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Outer space presents construction challenges that are completely different from the terrestrial environment. They should be characterized by high resilience and indefinite durability because there is no possibility of repair during exploitation. There are drives in spacecraft control systems that are necessary to move solar panels, robotic arms, and manipulators, and also to position antennas. In these devices, they have applications where harmonic drives are characterized by high kinematic accuracy but relatively low mechanical strength. The analysis presented in this study is aimed at modifying the shape of the harmonic drive to increase its durability and reliability. In this study, the most vulnerable damage element of the harmonic drive is the flexspline. The calculation was carried out using the finite element method (FEM) in the computer program ABAQUS. A standardized shape was tested as a basic model, and several other design solutions were proposed. For each of them, the mechanical strength was determined, which allowed the selection of the most preferred shape for the flexspline of the harmonic drive. The specific environmental requirements of the expectations for sand for gear used in spacecraft control systems were included in the analysis. The selected construction solutions of the flexspline allow for longer work and transfer of greater loads by the harmonic driver than the solutions currently used. The choice of harmonic driver design shape allows for failure-free and maintenance-free work in space vehicle control systems. [ABSTRACT FROM AUTHOR]

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This article briefly considers the basic principles of the modern digital systems of controlling space vehicles and the methods of their provision in solving the thermal control problems. For the generalized model of heat exchange with lumped parameters, the problem of nonterminal control of the heat power of internal sources has been formulated and solved by the variational-iterative method. The developed procedure has been verified using the simplest model of heat transfer in a nonhermetic space vehicle. [ABSTRACT FROM AUTHOR]

Space mission design places a premium on cost and operational efficiency. The search for new science and life beyond Earth calls for spacecraft that can deliver scientific payloads to geologically rich yet hazardous landing sites. At the same time, the last four decades of optimization research have put a suite of powerful optimization tools at the fingertips of the controls engineer. As we enter the new decade, optimization theory, algorithms, and software tooling have reached a critical mass to start seeing serious application in space vehicle guidance and control systems. This survey paper provides a detailed overview of recent advances, successes, and promising directions for optimization-based space vehicle control. The considered applications include planetary landing, rendezvous and proximity operations, small body landing, constrained attitude reorientation, endo-atmospheric flight including ascent and reentry, and orbit transfer and injection. The primary focus is on the last ten years of progress, which have seen a veritable rise in the number of applications using three core technologies: lossless convexification, sequential convex programming, and model predictive control. The reader will come away with a well-rounded understanding of the state-of-the-art in each space vehicle control application, and will be well positioned to tackle important current open problems using convex optimization as a core technology. [ABSTRACT FROM AUTHOR]

This paper addresses the approaching trajectory control problem for the final phase of non-cooperative spacecraft proximity operations in the presence of disturbances and even input saturation. As a stepping stone, a novel line-of-sight tracking model is developed to describe attitude maneuver towards the target in the proximity. Then, a two-stage artificial potential function is employed to enable the pursuer spacecraft to comply the safety trajectory during approaching the target. In each stage, gradients of potential function are analyzed and the local minimum problem associated with potential function can be addressed. Furthermore, a saturated control law is proposed to ensure a reliable real-time tracking measurement of the target, and the controller rigorously enforces safe path and actuator magnitude constraints. The associated stability proof is accomplished through Lyapunov method. Simulations are carried out to evaluate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The problem of the attitude-tracking control of spacecraft subject to additive measurement uncertainty, control saturation, parametric uncertainty, and unknown external disturbances is studied in this paper. Because of the mismatched disturbance generated by measurement uncertainty in attitude kinematics, the normwise adaptive estimations for the bounds of the mismatched disturbance are involved in the kinematical controller; then, a saturated adaptive hierarchical fuzzy controller for attitude dynamics is developed in the backstepping design, where an antiwindup compensator is used to deal with the effect of control saturation, elementwise adaptive estimations are used to compensate parametric uncertainty, and direct adaptive hierarchical fuzzy control is designed to compensate the lumped disturbances in attitude dynamics. It is proved via Lyapunov analysis that the attitude-tracking errors eventually converge to adjustable small neighborhoods of zero. Simulation results validate the effectiveness and performance of the proposed control approach. [ABSTRACT FROM AUTHOR]

