Comparison of Precipitation Observations from a Prototype Space-based Cloud Radar and Ground-based Radars

A prototype space-based cloud radar has been a precipitation system over Tianjin, China in July developed and was installed on an airplane to observe 2010. Ground-based S-band and Ka-band radars were used to examine the observational capability of the prototype. A cross-comparison algorithm between different wavelengths, spatial resolutions and platform radars is presented. The reflectivity biases, correlation coefficients and standard deviations between the radars are analyzed. The equivalent reflectivity bias between the S- and Ka-band radars were simulated with a given raindrop size distribution. The results indicated that reflectivity bias between the S- and Ka-band radars due to scattering properties was less than 5 dB, and for weak precipitation the bias was negligible. The prototype space-based cloud radar was able to measure a reasonable vertical profile of reflectivity, but the reflectivity below an altitude of 1.5 km above ground level was obscured by ground clutter. The measured refiectivity by the prototype space-based cloud radar was approximately 10.9 dB stronger than that by the S-band Doppler radar （SA radar）, and 13.7 dB stronger than that by the ground-based cloud radar. The reflectivity measured by the SA radar was 0.4 dB stronger than that by the ground-based cloud radar. This study could provide a method for the quantitative examination of the observation ability for space-based radars.

Robust Adaptive Gain Higher Order Sliding Mode Observer Based Control-constrained Nonlinear Model Predictive Control for Spacecraft Formation Flying

This work deals with the development of a decentralized optimal control algorithm, along with a robust observer,for the relative motion control of spacecraft in leader-follower based formation. An adaptive gain higher order sliding mode observer has been proposed to estimate the velocity as well as unmeasured disturbances from the noisy position measurements.A differentiator structure containing the Lipschitz constant and Lebesgue measurable control input, is utilized for obtaining the estimates. Adaptive tuning algorithms are derived based on Lyapunov stability theory, for updating the observer gains,which will give enough flexibility in the choice of initial estimates.Moreover, it may help to cope with unexpected state jerks. The trajectory tracking problem is formulated as a finite horizon optimal control problem, which is solved online. The control constraints are incorporated by using a nonquadratic performance functional. An adaptive update law has been derived for tuning the step size in the optimization algorithm, which may help to improve the convergence speed. Moreover, it is an attractive alternative to the heuristic choice of step size for diverse operating conditions. The disturbance as well as state estimates from the higher order sliding mode observer are utilized by the plant output prediction model, which will improve the overall performance of the controller. The nonlinear dynamics defined in leader fixed Euler-Hill frame has been considered for the present work and the reference trajectories are generated using Hill-Clohessy-Wiltshire equations of unperturbed motion. The simulation results based on rigorous perturbation analysis are presented to confirm the robustness of the proposed approach.

Review on strategies of space-based optical space situational awareness

Space-based optical(SBO)space surveillance has attracted widespread interest in the last two decades due to its considerable value in space situation awareness(SSA).SBO observation strategy,which is related to the performance of space surveillance,is the top-level design in SSA missions reviewed.The recognized real programs about SBO SAA proposed by the institutions in the U.S.,Canada,Europe,etc.,are summarized firstly,from which an insight of the development trend of SBO SAA can be obtained.According to the aim of the SBO SSA,the missions can be divided into general surveillance and space object tracking.Thus,there are two major categories for SBO SSA strategies.Existing general surveillance strategies for observing low earth orbit(LEO)objects and beyond-LEO objects are summarized and compared in terms of coverage rate,revisit time,visibility period,and image processing.Then,the SBO space object tracking strategies,which has experienced from tracking an object with a single satellite to tracking an object with multiple satellites cooperatively,are also summarized.Finally,this paper looks into the development trend in the future and points out several problems that challenges the SBO SSA.

Iterative Learning Disturbance Observer Based Attitude Stabilization of Flexible Spacecraft Subject to Complex Disturbances and Measurement Noises

To realize high-precision attitude stabilization of a flexible spacecraft in the presence of complex disturbances and measurement noises,an iterative learning disturbance observer(ILDO)is presented in this paper.Firstly,a dynamic model of disturbance is built by augmenting the integral of the lumped disturbance as a state.Based on it,ILDO is designed by introducing iterative learning structures.Then,comparative analyses of ILDO and traditional disturbance observers are carried out in frequency domain.It demonstrates that ILDO combines the advantages of high accuracy in disturbance estimation and favorable robustness to measurement noise.After that,an ILDO based composite controller is designed to stabilize the spacecraft attitude.Finally,the effectiveness of the proposed control scheme is verified by simulations.

