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This paper examines the velocity-free attitude stabilization of an underactuated nadir-pointing rigid spacecraft in a circular orbit in the presence of time-varying disturbances. A nonlinear observer is used to estimate the angular velocity of the spacecraft to complete an output feedback control law. A sliding-mode control strategy is proposed for this stabilization problem in conjunction with a nonlinear observer. The local ultimate boundedness of the closed-loop system is rigorously proven. Numerical examples demonstrate the performance of the controller in the presence of time-varying disturbances, nonzero orbital eccentricity, inertia matrix uncertainties, and measurement noise in a sampled data implementation. In particular, even though the theoretical ultimate boundedness guarantees are local, the numerical examples demonstrate ultimate boundedness with large initial attitude errors. [ABSTRACT FROM AUTHOR]

Spacecraft relative motion planning is concerned with the design and execution of maneuvers relative to a nominal target. These types of maneuvers are frequently used in missions such as rendezvous and docking, satellite inspection, and formation flight, where exclusion zones representing spacecraft or other obstacles must be avoided. The presence of these exclusion zones leads to nonlinear and nonconvex constraints that must be satisfied. In this paper, a novel approach to spacecraft relative motion planning with obstacle avoidance and thrust constraints is developed. This approach is based on a graph search applied to a virtual net of closed (periodic) natural motion trajectories, where the natural motion trajectories represent virtual net nodes (vertices), and adjacency and connection information is determined by conditions defined in terms of safe, positively invariant lubes built around each trajectory. These conditions guarantee that transitions from one natural motion trajectory to another natural motion trajectory can be completed without constraint violations. The proposed approach improves the flexibility of a previous approach based on the use of forced equilibria and has other advantages in terms of reduced fuel consumption and passive safety. The resulting maneuvers, if planned onboard, can be executed directly or, if planned off-board, can be used to warm start trajectory optimizers to generate further improvements. [ABSTRACT FROM AUTHOR]

The design of National Aeronautics and Space Administration Orion's prelaunch navigation system is introduced, both for the first flight test, Exploration Flight Test 1, and for the first planned Orion mission, Exploration Mission 1. A detailed tradeoff of possible design decisions is discussed, and the choices made by Orion are presented. The actual performance of the navigation system during Exploration Flight Test 1 is presented together with the navigation flight-software data provided by Orion to the ground controllers in telemetry. [ABSTRACT FROM AUTHOR]

The article presents a study related to spacecraft attitude stabilization with the use of magnetorquers with separation between measurement and actuation. Topics discussed include actuation mechanisms related to spacecraft attitude stabilization; ways to spacecraft attitude control; and use of electromagnetic actuators for generation of attitude control torques on satellites flying in low earth orbits.

The article presents a study related to mass ratio of electrodynamics tether to spacecraft on deorbit stability and efficiency. Topics discussed include ways in which space activities in low Earth orbit have produced a large population of dead bodies, or debris; guidelines to remove or mitigate the space debris; and parametrical analysis of deorbit performance.

The article presents a study related to spacecraft attitude stabilization with the use of magnetorquers with separation between measurement and actuation. Topics discussed include actuation mechanisms related to spacecraft attitude stabilization; ways to spacecraft attitude control; and use of electromagnetic actuators for generation of attitude control torques on satellites flying in low earth orbits.

The article presents a study related to adaptive attitude stabilization control design for spacecraft under physical limitations. Topics discussed include role of Attitude stabilization control is an important orbital mission for spacecraft; the saturation problem for attitude control; and use of adaptive attitude tracking controller.

The high complexity of modern autonomous aerospace systems has created a need for new, powerful analysis techniques for control design and validation. This paper exploits the capabilities of continuation algorithms to derive a numerical analysis method that provides a systematic way to investigate how system properties local to the equilibrium condition around which the system is operated change throughout the operational envelope. The method is based on computing subsets of the system's equilibrium manifold with particular properties that are specified by equality constraints. The contribution of this work lies in the derivation of extended systems of equations that enable the definition of constraints on the eigenvalues and eigenvectors of the linearized dynamics, so that sets of equilibrium points with particular characteristics of the local dynamical behavior can be computed. The capabilities of the method are demonstrated through application to a fully nonlinear aircraft model with six degrees of freedom. [ABSTRACT FROM AUTHOR]

This paper investigates the velocity-measurement-free feedback control problem associated with attitude tracking for a rigid spacecraft in the presence of both actuator failures and actuator saturation. The decreased reaction torque fault and the increased bias torque fault in the reaction wheel are handled. By using only the measurable attitude, a terminal sliding-mode-based observer is developed to reconstruct all the states of the attitude system in finite time. With the reconstructed information, a novel attitude tracking controller is developed. A Lyapunov analysis shows that the desired attitude trajectories are followed even in the presence of external disturbances. The key features of the proposed control approach are that it is independent from the knowledge of actuator faults and fault-tolerant control is achieved without the need of angular velocity. The attitude tracking performance using the proposed strategy is evaluated through a numerical example. [ABSTRACT FROM AUTHOR]