This paper designs an active fault-tolerant control system for spacecraft attitude control in the presence of actuator faults, fault estimation errors, and control input constraints. The developed fault-tolerant control system is able to detect the actuator fault without false alarms caused by external disturbances, and also estimate the total fault effects accurately through an indirect fault identification approach, in which an auxiliary variable is utilized to build the relation between fault and system states. Once the fault identification is completed with certain degree of reconstruction accuracy, a fault-tolerant backstepping controller using the nonlinear virtual control input is reconfigured to accommodate the detected actuator faults effectively, in spite of actuator saturation limitations and fault estimation errors. Numerical simulation is carried out to demonstrate that the proposed active fault-tolerant control system is successful in fault detection, identification, and controller reconfiguration for handling actuator faults in attitude control systems. [ABSTRACT FROM AUTHOR]

Abstract A saturated fault-tolerant attitude tracking controller for disturbed rigid spacecraft is derived using nonlinear state feedback control method. The proposed controller achieves the constraints of control inputs by directly using the bounded function instead of the traditional saturation compensator technique, and the active tolerance to the partial loss of actuator effectiveness is also achieved by directly using the known bounds of the actuator faults in the controller. Specifically, compared with the traditional saturated control methods, a continuously bounded nonlinear function in the proposed controller is used to guarantee that the actuator outputs are smoothly bounded under the prescribed constraints. Based on some properties of the attitude tracking dynamics, the proposed controller can ensure the attitude tracking errors converge to small neighborhoods of zero via stability analysis in the Lyapunov framework. Simulation results are presented to illustrate the effectiveness of the control scheme. [ABSTRACT FROM AUTHOR]

A robust adaptive control scheme with prescribed performance is proposed for attitude maneuver and vibration suppression of flexible spacecraft, in which the parametric uncertainty, external disturbances and unmeasured elastic vibration are taken into account simultaneously. On the basis of the prescribed performance control (PPC)theory, the prescribed steady state and transient performance for the attitude tracking error can be guaranteed through the stabilization of the transformed system. This controller does not need the knowledge of modal variables. The absence of measurements of these variables is compensated by appropriate dynamics of the controller which supplies their estimates. The method of sliding mode differentiator is introduced to overcome the problem of explosion of complexity inherent in traditional backstepping design. In addition, the requirements of knowing the system parameters and the unknown bound of the lumped uncertainty, including external disturbance and the estimate error of sliding mode differentiator, have been eliminated by using adaptive updating technique. Within the framework of Lyapunov theory, the stability of the transformed system is obtained. Finally, numerical simulations are carried out to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In this paper, a robust gain-scheduling attitude control scheme for spacecrafts with large rotational appendages is proposed. First, by introducing the higher-order singular value decomposition (HOSVD) method, a polytopic linear parameter varying (LPV) model with a family of weighting coefficients is developed based on the kinetics of a flexible spacecraft. This model eliminates the need of verifying all the gridding points, which is required in conventional controller synthesis process, and reduces the calculation complexity. Second, a generalized plant is derived to guarantee both the system robust stability and the tracking performances. Based on the LPV control theory, a less conservative controller synthesis condition for the polytopic LPV system is deduced. With an online tuning unit, the convex combination of every vertex controller is obtained. For control implementation, the present scheduling parameter is taken as an input for the tuning unit. Numerical results demonstrate the effectiveness and efficiency of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Control moment gyros (CMG) have been widely used in spacecraft attitude control and large angle slewing maneuvers over the years. Understanding and suppressing high-frequency disturbances in CMG’s output torques is a crucial factor to achieving the desired level of payload performance. Output torque modeling of a single gimbal CMG (SGCMG) with nonlinear rolling bearing supports is conducted in this paper. Taking the installation errors and micro-vibrations of the flywheel into account, three axis output torques of a SGCMG are derived based on Newton-Euler approach and theorem of moment of momentum. Dynamic model is then constructed to obtain the micro-vibration responses of the rotary flywheel. Mass imbalances of the flywheel, flexibility of supporting structures and nonlinearity induced by one pair of angular contact ball bearings are considered in the dynamic model. Especially for the rolling bearing, an improved load distribution analysis is proposed to more accurately obtain the contact deformations and angles between the rolling balls and raceways. Various factors, including the preload condition, surface waviness, Hertz contact and elastohydrodynamic lubrication, are included in the analysis. The bearing restoring forces are then obtained through iteratively solving the load distribution equations at every time step. Dynamic tests on a typical SGCMG supported by angular contact ball bearings are conducted to verify the output torque model. The effects of flywheel dynamic/static eccentricities, inner/outer surface waviness amplitudes, bearing axial preload and installation skew angles on the dynamic output torques are discussed. The obtained results would be useful for the optimal design and vibration control of the SGCMG system. [ABSTRACT FROM AUTHOR]

