On-orbit Servicing Missions Research Articles

Model Predictive Control based Coordinated Control for Free-Flying Space Manipulator Systems

Space Manipulator Systems (SMS) used in On-Orbit Servicing (OOS) missions are over-actuated, and safety critical systems that perform complex tasks in space. We propose a coordinated control for a free-flying SMS, which is based on a cascaded output tracking nonlinear model predictive control, to simultaneously control the spacecraft-base and manipulator of the SMS. The output tracks a given end-effector pose trajectory, and by using an artificial reference the controller stays feasible even for an unreachable trajectory. Moreover, the systems over-actuation is optimally resolved, such that the base of the SMS can be utilized to generate an unrestricted workspace for robotic operations. Throughout the entire mission, critical constraints related to system and safety, including collision avoidance, field of view of sensor devices, joint limitations, and actuator saturations, are integrated as hard constraints. Simulations of the most challenging mission phases, such as the approach and detumbling of a target spacecraft, as well as an escape maneuver verify the performance, reliability, and flexibility of our coordinated control concept.

Ground Experiment of Safe Proximity Control for Complex-Shaped Spacecraft

The safe proximity to spacecraft is a prerequisite for most on-orbit service missions. However, performing proximity operations on orbit is dangerous and high-cost. Therefore, pre-verification of the proximity control algorithm through ground experiments can determine the reliability and promote the success of actual on-orbit missions. This paper constructs an air floating experiment system and a guidance algorithm for the safe proximity to a complex-shaped tumbling spacecraft. The proposed control algorithm consists of two parts: one part is an improved Gaussian mixture model, with which a novel artificial potential function is designed to accurately describe the complex shape of the target and provide 3D collision avoidance constraints; the other part is to use the fixed time control method to ensure that the service spacecraft can reach the desired position within a fixed time. The numerical simulation and experiments are implemented to verify the effectiveness of the proposed algorithm.

An optimization framework with improved auction-based initialization for highly constrained on-orbit servicing mission planning

The on-orbit servicing (OOS) mission planning problem for multiple geosynchronous earth orbit (GSO) satellites is a critical issue due to its huge economic value. However, given the complexity of the space scenario and the subjective nature of the formulation process, the problem is highly constrained in actual use, and the performance of conventional optimization algorithms decreases due to fuel or mission deadline constraints. With the goal of enhancing solution quality and speeding up the solution process, this paper proposes a novel optimization framework. The problem is decomposed into three sub-problems and then solved using the proposed three-level algorithm to lessen its complexity. In addition, a new initialization method is embedded to improve global search capabilities. Notably, considering the high computation efficiency of the auction algorithm (AA), an improved version is designed to generate primary solutions, which introduces the grouping and reallocation mechanism. Then, several variations of the primary solutions are used as the initial variables in the optimization. Numerical simulations demonstrate that the proposed methodology has higher computational efficiency and convergence performance than conventional strategies.

A Tangent Release Manipulation Controlled by a Dual-Arm Space Robot

As people further develop space with advanced technology, space robots have played a significant role in on-orbit servicing missions. Space robots can carry out more risky and complicated missions with less cost than astronauts. Dual-arm space robots can perform complex on-orbit space missions more effectively than single-arm space robots. Since the coupled dynamics between the free-floating base and the arms exist in space robots, accurate coordinate control of the base and the arms is essential. Spacecraft release missions have been proposed to berth/deberth a spacecraft to a space station. Based on the existing release missions, a tangent release strategy is introduced in this paper, which can release a space object in the tangent direction of the final link of a space manipulator. This strategy can control a dual-arm space robot to deploy cargo/spacecraft in variable directions in 3D space without thrusters and the associated fuel consumption. For instance, this tangent release operation can transport cargo or modules of large-scale spacecraft needing on-orbit assembly. Considering model uncertainties, robust controllers again model uncertainties that are used to control the dual-arm space robot with high accuracy. Hence, a robust sliding mode controller (SMC) is utilized to accurately control the space robot to carry out the proposed tangent release strategy. For comparison, we select a conventional computed torque control (CTC) implemented by a PD-type controller. In the simulations, the SMC performs better in tracking accuracy and robustness against the model uncertainties than the PD controller. Numerical simulations indicate the feasibility and effectiveness of the tangent release manipulation of a space object by a dual-arm space robot.

