Spacecraft Pose Estimation Based on Different Camera Models

Spacecraft pose estimation is an important technology to maintain or change the spacecraft orientation in space.For spacecraft pose estimation,when two spacecraft are relatively distant,the depth information of the space point is less than that of the measuring distance,so the camera model can be seen as a weak perspective projection model.In this paper,a spacecraft pose estimation algorithm based on four symmetrical points of the spacecraft outline is proposed.The analytical solution of the spacecraft pose is obtained by solving the weak perspective projection model,which can satisfy the requirements of the measurement model when the measurement distance is long.The optimal solution is obtained from the weak perspective projection model to the perspective projection model,which can meet the measurement requirements when the measuring distance is small.The simulation results show that the proposed algorithm can obtain better results,even though the noise is large.

一种用于行星间转移轨迹优化的混合优化方法

本文针对行星间转移轨迹优化问题,提出一种结合Gauss伪谱法和遗传算法的混合优化方法。这种混合优化方法通过遗传算法全局寻优获得Gauss伪谱法中所用的状态变量及控制变量初值,然后Gauss伪谱法根据遗传算法获得的初值进一步寻优,从而克服了传统间接优化法和直接优化法对初值的依赖。本文以地球至火星低推进过渡轨迹优化问题为例,对所提出的混合优化方法进行验证。仿真结果表明,这种优化方法既具有遗传算法的全局寻优能力又具有Gauss伪谱法的局部强收敛特性,能够在无任何初值猜测的情况下完成对近似全局最优解的搜索。

Guidance laws for attacking defended target

In this paper,two new guidance laws based on differential game theory are proposed and investigated for the attacker in an attacker-defender-target scenario.The conditions for the attacker winning the game are analyzed when the target and defender using the differential game guidance law based on the linear model.The core ideas underlying the two guidance laws are the attacker evading to a critical safe boundary from the defender,and then maintaining a critical miss distance.The guidance law more appropriate for the attacker to win the game differs according to the initial parameters.Unlike other guidance laws,when using the derived guidance laws there is no need to know the target and the defender’s control efforts.The results of numerical simulations show that the attacker can evade the defender and hit the target successfully by using the proposed derived guidance laws.

Evasion and pursuit guidance law against defended target

This paper investigates a new approach for a scenario in which an Attacker attempts to intercept a defended aerial Target. The problem is formulated as a game among three players, an Attacker, a Defender, and a Target, with bounded controls. In the considered pursuit–evasion problem, the Target uses an optimal evasion strategy and the Defender uses an optimal pursuit strategy.The proposed approach focuses on the miss distance as the outcome of the conflict. The infeasible region for the initial Zero-Effort-Miss（ZEM） distance between the Attacker and the Defender, for a scenario in which the Attacker evades the Defender, is analyzed, assuming that the Attacker uses a control effort chosen from the permitted control region. The sufficient conditions are investigated under which, for ideal players, the Attacker can pursue the Target while evading the Defender launched by the Target. The guidance provided on how the Attacker can accomplish the task is divided into two parts. During the final time between the Attacker and the Defender, the Attacker chooses the control effort that guarantees the miss distance, and then uses the optimal pursuit strategy to accomplish the task. The derived guidance law is verified by nonlinear simulation.

Fast cooperative trajectory optimization and test verification for close-range satellite formation using Finite Fourier Series method

The process of formation reconfiguration for close-range satellite formation should take into account the risk of collisions between satellites.To this end,this paper presents a method to rapidly generate low-thrust collision-avoidance trajectories in the formation reconfiguration using Finite Fourier Series(FFS).The FFS method can rapidly generate the collision-avoidance threedimensional trajectory.The results obtained by the FFS method are used as an initial guess in the Gauss Pseudospectral Method(GPM)solver to verify the applicability of the results.Compared with the GPM method,the FFS method needs very little computing time to obtain the results with very little difference in performance index.To verify the effectiveness,the proposed method is tested and validated by a formation control testbed.Three satellite simulators in the testbed are used to simulate two-dimensional satellite formation reconfiguration.The simulation and experimental results show that the FFS method can rapidly generate trajectories and effectively reduce the risk of collision between satellites.This fast trajectory generation method has great significance for on-line,constantly satellite formation reconfiguration.