In this study, an adaptive critics design based on a support vector machine (SVM) is adopted to design a finite-horizon optimal feedback controller. The adaptive critics design consists of actor and critic networks. The actor (control input) and critic (cost-to-go) network are trained off-line with respect to various initial states and final times within a finite step. Using the well-trained actor-critic, the near-optimal feedback control solution can be obtained online. In the process of applying SVM to the adaptive critics, an adequate kernel function and parameters depending on the kernel function must be selected. In this study, a polynomial function and radial basis function are used for the SVM kernel function to implement the algorithm. A minimum control effort problem with final constraints for spacecraft rendezvous is considered to demonstrate the performance of the proposed the developed algorithm with respect to each kernel function and to show its potential for designing an optimal controller. [ABSTRACT FROM AUTHOR]

The problem of the artificial frozen orbits around the Earth is investigated. To design such orbits with arbitrary orbital parameters, six control strategies are discussed taking into account the main zonal harmonics up to J4 and solar radiation pressure (SRP). The effect in the argument of periapsis due to the main zonal harmonics is revealed to be about three orders of magnitude greater than that of SRP. Two comparisons are given. To obtain longer lifespan of the spacecraft, continuous variable low-thrust control is implemented. Radial and/or transverse thrusts are used to achieve these orbits. Furthermore, explicit formulas for refinement of the control thrusts are given in order to overcome errors due to approximations arising from long and short periodic variations of a and e. Some concluding remarks can be inferred from the presented simulation; e.g. the energy consumed is saved efficiently when using the root mean squares rather than using averages, also when using coupled radial and transverse thrusts instead of using single radial or transverse thrust. To minimize the fuel consumption more efficiently, three new techniques are suggested. The calculations illustrated that AFOT-F Technique, where the transverse control thrust is constant, has the minimum control thrust required. [ABSTRACT FROM AUTHOR]

Abstract Motions of spacecraft in non-synchronized binary asteroid systems are facing significant dynamics problems due to the irregular-shaped primaries and time-varying gravitational field. This paper presents a novel method for the design and maintenance of bounded trajectory in the vicinity of non-synchronized binary systems. A non-synchronized model with considering both the planar full two-body problem and the elliptic restricted three-body problem is developed for the bounded motion design. This model can better reflect the properties of motion in non-synchronized binary systems. Based on this non-synchronized model, bounded motions are designed with predetermined stability using the modified grid search and two-level correction strategy. Further, considering the model uncertainty and unmodeled perturbations in the high-fidelity model, a self-adaptive control method is proposed to maintain the trajectories. Simulations for the binary asteroid 1999 KW4 validate the stability and robustness of the proposed controller. Compared with periodic orbits using the synchronized model, the proposed strategy by taking the bounded trajectories as reference trajectories can effectively improve the tracking accuracy and reduce the fuel consumption. This provides a more effective solution for mission trajectory design in future explorations to binary asteroid systems. Highlights • The spacecraft dynamics in the non-synchronized binary system is developed. • The bounded trajectory design for the non-synchronized binary system is proposed. • A self-adaptive control method is studied for orbit maintenance in binary systems. [ABSTRACT FROM AUTHOR]