ORBITAL STRUCTURE OPTIMIZATION TECHNIQUE OF THE LOW-ORBIT COMPLEX OF ON-ORBIT SERVICE

Most of the currently planned on-orbit servicing (OOS) missions involve the use of disposable OOS spacecraft. The use of disposable OOS spacecraft may be profitable in the near future. But it is not a reliable solution for OOS in the long term. As an alternative, a more useful concept is the use of reusable OOS complexes, which allow responding to scheduled and random requests from OOS clients. This concept can ensure the timeliness and efficiency of OOS implementation during planned services and random requests of OOS clients. However, despite the potential advantage of a reusable OOS, the design of its orbital structure and operational maintenance is much more complicated in comparison with the traditional concept of the organization of OOS. This is because when planning the response of reusable OOS complexes to requests, it is necessary to distribute OOS client service operations between space vehicles of the reusable OOS complex. Now the space industry is switching its attention to the area of low Earth orbits. This causes an increase in deployed and planned low-orbit satellite groups, the number of satellites in them, the difference in structural schemes of satellite groups, and the significant influence of the environment on orbital parameters. As you know, the orbital parameters of low orbits of space vehicles can differ significantly, and the difference between them can reach tens or even hundreds of degrees in the longitude of the ascending node. This leads to unacceptably high energy costs for modern OOS spacecraft for active rotation of the planes of their original orbits to the planes of the destination orbits. In some works, the possibility of reducing these energy costs due to the use of the difference in the speed of the nodal precession of the parking and destination orbits of the OOS spacecraft due to the non-centrality of the Earth’s gravitational field is considered. However, due to the long wait of the OOS spacecraft in the parking orbit, the flight time with the wait between the parking and destination orbits increases significantly. Its reduction can be achieved by increasing the number and rational selection of the semi-major axis and inclination of the parking orbits of the OOS spacecraft. The purpose of the article is to develop a technique for the optimal synthesis of the orbital structure and optimal operational planning of the low-orbit OOS complex in near-Earth orbits with a small eccentricity. Methods for solving the problem are the averaging method, the branch-and-bound method, and the multiobjective optimization method. The novelty of the obtained results lies in the development of a technique for optimal synthesis of the orbital structure and optimal operational planning of the low-orbit space OOS complex in near-Earth orbits with low eccentricity. The developed technique can be used in the previous planning and design of space OOS complexes in low near-Earth orbits with a small eccentricity.

Application of the Relative Orbit in an On-Orbit Service Mission

To achieve an on-orbit service mission, the mission spacecraft must approach the target spacecraft first, which is based on the spacecraft’s relative motion. To enhance the safety and reliability of on-orbit service missions, the relative hovering orbit was proposed and needed to be studied further. A high-precision design method for hovering orbit is presented based on the relative dynamics model of spacecraft in this paper. Firstly, based on the stability analysis of the spacecraft relative dynamics model, a method to determine the initial value of periodic relative motion orbit is explored, and an example is given to verify the validity of the method. Then, through theoretical analysis, the formulae of control acceleration required during the hovering flying mission were put forward for both without considering perturbation and with considering J2 perturbation, and numerical simulations for hovering orbit were made to verify the feasibility of the approaches proposed. Simulation results show that the control acceleration curves are smooth, which indicates that the hovering flying mission is easier to achieve, and the control method based on sliding mode control theory is adopted for hovering control. The relative hovering method proposed can provide references in space on-orbit service missions for practical engineers.