A method is presented to optimize the orientation of directional sensors and instruments in a vehicle body-fixed frame. Directional dependence is included by creating a uniformly distributed set of directions in the body-fixed frame and formulating the objective as a function of these directions. The method is demonstrated by application to photodiodes for sun sensing, for which the covariance of the sun vector estimate is derived as a function of the photodiode configuration. The measured sun vector angular accuracy is then minimized as a function of the configuration, which enables the most accurate sun sensing with the given hardware. This technique maximizes subsystem performance and provides a design method to replace traditional, iterative design approaches to sensor placement. [ABSTRACT FROM AUTHOR]

A novel saturated proportional-derivative control law incorporated with closed-loop control allocation is proposed for spacecraft attitude stabilization in this paper. More specifically, a saturated proportional-derivative-based baseline nonlinear controller is designed to guarantee the globally asymptotic stability under control input/signal constraints. Then, a closed-loop constrained optimal control allocation scheme is employed to distribute the moments synthesized by the baseline controller over the redundant actuators in the presence of constraints due to actuator amplitude and rate constraints. The optimal control solution is to be found by penalizing the control allocation errors and rates using optimal quadratic programming algorithms. A significant feature of this work is that the asymptotic stability with the closed-loop control allocation is guaranteed theoretically. Simulation results are presented to illustrate the performance of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper develops the analytical solution to the nondrifting transfer problem for rendezvous in elliptical orbits. The transfer algorithm generates a nondrifting, passively safe trajectory between two spacecraft slates by finding the appropriate maneuvers and transfer time. The nondrifting solution is derived first for Cartesian state vectors, which is useful for transferring to a terminal approach point that is defined in terms of such a Cartesian state vector. Next, the transfer problem is solved in terms of differential orbital elements. This second solution lends itself well to perform close-range rendezvous in elliptical orbits. [ABSTRACT FROM AUTHOR]

Motivated by aerospace applications, this paper presents a methodology to use second-order cone programming to solve nonconvex optimal control problems. The nonconvexity arises from the presence of concave state inequality constraints and nonlinear terminal equality constraints. The development relies on a solution paradigm, in which the concave inequality constraints are approximated by successive linearization. Analysis is performed to establish the guaranteed satisfaction of the original inequality constraints, the existence of the successive solutions, and the equivalence of the solution of the original problem to the converged successive solution. These results lead to a rigorous proof of the convergence of the successive solutions under appropriate conditions as well as non-conservativeness of the converged solution. The nonlinear equality constraints are treated in a two-step procedure in which the constraints are first approximated by first-order expansions, then compensated by second-order corrections in each of the successive problems for enhanced convergence robustness. Applications in highly constrained spacecraft rendezvous and proximity operations, finite-thrust orbital transfers, and optimal launch ascent are provided to demonstrate the effectiveness of the methodology. [ABSTRACT FROM AUTHOR]

The feasibility of flying a crewed lifting body, such as the HL-20, during entry from low Earth orbit without the steady-state body flap deflections required for angle-of-attack control was evaluated. This entry strategy mitigates the severity of the aerothermal environment on the vehicle's body flaps and reserves control power for transient steering maneuvers. A real-time numeric predictor-corrector entry guidance algorithm was developed to accommodate the range of vehicle trim angle-of-attack profiles possible in the absence of angle-of-attack control. Results show that it is feasible to steer the vehicle from low Earth orbit to a desired target with real-time guidance while satisfying a reasonable suite of trajectory constraints, including limits on peak heat rate, peak sensed deceleration, and integrated heat load. Uncertainty analyses confirm this result and show that the vehicle maintains significant performance robustness to expected day-of-flight uncertainties. Additionally, parametric scans over mission design parameters of interest indicate that a high level of flexibility is available for the low-Earth-orbit return mission. Together, these results indicate that the proposed entry strategy is feasible: Crewed lifting bodies may be effectively flown without steady-state body flap deflections. [ABSTRACT FROM AUTHOR]

A program of research, development, test, and evaluation is planned for the development of spacecraft handling qualities guidelines. In this first experiment, the effects of reaction control system design characteristics and rotational control laws were evaluated during simulated proximity operations and docking. Also, the influence of piloting demands resulting from varying closure rates was assessed. The pilot-in-the-loop simulation results showed that significantly different spacecraft handling qualities result from the design of the reaction control system. In particular, cross coupling between translational and rotational motions significantly affected handling qualities as reflected by Cooper-Harper pilot ratings and pilot workload, as reflected by Task Load Index ratings. This influence is masked (but only slightly) by the rotational control system mode. Although rotational control augmentation using rate command/attitude hold can reduce the workload (principally, physical workload) created by cross coupling, the handling qualities are not significantly improved. The attitude and rate deadbands of the rate command/attitude hold introduced significant mental workload and control compensation to evaluate when deadband firings would occur, assess their impact on docking performance, and apply control inputs to mitigate that impact. [ABSTRACT FROM AUTHOR]