Two fractional proportional-integral-derivative controllers are proposed for rigid spacecraft rotational dynamics. In the first strategy, the controller is developed on the tangent bundle of SO(3), which is the Lie group of rigid-body rotational motion, using states consisting of a rotation matrix and an angular velocity vector in the fractional-order derivative and integral feedback terms. In the second strategy, the exponential coordinates are used to design the controller. The fractional derivative and integral feedback terms have adjustable orders that can be used to tune the closed-loop response. The performances of the controllers are studied numerically for different values of the derivative and integral fractional orders in terms of control effort, steady-state response characteristics, and robustness to unmodeled disturbances; and the results arc compared with those of an integer-order controller. It is shown that the inclusion of fractional orders in the controllers allows for a desired shape and spectrum of the controlled response to be obtained by tuning the fractional derivative and integral orders. For instance, the settling time and control effort may be simultaneously reduced, in contrast to the case of integer feedback control in which a tradeoff exists between these competing design specifications. The proposed fractional feedback attitude control strategics thus allow for a greater degree of flexibility over their integer-order analogs. [ABSTRACT FROM AUTHOR]

Feasibility of underactuated formation reconfiguration in elliptic orbits without radial or in-track thrust is investigated in this paper. For either underactuated case, by using a linear time-varying dynamical model of spacecraft formation, controllability and feasibility analyses are conducted, based on which the preconditions on reconfigurable formations are then derived. With the inherent coupling of system dynamics, the reduced-order sliding mode control technique is employed to design a closed-loop underactuated controller for either case. To ensure the stability of the closed-loop system, the conditions imposed on the controller parameters are derived, and the parameter adaptation laws are then solved analytically. Meanwhile, the explicit relationships between the steady accuracies of system states and the controller parameters are obtained via a Lyapunov-based approach. Numerical examples are simulated in a J2-perturbed environment to validate the theoretical analyses. The results indicate that by using the proposed control schemes, underactuated reconfiguration in elliptic orbits is still feasible even in the absence of radial or in-track thrust and in the presence of unmatched disturbances. [ABSTRACT FROM AUTHOR]

This paper aimed to investigate the feasibility of a contactless method to detumble space debris by using thruster plume impingement during proximity operation. To detumble the rotational debris, the force and torque applied to the debris must be precisely estimated by an onboard computer. The high-fidelity computational fluid dynamics (CFD) approach is one of the options for estimating the high precision force and torque applied on the debris, although it entails a huge computational cost. In this study, CFD results, under representative conditions with several relative attitudes and distances, were plotted on a multidimensional map. Then force and torque were derived from the interpolated discrete mapped data, thus reducing the computational time. The present study designed a guidance and control algorithm for debris removal spacecraft, in order to achieve safe and efficient detumbling using estimated force and torque. Dynamics simulations were conducted using this approach to detumble large space debris objects, assuming an upper-stage rocket. The results showed that the contactless detumbling of space debris can be achieved with reasonable fuel consumption and time. [ABSTRACT FROM AUTHOR]