Shadow removal of spacecraft images with multi-illumination angles image fusion

Spacecraft components recognition is essential for space on-orbit service missions such as space docking, on-orbit maintenance, and other applications. Compared with the expensive recognition methods based on the Light Detection and Ranging (LIDAR) system, the optical camera provides an economical alternative. However, owing to the adverse illumination in space, spacecraft images often suffer from shadow and poor visibility, which seriously damages the recognition accuracy. To overcome the image quality limitation, we proposed a novel concept “Multi-Illumination Angles Image Fusion” (MIAF), which aims to fuse the information of single spacecraft in different illumination angles and reconstruct the shadow-free spacecraft image. In this study, a new dataset containing spacecraft images under multiple illumination angles is created. Then a novel image fusion model for spacecraft image shadow removal is proposed. Different from existing image fusion methods that use handcrafted fusion rules, a weight subnet block is designed to learn the optimal fusion strategy automatically. Additionally, a novel loss function combining supervision and self-supervision is adopted to train the weight subnet. To verify the advantage, the proposed method is evaluated and compared with 8 competitive image fusion methods on the released dataset. The proposed method achieves the state-of-the-art performance in both qualitative and quantitative experiment results. The dataset and MIAF implementation code are open-sourced, which can be found in (https://github.com/xiang-ao-data/Spacefuse-shadow-removal/tree/master).

A parametric design method of nanosatellite close-range formation for on-orbit target inspection

This paper proposes an efficient design method for nano satellites formation flying near a large space target to perform ultra-close inspection missions. A parametric model for periodic relative motion between two satellites is firstly proposed through a detailed analysis of the relative orbital dynamics. It is proved that the existing periodic solutions of satellite relative motion such as in-plane 2:1 elliptic and circular periodic relative orbits both belong to the ellipsoid family of periodic relative orbits. The motion planes and their locations and orientations of the general periodic relative orbits are then determined as the analytic functions of the initial relative states. The maximal and minimal distances from the relative orbit to the origin are further analytically calculated too. A formation design algorithm is then proposed for optimal observation of feature points of the target considering various requirements of collision avoidance and observable distance by using this parametric model. Numerical examples about target inspection are introduced to quantitively evaluate and verify the models and methods. The simulation results are well consistent with the theoretical predictions, showing that the design proposed can be potentially applied for future practical on-orbit service missions.

Relative Navigation Strategy About Unknown and Uncooperative Targets

In recent years, space debris has become a threat for satellites operating in low Earth orbit. Even by applying mitigation guidelines, their number will still increase over the course of the century. As a consequence, active debris removal missions and on-orbit servicing missions have gained momentum at both academic and industrial level. The crucial step in both scenarios is the capability of navigating in the neighborhood of a target resident space object. This problem has been tackled many times in literature with varying level of cooperativeness of the target required. While several techniques are available when the target is cooperative or its shape is known, no approach is mature enough to deal with uncooperative and unknown targets. This paper proposes a hybrid method to tackle this issue called Coarse Model-Based Relative Navigation (CoMBiNa). The main idea of this algorithm is to split the mission into two phases. During the first phase, the algorithm constructs a coarse model of the target. In the second phase, this coarse model is used as a reference for a relative navigation technique, effectively shifting the focus toward state and inertia estimation. In addition, this paper proposes a strategy to leverage the structure of the selected navigation method to detect and reject outliers. To conclude, CoMBiNa is tested on a simulated environment to highlight its benefits and its shortcomings, while also assessing its applicability on a limited-resource single-board computer.

