection transformation, that projects a three-dimensional shape onto a two-dimensional plane. In this study, Vector Code Correlation algorithm for correlation calculation of template matching is used. Therefore, the correlation of images can be calculated via XOR operations suitable for FPGA. For these reasons, three-dimensional positions of the spacecraft can be estimated in real-time by utilizing the shape model. The estimation accuracy and computational time are evaluated by comparing the proposed method with other methods. As a result, the high estimation accuracy of several image resolution in real-time is achieved. The authors believe that the proposed method will be a key technology for distant small body explorations.

© 2020 by the International Astronautical Federation (IAF). All rights reserved. A solar sail is not only propelled by solar radiation pressure (SRP), but also generates torque using SRP. When a sail membrane's shape is not flat, unexpected torque due to SRP is generated. It is necessary to control the sail membrane's shape and the resulting SRP torque to operate a solar sail for long periods and to reduce fuel consumption. Membrane shape control is a common issue not only in solar sails but also in space membrane structures. Therefore, we propose a method to actively control the in-plane and out-of-plane torque by deformation of the membrane shape using Shape Memory Alloy wires (SMA wires). An SMA wire is a type of a soft actuator that contracts when heated. In the proposed method, the SMA wires are attached at several locations on the sail membrane, allowing the entire membrane to deform in out-of-plane at the locations where the SMA wires contract. Furthermore, it is possible to control the membrane shape depending on the situation by selectively contracting subsets of SMA wires. In this paper, a finite element analysis and an experiment were conducted under the same condition to validate the numerical modeling for the SMA wire actuation. As a result, the validity of the membrane shape analysis could be shown since the tendency of the membrane shape deformation was the same in the experiment and the analysis. Next, the relationship between the SMA wire positions and the resulting SRP torque is clarified via another numerical analysis. Results show that in-plane and out-of-plane torques can be produced, respectively by contracting the SMA wires in line-symmetric and point-symmetric manners.

© 2020 by the International Astronautical Federation (IAF). All rights reserved. Two novel approaches to maximise structural flatness and minimise perturbation solar radiation torque on 100 m class interplanetary solar sails are presented. First, allowing the length of central tethers to be actuated to maximise sail flatness in orbit. Second, replacing the central tether with rigid booms with controllable length and extension angle, to move the centre of solar radiation pressure with respect to the centre of mass. The performance of the above two methods are analysed by using analytical and numerical techniques. While the large sail size requires a high degree of sail flatness (<1 degree), the results show that the two methods can satisfy this requirement, paving the way towards 100 m class deployable space structures, and an entirely new class of space missions.

Copyright © 2020 by the International Astronautical Federation (IAF). All rights reserved. The removal of space debris from an orbit is one of the definitive solutions to the increasing Earth-orbiting satellites issue. To achieve active removal of space debris, accurate estimation of the state of motion of the target is crucial. In addition to that, a safe approach is preferable. Many strategies have been proposed for estimation of motion of a non-cooperative target, but there are still problems with regards to conducting precise, robust and cost-effective estimation in real-time. This study proposes a new strategy to solve this issue - a real-time and practical method based on optical navigation around small bodies. A computer simulation results show the efficacy of the proposed method and verifies that it can be a viable option for use in the capture of non-cooperative space debris targets.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. This paper describes an autonomous optical navigation to estimate the relative position of a spacecraft with a target body for deep space explorations. The asteroid exploration spacecraft Hayabusa2 touched down on the asteroid Ryugu with pin-point accuracy in February 2019. In the case of Hayabusa2, the asteroid-relative position is estimated by ground operators. On the other hand, in the case of explorations to small bodies farther than Main-belt, the communication delay is unacceptably large for the asteroid-relative feedback guidance. This situation becomes worse for larger asteroids because the time constant of the dynamics becomes faster. Therefore, importance of high-seed on-board optical navigation is highlighted for explorations to far distant bodies. To accomplish the high-precision and high-speed on-board optical navigation that can be applied to various bodies, the Vector Code Correlation (VCC) algorithm suitable for field-programmable gate array (FPGA) is focused on. This method is a type of correlation calculation for the template matching that finds the most similar part of 2 images by comparing them. In the case of the VCC algorithm, by discretization of luminance gradient into 3 patterns, data size of each pixel can be reduced from 8 to 4 bit without losing feature amount. Accordingly, their correlation can be calculated at high speed via XOR operations on FPGA. In this study, the VCC-based position estimation method was developed and implemented on an FPGA board. The proposed method mainly consists of 2 steps: generation of a reference image from nominal position by rendering; estimation of the deviation between a captured image and a reference image by the VCC algorithm. In addition, the VCC algorithm on multiple planes in images was implemented in order to improve the estimation accuracy at low altitudes, where the outlines of the target bodies cannot be seen in the images. The estimation accuracy and computational time werw evaluated by comparing the proposed method with other methods. As a result, the position estimation accuracy of approximately image resolution size (1pixel size) in real space is achieved. Finally, by demonstration of the proposed method to the flight data of Hayabusa2 in landing phase, it was found that it can be applied to a real mission environment from the aspect of estimation accuracy and computational time.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. The membrane dynamics of spinning solar sails have a special relevance when considering attitude control of the spacecraft. So as to model the system accurately, the bending stiffness of the membrane has been included in the numerical approach, as it is believed to have strong effects on the behavior of the sail. First, this study shows that the influence of the bending moment on the attitude of the sail during its spin axis reorientation should not be neglected. Given the difficulty of measuring the actual bending stiffness of the membrane, finding a control system capable to perform the same regardless of its value is necessary. Therefore, this paper presents a new control system and its corresponding logic to lower the influence of the bending parameter on this attitude maneuver performance. Finally, a frequency analysis on the vibrations arising in the membrane when considering different bending stiffness values is done. This last analysis shows the shift in the natural frequencies obtained, remarking the importance of the bending stiffness when considering the dynamics of a mast-free sail.

Copyright © 2019 by the International Astronautical Federation (IAF). All rights reserved. As for missions that explore small bodies such as asteroids or comets, landmark-based optical navigation is widely used in such operations as descent or landing. The Japanese asteroid explorer Hayabusa2 successfully performed two touchdowns on the asteroid Ryugu in 2019, using one of the landmark-based optical navigation. Hayabusa2 realized the guidance, navigation, and control with accuracy of less than 5 m at the touchdowns. On the other hand, this navigation method strongly depends on the terrain surface of the target celestial bodies, and also requires laborious work to detect sufficient number of landmarks in images. This paper presents an optical navigation method that is independent of landmarks as an advanced study for future missions. The movement of a global surface, rather than a local point, is focused to enable visual tracking without relying on landmarks. The result of the visual tracking yields the pose of the probe via perspective projection equation. The function of the developed method is simulated using the flight data of Hayabusa2.

We present an onboard navigation system for approaching and landing on an asteroid in deep space. The focus of this research is to apply the heuristic optical navigation method which is used in Hayabusa2 called GCP-NAV into an onboard processable algorithm. This technique is used to enable a spacecraft to touchdown correctly on a target point of a planetary surface during deep space mission operations. The focus of our approach is to estimate the position of the spacecraft using an asteroid shape model and imagery data obtained in real-time during spacecraft orbiting, descent, or landing. This novel approach will make possible to plan missions on asteroids farther than 3 AU. As a result, the estimation result that fits the error of up to 1 pixel on the image coordinates was obtained. Furthermore, the calculation time was decreased under 1/10 compared to calculation time on CPU.