This paper has proposed a robust control for the spacecraft rendezvous system by considering the parameter uncertainties and actuator unsymmetrical saturation based on the discrete gain scheduling approach. By changing of variables, we transform the actuator unsymmetrical saturation control problem into a symmetrical one. The main advantage of the proposed method is improving the dynamic performance of the closed-loop system with a region of attraction as large as possible. By the Lyapunov approach and the scheduling technology, the existence conditions for the admissible controller are formulated in the form of linear matrix inequalities. The numerical simulation illustrates the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper addresses the integrated attitude and position control problem for the final phase proximity operations of spacecraft autonomous rendezvous and docking, in which important motion constraints of the chaser spacecraft are considered. On the one hand, to ensure reliable real-time measurements of the relative attitude and position information between two spacecraft, the relevant sensor system of the chaser spacecraft is required to continuously point toward the target; on the other hand, for the proximity safety concerns, the chaser also needs to follow a specified approach path constraint. A special dual-quaternion-based artificial potential function is presented to encode information regarding these motion constraints. Using this potential function, a novel six-degree-of-freedom control method is proposed to ensure the arrival of the chaser at the docking port of the target with a desired relative attitude, while strictly complying with all constraints. The closed-loop stability is demonstrated by a Lyapunov-based method in conjunction with the special properties of the artificial potential function. The local minimum problem associated with the artificial potential function can be addressed by selection of control parameters that satisfies a mild condition. Simulation results of prototypical spacecraft rendezvous and docking missions are provided to illustrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper describes a novel uplink scheduling (ULSC) algorithm for the Galileo system. The algorithm is designed to meet the stringent dissemination requirements that are currently foreseen for the provision of the Galileo Commercial Service (CS). In particular, these include “link availability” (LA) requirements that call for a minimum percentage of time (“availability”) when a minimum number of satellites (“space vehicles,” SVs), connected with an uplink station antenna, shall be in view with a generic user at a minimum elevation angle. LA requirements for CS were not fully considered in the design phase of the ULSC algorithm, which is currently implemented at the Mission and Uplink Control Facility (MUCF) of the Galileo Ground Segment. Similarly, this type of requirements is not considered in the ULSC algorithms available in the scientific literature. In particular, current solutions take into account only specifications about 1) the maximum “gap” duration between two subsequent contacts and 2) the minimum contact duration (MCD). In the new ULSC algorithm, LA specifications are effectively dealt with by a proper selection of the subset of the SVs to be allocated. In particular, the proposed selection scheme takes into account the specifications about both maximum gap duration and MCD while maximizing the “spatial diversity” of the solution. Such a diversity is defined as the union of the coverage areas of the allocated satellites. The performance of the new ULSC algorithm is evaluated based on the final deployment stage (full operational capability) of the Galileo system. The performance is also compared with that of the ULSC algorithm currently implemented at the Galileo MUCF (“reference” algorithm). Numerical results show that, compared to the reference algorithm, the average disconnection duration is reduced by more than 50%. In addition, the availability of the links needed for the provision of CS is significantly improved (up to 7%). As a result, preliminary LA requirements for CS are met using the new ULSC algorithm, while the reference algorithm has limitations to meet the new stringent LA requirements. [ABSTRACT FROM PUBLISHER]

Modern spacecraft are usually equipped with various flexible appendages, which bring great challenges to the design of high-performance control systems. This paper proposes a novel approach to provide active damping to spacecraft flexible appendages. Vibrations of flexible appendages are actively attenuated using a base-mounted angular displacement mechanism and tip-mounted non-collocated sensors. Dynamic models are presented and the controller design techniques provided. Effectiveness of the approach is demonstrated through an experimental apparatus, in which the damping ratio of the target beam is increased to be better than 40%. Furthermore, an active damping-control scheme using no extra sensors beyond attitude sensors onboard the spacecraft bus is proposed and demonstrated through numerical simulations. [ABSTRACT FROM AUTHOR]

The paper deals with the problem of time-optimal spacecraft reorientation maneuvers by means of reaction wheels, with boundary and path constraints. When searching for solutions to optimal attitude-control problems, spacecraft can be easily modeled as controlled by external torques. However, when using actuators such as reaction wheels, conservation of the total angular momentum must be taken into account and the wheel dynamics must be included. A rest-to-rest slew maneuver is considered where an optical sensor cannot be exposed to sources of bright light such as the Earth, the Sun and the Moon. The motion must be constrained to prevent the sensor axis from entering into established keep-out cones. The minimum-time solution is proposed using the Inverse Dynamics Particle Swarm Optimization technique. The attitude and the kinematics of the satellite evolve, leading to the successive attainment of the wheel control input via fixed-step numerical integration. Numerical results are evaluated over different scenarios. It is established that the computation of minimum time maneuvers with the proposed technique leads to near optimal solutions, which fully satisfy all the boundary and path constraints. The ability to converge in a variety of different scenarios always requiring the same computational effort characterizes the proposed technique as a feasible future on-board path-planner. [ABSTRACT FROM AUTHOR]