EKF enhanced MPC for rapid attitude stabilization of space robots with bounded control torque in postcapture

Space robots play a more and more important role in on-orbit service missions. It is essential for a space robot to stabilize its attitude rapidly after capturing an unknown target, because of its residual momentum. Regarding uncertainties in inertial parameters of the target, process and measurement, and insufficient measurements, this paper first presents extended Kalman filter (EKF) enhanced model predictive control (MPC) strategy for rapid attitude stabilization of space robots with bounded control torque of the actuator and the manipulator in postcapture, in which motion of the manipulator is planned to make up for the limitation of the actuator. Thanks to the Jacobian matrix, which is required for EKF, the linear varying time-MPC is applied to handle the constraints of the drive torque, which can be solved by the online active set method effectively and efficiently. Simulation has been done for the 2D and the 3D models, and the results show that the whole system, including the base and the manipulator can be stabilized, and inertial parameters of the target can converge to their ideal values with the constraints. In addition, robustness of the presented method has been verified by Monte Carlo simulation.

Possible approach to establish international rules of emerging space activities - risk-based approach and adaptive governance

In May 2021, the Supplementary Requirements for a License to Operate a Spacecraft Performing On-Orbit Servicing were established and published in Japan. The Supplementary Requirements were made as the Rules for a person who intends to carry out on-orbit servicing mission including active debris removal. The Rules were to be reflected in the licensing schemes within the already existent framework of the Space Activities Act, which provides the requirements to obtain a license to control a spacecraft in Japan. Consisting of legal, technical, and organizational requirements, the Rules provide a framework to carry out on-orbit servicing mission based on the agreements and consents among the stakeholders. The Rules address the legal and technical risks derived especially from the rendezvous and proximity operations phases in the on-orbit servicing mission profile. This framework implicates the principles of safe and transparent operations as the norms for the on-orbit servicing missions, and its approach demonstrates adaptive governance in the sense that the Rules were deliberated among multi-sector stakeholders, they are expected to be introduced and extended internationally, and they are flexible to technology development. This paper explains the Rules from the points of view of risk-based approach and adaptive governance and aims to gain the perspectives on the possible approaches to establish the international rules and norms for emerging space activities.

Coupled position and attitude control of a servicer spacecraft in rendezvous with an orbiting target

Rendezvous is one of the fundamental phases of on-orbit servicing (OOS) missions. Since it requires high accuracy and safety, modeling is an indispensable part. Therefore, this article puts forward an approach for boosting the exactitude of the final proximity phase of a servicer spacecraft using precise modeling. Unlike other similar works that solely use linear models to design controllers, this paper employs a fully nonlinear model and considers most possible uncertainties and disturbances. In this regard, first a complete nonlinear relative pose (i.e., concurrent position attitude) motion dynamic is developed, which includes (1) the role of the reaction wheels and (2) the major environmental force and torque model. Second, taking the thruster's adverse torque into account, two sliding mode-based control techniques with different nonlinear sliding surfaces are designed. Moreover, the Lyapunov stability criterion is used to handle high nonlinearity effects, control input saturation, actuator misalignment, external disturbance torque/force, measurement error, uncertainties of both inertia parameters, and control inputs. Even the PWPF modulator of the thrusters has been considered to make the outcomes more realistic. Finally, three different scenarios are comprehensively simulated to illustrate the feasibility and efficiency of the designed scheme. The results prove that the proposed closed-form controller is more executable to implement than other existing approaches.

Design and Testing of Torveastro: An Outer Space Service Robot

Space robots are one of the most promising solutions for on-orbit servicing (OOS) duties like docking, berthing, refueling, re-pairing, upgrading, transporting, rescuing, and orbital trash disposal. Numerous enabling techniques and technological demonstration missions have been developed and completed over the past two decades. There have been several successful manned on-orbit service missions, but unmanned service missions have not yet been conducted. Robotic maintenance continues to be an important area of investigation with numerous technical challenges. This report outlines the design and initial testing of Torveastro, an astronaut service robot. The specifications are provided concurrently with the design and simulation. In comparison with the simulation results, preliminary tests demonstrated promising behavior for future development.