An optimal one-way cooperative strategy for two defenders against an attacking missile

This paper investigates a cooperative strategy for protecting an aerial target.The problem is solved as a game among four players（a target,two defenders,and a missile）.In this scenario,the target launches two defenders（defender-1 and defender-2）simultaneously,to establish a oneway cooperation system（OCS）against an attacking missile.A new optimal evasion strategy for the target is also derived.During the engagement,the target takes into account the reaction of the attacking missile,and guides defender-1 to the interception point by receiving information from defender-1.Depending on the control effort of the target,defender-2 can choose appropriate launch conditions and use very limited maneuvering capability to intercept the missile.For adversaries with first-order dynamics,simulation results show that the OCS allows two defenders to intercept the missile.During the engagement,even if one defender or communication channel is broken,the OCS still allows an interception to be made,thus increasing the target’s survivability.

Capture condition for endo-atmospheric interceptors steered by ALCS and ARCS

This contribution deals with capture condition for interceptor missiles steered by aero-lift control system （ALCS） and attitude reaction-jet control system （ARCS）. With the guidance law derived from bounded differential game formulation, existence condition of capture zone is studied for the case that the interceptor has advantage on maneuverability and disadvantage on agility. For the existence of the open capture zone, ARCS can only close after the engagement terminates. Moreover, ARCS also needs to contribute to maneuverability over the minimum required value. More fuel will be required if ARCS increases its contribution to maneuverability. The minimum required fuel occurs at the tangent point of two curves： the curve of critical parameters and a candidate constraint curve, which is also true even for the complex propellant constrain. The validity of these results is also demonstrated by simulations.

Coupled attitude-vibration analysis of an E-sail using absolute nodal coordinate formulation

In this study,the effects of solar wind on an electric sail(E-sail)are modeled and analyzed using an absolute nodal coordinate formulation(ANCF).First,the thrust of the charged metal tether that makes up the E-sail was analyzed and a model was established.Numerical simulations of a single metal tether were performed.Then,an overall E-sail model was established using the connection matrix,and E-sails subjected to different angular velocities were compared.Simulation results of the ANCF model and a dumbbell model were compared at different angular velocities.The results confirm that with a relatively high angular velocity,the flexible metal chain can be approximately regarded as a rigid body.However,with a small angular velocity,the flexibility of the metal chain cannot be ignored.

Feature-aided pose estimation approach based on variational auto-encoder structure for spacecrafts

Real-time 6 Degree-of-Freedom(DoF)pose estimation is of paramount importance for various on-orbit tasks.Benefiting from the development of deep learning,Convolutional Neural Networks(CNNs)in feature extraction has yielded impressive achievements for spacecraft pose estimation.To improve the robustness and interpretability of CNNs,this paper proposes a Pose Estimation approach based on Variational Auto-Encoder structure(PE-VAE)and a Feature-Aided pose estimation approach based on Variational Auto-Encoder structure(FA-VAE),which aim to accurately estimate the 6 DoF pose of a target spacecraft.Both methods treat the pose vector as latent variables,employing an encoder-decoder network with a Variational Auto-Encoder(VAE)structure.To enhance the precision of pose estimation,PE-VAE uses the VAE structure to introduce reconstruction mechanism with the whole image.Furthermore,FA-VAE enforces feature shape constraints by exclusively reconstructing the segment of the target spacecraft with the desired shape.Comparative evaluation against leading methods on public datasets reveals similar accuracy with a threefold improvement in processing speed,showcasing the significant contribution of VAE structures to accuracy enhancement,and the additional benefit of incorporating global shape prior features.