Due to configurability feature and increasingly complex architecture, FPGAs have brought advantages to many applications such as avionics and safety critical aerospace, allowing in system reconfiguration after launch. Commercial FPGAs suffer from radiation-induced failures, which are provoked by high-energy particles in space; for this reason, fault tolerant techniques are necessary to harden these devices. This paper presents a design of a fault tolerant multicore processor architecture based on a novel modular voting strategy that fits in FPGAs and System-on-Chip (SoC) devices with an even number of processors. This architecture is implemented within a Commercial off-the-shelf (COTS) SoC that will allow to be used safely in space missions. To harden the fault tolerance of the embedded multicore processor architecture different fault tolerance techniques are combined. [ABSTRACT FROM AUTHOR]

A rotary single inverted pendulum (RSIP) typically represents a space booster rocket, Segway and similar systems with unstable equilibrium. This paper proposes a novel two degree of freedom (2-DOF) fractional control strategy based on 2-loop topology for RSIP system which can be extended to control the systems with unstable equilibrium. It comprises feedback and feed-forward paths. Primary controller relates the perturbation attenuation while the secondary controller is accountable for set point tracking. To tune the parameters of proposed fractional controller a simple graphical tuning method based on frequency response is used. The study will serve the outstanding experimental results for both, stabilization and trajectory tracking tasks. The study will also serve to present a comparison of the performance of the proposed controller with the 1-DOF FOPID controller and sliding mode controller (SMC) for the RSIP system. Further to confirm the usability of the proposed controller and to avoid the random perturbations sensitivity, robustness, and stability analysis through fractional root-locus and Bode-plot is investigated. [ABSTRACT FROM AUTHOR]

This study is concerned with vibration control of a flexible spacecraft system in the presence of disturbances and input saturation constraint. The dynamics of the spacecraft consisting of a rigid body and two flexible appendages is presented by the coupled partial differential equations and ordinary differential equations. The first boundary state feedback control is adopted when the spacecraft system states can be measured. The second boundary output feedback control is employed when there exists the unmeasurable state in the proposed control law. A high-gain observer is developed to estimate the unmeasurable system state, two auxiliary systems are designed to prevent the presence of input saturation from destroying the system performance and two disturbance observers are established to handle the unknown boundary disturbances. The well-posedness and stability of the spacecraft system are proved. Simulation results are presented to demonstrate the effectiveness of the proposed control schemes. [ABSTRACT FROM AUTHOR]

A novel star identification algorithm based on voting scheme is presented in this paper. In the proposed algorithm, the global distribution and local distribution of sensor stars are fully utilized, and the stratified voting scheme is adopted to obtain the candidates for sensor stars. The database optimization is employed to reduce its memory requirement and improve the robustness of the proposed algorithm. The simulation shows that the proposed algorithm exhibits 99.81% identification rate with 2-pixel standard deviations of positional noises and 0.322-Mv magnitude noises. Compared with two similar algorithms, the proposed algorithm is more robust towards noise, and the average identification time and required memory is less. Furthermore, the real sky test shows that the proposed algorithm performs well on the real star images. [ABSTRACT FROM AUTHOR]

This paper presents a trajectory-optimization method for a chaser spacecraft operating in ultra-close proximity to a failed satellite. Based on the combination of active and passive trajectory protection, the constraints in the optimization framework are formulated for collision avoidance and successful docking in the presence of any thruster failure. The constraints are then handled by an adaptive Gauss pseudospectral method, in which the dynamic residuals are used as the metric to determine the distribution of collocation points. A finite-time feedback control is further employed in tracking the optimized trajectory. In particular, the stability and convergence of the controller are proved. Numerical results are given to demonstrate the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