Adaptive Control on SE(3) for Spacecraft Pose Tracking With Harmonic Disturbance and Input Saturation

This article investigates the coupled attitude and orbit tracking maneuver control problem of a spacecraft system in close space missions with model uncertainty, unknown disturbance, and input saturation. An adaptive compact control scheme with featuring of disturbance rejection and antiwindup is reported. First, the exponential coordinates on Lie group SE(3) are employed to describe the relative pose, which includes the relative position and attitude, and the tracking error system is established by introducing an instrumental error variable, which contains exponential coordinates and velocity tracking error. Then, a dynamic disturbance compensator is constructed based on the idea of internal model design to finely compensate for a class of unknown harmonic disturbances. Furthermore, a nonsingular auxiliary system is proposed to overcome input saturation, in which a time-varying parameter is introduced to improve the performance of the antiwindup auxiliary system. This parameter makes the closed-loop system possess two properties: the timeliness of saturation compensation and the asymptotic convergence of tracking errors. With the developed control law, the stability of the obtained feedback system is strictly proved by means of Lyapunov stability theory. Finally, the numerical simulation is carried out for an on-orbit servicing mission, which validates the effectiveness of the proposed integrated controller in solving the pose tracking control problem.

A model-free method for attitude estimation and inertial parameter identification of a noncooperative target

Determining the attitude and inertial parameters of a noncooperative target is essential in an on-orbit servicing mission. Various methods based on machine vision have been proposed, but most of them require the 3D model of the target. This paper proposes a model-free method through sequentially registering point clouds captured by a depth camera. Our main contributions are the avoidance of the ambiguity in registration, and the combination of the multiplicative extended Kalman filter and the pose graph optimization to reduce the effect of measurement noise and drift error. A hardware experiment was performed to generate the sequence of point clouds of a three-axis free-floating target and validate our method. The result shows that the proposed method outperforms existing methods and effectively identifies the inertial parameters, including the normalized principal moments of inertia and the orientation of principal axes.

Efficient pose and motion estimation of non-cooperative target based on LiDAR.

In on-orbit servicing missions, autonomous close proximity operations require knowledge of the target's pose and motion parameters. Due to the lack of prior information about the non-cooperative target in an unknown environment, the pose and motion estimation of an uncooperative target is a challenging task. In this paper, a relative position and attitude estimation method is proposed using consecutive point clouds. First, a fast plane detection method is used to extract the global features of non-cooperative targets. Compared with some other local feature-detection methods, the method mentioned in this paper is faster. Then a two-stage angle adjustment method and iterative closest point algorithm are used to register the two adjacent point clouds. Finally, an unscented Kalman filter is designed to estimate the relative pose and motion parameters (velocity and angular velocity) of the target. Experiments show that the proposed measurement method of pose and motion parameters has acceptable accuracy and good stability.

On-Orbit Servicing System Architectures for Proliferated Low-Earth-Orbit Constellations

On-orbit servicing (OOS) presents new opportunities for refueling, inspection, repair, maintenance, and upgrade of spacecraft (s/c). OOS missions for s/c in a geostationary orbit (GEO) are currently underway. However, there are currently no plans for OOS of low-Earth-orbit (LEO) s/c, aside from technology demonstrations, because of their shorter design life and lower cost. Designing OOS systems for LEO constellations differs from that of GEO-based systems; this difference is attributed to LEO’s proliferation of satellites, environmental effects (J2 nodal precession, drag), and different constellation patterns. Satellite constellations in LEO are becoming more distributed due to increased access, distributed risk, flexibility, and cost. OOS of s/c may enable the reduction of requirements on subsystems such as safety and the need for redundancy. These requirement reductions will enable lower risks, lower costs, and increased system resilience. This paper analyzes the benefits of OOS in proliferated LEO constellations. Several OOS system architectures are modeled in various scenarios, and we will evaluate the utility tradespace in which OOS is beneficial. OOS provides higher utility over the comparative alternative of using spare satellites in some scenarios. The utility of OOS provides even greater utility when accepting higher failure rates of satellites, which can be mitigated by an OOS system compared to the comparative alternative of orbital spares.

