Mission Planning And Optimal Attitude Control Of The Agile Spacecraft For Staring-mode Observation

For the complex remote sensing missions,the spacecraft is required to observe several targets respectively within an orbit period.Moreover,the maneuver ability of the spacecraft has been enhanced.The non-agile spacecraft gradually has been replaced by agile spacecraft endowed with three-axis attitude maneuver ability.How to take the advantage of the maneuver ability such that more targets could be observed is critical to improve the observation efficiency and is a promising issue to be studied.Besides,since the fuel on the spacecraft is limited,it is essential to enhance the efficiency of the attitude maneuver with less energy consumption such that the spacecraft can work longer.Therefore,this thesis addresses the problem of mission planning and optimal attitude control of the agile spacecraft operating in staring mode.Simple and efficient planning and control schemes are proposed to enhance the observation efficiency of the spacecraft.Considering the property of the staring-mode observation missions,a novel attitude representation is presented.Different from the traditional push-brooming imaging mode,the staring-mode only requires the optical axis aligned with the target direction and thus there are two DOF(Degree of Freedom)constraints for the final desired attitude.Several attitude representations,including Euler angle and quaternion are used to describe the desired attitude,and the characteristics of these representations are discussed.Because the final desired attitude is not specifically decided in advance when using quaternion or Euler angle,it is not a trivial to design the controller.To address this issue,a novel attitude representation is presented where the angle between the optical axis and the target direction is used and the corresponding kinematics for the pointing error is established.For the multi-targets observation mission,an intelligent and credible planning approach is proposed to automatically and intelligently decide the observation sequence and observation start time.The sequence can be used for the attitude control as a desired signal.First,a task clustering strategy based on minimum clique partition is presented and a series of cliques are built.Then,an improved ant colony optimization(ACO)algorithm is presented where a heuristic decision method is introduced which synthesizes the priorities and time windows of targets.In this way,the optimization ability of the algorithm can be enhanced.Since the minimum attitude maneuver time between any two adjacent targets is important for designing the ACO algorithm,a novel and simple near time-optimal attitude maneuver approach is presented.The minimum maneuver time can be explicitly expressed as a function of the maneuver angle and control torque constraints.The analytic solution of the time optimal maneuvers enhances the efficiency of the algorithm.The rapid attitude maneuver between the targets is of paramount for the remote sensing.The optimality and robustness of the system is urgently required to be improved.In the thesis,a composite control scheme is proposed that combines the inverse optimal theory with the CLF(Control Lyapunov Function)method where the observation sequence obtained by using the mission planning approach is used as the desired attitude signal.This control approach could drive the spacecraft to point to the targets in sequence with high precision.First,the linear and terminal sliding mode surfaces are designed respectively and the corresponding CLF function is constructed such that the sliding surface is equivalent to the switch condition of the traditional CLF approach.Then,the CLF control is applied in the case of the states far from the sliding manifold to improve the optimality of the system.Once the states are near the manifold,the controller is switched to the sliding mode control by which the states are driven to converge towards the origin robustly.Since the spacecraft is required to observe the targets in sequence planned by the ant colony algorithm,but the aforementioned control approach only guarantees that the spacecraft points to the target before the specified time instead of at the exact time.To address this issue,two exact-time attitude repointing maneuver approaches are presented such that the optical axis of the spacecraft is aligned with the target direction at the exact time.The first control method is based on the sliding mode control theory and the reaching law is replaced by the constant angular acceleration along the eigenaxis.The control parameters can be conveniently decided according to the desired convergence time and initial attitude errors.However,this method is only able to deal with the rest-to-rest maneuvers.For the real staring-mode observations,the optical axis should point to the target with a non-zero angular velocity.An improved State-Dependent Riccati Equation(SDRE)approach with a waypoint is proposed.The whole time span is divided into two equal intervals and the optimal state at the middle of the time span is calculated to minimize the cost index.In this way,the spacecraft could stare at the target with a specific velocity at any prescribed time.