The deployment dynamics of a spin stabilized electric sail (E-sail) with a hub-mounted control actuator are investigated. Both radial and tangential deployment mechanisms are considered to take the electric sail from a post-launch stowed configuration to a fully deployed configuration. The tangential configuration assumes the multi-kilometer tethers are wound up on the exterior of the spacecraft hub, similar to yo-yo despinner configurations. The deployment speed is controlled through the hub rate. The radial deployment configuration assumes each tether is on its own spool. Here both the hub and spool rate are control variables. The sensitivity of the deployment behavior to E-sail length, maximum rate and tension parameters is investigated. A constant hub rate deployment is compared to a time varying hub rate that maintains a constant tether tension condition. The deployment time can be reduced by a factor of 2 or more by using a tension controlled deployment configuration. [ABSTRACT FROM AUTHOR]

This paper proposes a novel feedback control law for spacecraft to deal with attitude constraint, input saturation, and stochastic disturbance during the attitude reorientation maneuver process. Applying the parameter selection method to improving the existence conditions for the repulsive potential function, the universality of the potential-function-based algorithm is enhanced. Moreover, utilizing the auxiliary system driven by the difference between saturated torque and command torque, a backstepping control law, which satisfies the input saturation constraint and guarantees the spacecraft stability, is presented. Unlike some methods that passively rely on the inherent characteristic of the existing controller to stabilize the adverse effects of external stochastic disturbance, this paper puts forward a nonlinear disturbance observer to compensate the disturbance in real-time, which achieves a better performance of robustness. The simulation results validate the effectiveness, reliability, and universality of the proposed control law. [ABSTRACT FROM AUTHOR]

Observability analysis is a method for determining whether a chosen state of a system can be determined from the output or measurements. Knowledge of state information availability resulting from observability analysis leads to improved sensor tasking for observation of orbital debris and better control of active spacecraft. This research performs numerical observability analysis of artificial near-Earth objects. Analysis of linearization methods and state transition matrices is performed to determine the viability of applying linear observability methods to the nonlinear orbit problem. Furthermore, pre-whitening is implemented to reformulate classical observability analysis. In addition, the state in observability analysis is typically composed of position and velocity; however, including object characteristics beyond position and velocity can be crucial for precise orbit propagation. For example, solar radiation pressure has a significant impact on the orbit of high area-to-mass ratio objects in geosynchronous orbit. Therefore, determining the time required for solar radiation pressure parameters to become observable is important for understanding debris objects. In order to compare observability analysis results with and without measurement noise and an extended state, quantitative measures of observability are investigated and implemented. [ABSTRACT FROM AUTHOR]

In this paper, the attitude stabilization and tracking control problem is investigated for spacecraft with consideration of unknown dynamics and external disturbance. A low-complexity prescribed performance attitude control scheme is presented to solve this problem. Different from the existing works, there are threefold prominent advantages. The first one is the transient performance and the steady performance of the system is guaranteed by a user-defined function rather than depending on repeated adjustment of controller parameters. The second is that no information of the system and external disturbance is necessary in the developed control scheme, which means the method is model-free. Moreover, the developed low-complexity controller is calculated without any time-consuming iterative operations; thus it's significantly advantageous in engineering applications. It is proved that the state variables converge to the prescribed region at a prescribed exponential rate under the proposed control scheme. Four groups of numerical simulations are organized to validate the effectiveness of the method. [ABSTRACT FROM AUTHOR]