Instance Segmentation for Feature Recognition on Noncooperative Resident Space Objects

Active debris removal and unmanned on-orbit servicing missions have gained interest in the last few years, along with the possibility to perform them through the use of an autonomous chasing spacecraft. In this work, new resources are proposed to aid the implementation of guidance, navigation, and control algorithms for satellites devoted to the inspection of noncooperative targets before any proximity operation is initiated. In particular, the use of convolutional neural networks (CNN) performing object detection and instance segmentation is proposed, and its effectiveness in recognizing the components and parts of the target satellite is evaluated. Yet, no reliable training images dataset of this kind exists to date. A tailored and publicly available software has been developed to overcome this limitation by generating synthetic images. Computer-aided design models of existing satellites are loaded on a three-dimensional animation software and used to programmatically render images of the objects from different points of view and in different lighting conditions, together with the necessary ground truth labels and masks for each image. The results show how a relatively low number of iterations is sufficient for a CNN trained on such datasets to reach a mean average precision value in line with state-of-the-art performances achieved by CNN in common datasets. An assessment of the performance of the neural network when trained on different conditions is provided. To conclude, the method is tested on real images from the Mission Extension Vehicle-1 on-orbit servicing mission, showing that using only artificially generated images to train the model does not compromise the learning process.

Quick-response attitude takeover control using multiple servicing spacecraft based on inertia properties identification

Multi-spacecraft is essential to on-orbit servicing (OOS) which leads to spacecraft life prolongation thus reducing space debris. However, the abrupt change of inertia parameters caused by the attachment of each servicing spacecraft is a critical obstacle to the stable and high-accuracy attitude takeover control of a target spacecraft during the entire servicing process using multiple servicing spacecraft. Therefore, a quick-response attitude takeover control method based on inertia properties identification is proposed in this paper. Combining the Euler dynamical equations of combined spacecraft with space environment moment model, a novel iterative identification equation integrating inertia matrix is established to eliminate ill-conditioned identification and realize the precise identification of inertia parameters using only single sampling data for each update. With the quickly and precisely estimated inertia parameters, a Lyapunov-based attitude controller and an optimal torque allocation method are designed to actualize the high-accuracy and globally stable attitude takeover control with the minimum energy consumption. The proposed method can actualize the quick-response and stable attitude takeover control with low computational cost, even at the moment of attachment of servicing spacecraft. Hence, it is greatly appropriate for the OOS missions with multiple servicing spacecraft. The ground experiments and numerical simulations were conducted for demonstrating the feasibility and effectiveness of the proposed method. The experiment result indicated that the trajectory tracking error could converge to ±0.02° and ±0.02°/s and the parameters identification error was less than 5.5%.

The Modular Relative Orbit Design Method for Spacecraft Proximity Motion

Abstract This paper presents a modular relative motion trajectory design tool for proximity operation of on-orbit service spacecrafts. First, a series of commonly used relative motion trajectories are defined. The dynamics are solved such that each type of trajectory is encapsulated into different modules. The input parameters of each module include the key geometrical features, such as the initial position, desired final position, and flying time. Different modules are connected via the same position and velocity. The modular orbit design tool is able to cover most of the typical proximity operation scenarios. Following that, two notational on-orbit service missions are given as examples to describe the specific usage process of the tool. It includes three major steps, namely, determining the appropriate module sequence according to the mission requirements, designing the pending input parameters of each module by taking consideration of constraints, and computing the details of module sequence to generate relative trajectories. Compared with the traditional maneuver-optimized orbit design approach, the modular method has fewer parameters to solve. Additionally, the modular application tool is versatile and can be used for a variety of different on-orbit service missions. This tool is of great importance in standardizing the space operations.