In this paper, a robust inertia-free attitude takeover control scheme with guaranteed prescribed performance is investigated for postcapture combined spacecraft with consideration of unmeasurable states, unknown inertial property and external disturbance torque. Firstly, to estimate the unavailable angular velocity of combination accurately, a novel finite-time-convergent tracking differentiator is developed with a quite computationally achievable structure free from the unknown nonlinear dynamics of combined spacecraft. Then, a robust inertia-free prescribed performance control scheme is proposed, wherein, the transient and steady-state performance of combined spacecraft is first quantitatively studied by stabilizing the filtered attitude tracking errors. Compared with the existing works, the prominent advantage is that no parameter identifications and no neural or fuzzy nonlinear approximations are needed, which decreases the complexity of robust controller design dramatically. Moreover, the prescribed performance of combined spacecraft is guaranteed a priori without resorting to repeated regulations of the controller parameters. Finally, four illustrative examples are employed to validate the effectiveness of the proposed control scheme and tracking differentiator. [ABSTRACT FROM AUTHOR]

This paper comprehensively reviews the state-of-the-art development in formation control of small satellites. Satellite formation flying, distributed satellite systems, and fractionated satellite formation are discussed first. Various formation control architectures and methods of small satellites are then introduced, including the leader-following method, the behavior-based method, the virtual structure method, the cyclic pursuit method, the artificial potential function method, the algebraic graph method, and the noncontact force method. Coordinative control of multiple small satellites is also reviewed, covering coordinative control of satellite formation, coordinative attitude control of satellite formation, and coordinative coupled attitude and orbit control of satellite formation. The achievements and development trends of the formation control of small satellites are considered and analyzed. [ABSTRACT FROM PUBLISHER]

This paper presents an adaptive neural networks-based control method for spacecraft formation with coupled translational and rotational dynamics using only aerodynamic forces. It is assumed that each spacecraft is equipped with several large flat plates. A coupled orbit-attitude dynamic model is considered based on the specific configuration of atmospheric-based actuators. For this model, a neural network-based adaptive sliding mode controller is implemented, accounting for system uncertainties and external perturbations. To avoid invalidation of the neural networks destroying stability of the system, a switching control strategy is proposed which combines an adaptive neural networks controller dominating in its active region and an adaptive sliding mode controller outside the neural active region. An optimal process is developed to determine the control commands for the plates system. The stability of the closed-loop system is proved by a Lyapunov-based method. Comparative results through numerical simulations illustrate the effectiveness of executing attitude control while maintaining the relative motion, and higher control accuracy can be achieved by using the proposed neural-based switching control scheme than using only adaptive sliding mode controller. [ABSTRACT FROM AUTHOR]

This paper presents a robust fuzzy tracking controller for position and attitude control, and for vibration suppression in a flexible spacecraft. The robust fuzzy controller guarantees H\infty tracking control performance while satisfying the actuators’ amplitude constraints. The design problem of the fuzzy tracking controller is formulated in terms of linear matrix inequalities. The stability, performance, and robustness of the proposed fuzzy controller are examined and compared with a baseline nonlinear controller such as the model-reference adaptive controller with $\sigma$-modification via numerical simulations. [ABSTRACT FROM AUTHOR]

The capture of a free-floating tumbling object using an autonomous vehicle is a key technology for many future orbital missions. Spacecraft proximity operations will play an important role in guaranteeing the success of such missions. In this paper, we technically propose a tracking control scheme for proximity operations between a target and a pursuer spacecraft that ensures accurate relative position tracking as well as attitude synchronization. Specifically, an integrated six degrees of freedom dynamics model is first established to describe the relative motion of the pursuer with respect to the target. Then, a robust fault-tolerant controller is derived by combining the sliding mode control and the adaptive technique. The designed controller is proved to be not only robust against unexpected disturbances and adaptive to unknown and uncertain mass/inertia properties of the pursuer, but also able to accommodate a large class of actuator faults. In particular, by incorporating a novel time-varying forcing function into the sliding dynamics, the proposed control algorithm is able to guarantee the finite-time convergence of the translational and rotational tracking errors, and the convergence time as an explicit parameter can be assigned a priori by the designers. Furthermore, a theoretical analysis is also presented to assess the fault tolerance ability of the designed controller. Finally, numerous examples are carried out to evaluate the effectiveness and demonstrate the benefits of the overall control approach. [ABSTRACT FROM AUTHOR]