A non-contact spacecraft architecture with extended stochastic state observer based control for gravity mission

A novel non-contact spacecraft architecture with the extended stochastic state observer for disturbance rejection control of the gravity satellite is proposed.First,the precise linear driving non-contact voice-coil actuators are used to separate the whole spacecraft into the non-contact payload module and the service module,and to build an ideal loop with precise dynamics for disturbance rejection control of the payload module.Second,an extended stochastic state observer is enveloped to construct the overall nonlinear external terms and the internal coupled terms of the payload module,enabling the controller design of the payload module turned into the linear form with simple bandwidth-parameterization tuning in the frequency domain.As a result,the disturbance rejection control of the payload module can be explicitly achieved in a timely manner without complicated tuning in actual implementation.Finally,an extensive numerical simulation is conducted to validate the feasibility and effectiveness of the proposed approach.

Fault diagnosis using robust cascade observers with application to spacecraft attitude control

This paper proposes a new gyro and star sensor fault diagnosis architecture that designs two groups of cascade H∞ optimal fault observers using LMI for spacecraft attitude control systems.The basic idea of the approach is to identify the gyro fault to good effect first and then makes a further diagnosis for the star sensor based on the former.The H∞ optimal fault observer in design has the robustness with respect to model uncertainties and diagnosis uncertainties.Its robustness to unknown inputs is as a special study in frequency domain.Finally,simulation results demonstrate the effectiveness and feasibility of the proposed control algorithm.

Robust Adaptive Attitude Maneuvering and Vibration Reducing Control of Flexible Spacecraft Under Input Saturation

A robust adaptive control scheme is proposed for attitude maneuver and vibration suppression of flexible spacecraft in situations where parametric uncertainties,external disturbances,unmeasured elastic vibration and input saturation constraints exist. The controller does not need the knowledge of modal variables but the estimates of modal variables provided by appropriate dynamics of the controller. The requirements to know the system parameters and the bound of the external disturbance in advance are also eliminated by adaptive updating technique. Moreover,an auxiliary design system is constructed to analyze and compensate the effect of input saturation,and the state of the auxiliary design system is applied to the procedure of control design and stability analysis. Within the framework of the Lyapunov theory,stabilization and disturbance rejection of the overall system are ensured. Finally,simulations are conducted to study the effectiveness of the proposed control scheme,and simulation results demonstrate that the precise attitude control and vibration suppression are successfully achieved.

Robust Adaptive Attitude Maneuvering and Vibration Reducing Control of Flexible Spacecraft with Prescribed Performance

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.

Cooperative Angles-Only Relative Navigation Algorithm for Multi-Spacecraft Formation in Close-Range

As to solve the collaborative relative navigation problem for near-circular orbiting small satellites in close-range under GNSS denied environment,a novel consensus constrained relative navigation algorithm based on the lever arm effect of the sensor offset from the spacecraft center of mass is proposed.Firstly,the orbital propagation model for the relative motion of multi-spacecraft is established based on Hill-Clohessy-Wiltshire dynamics and the line-of-sight measurement under sensor offset condition is modeled in Local Vertical Local Horizontal frame.Secondly,the consensus constraint model for the relative orbit state is constructed by introducing the geometry constraint between the spacecraft,based on which the consensus unscented Kalman filter is designed.Thirdly,the observability analysis is done and the necessary conditions of the sensor offset to make the state observable are obtained.Lastly,digital simulations are conducted to verify the proposed algorithm,where the comparison to the unconstrained case is also done.The results show that the estimated error of the relative position converges very quickly,the location error is smaller than 10m under the condition of 10−3 rad level camera and 5m offset.

液体大幅晃动充液航天器分数阶滑模姿态机动控制

研究部分充液航天器存在系统参数不确定、外部未知扰动、液体燃料大幅晃动的姿态机动控制问题。首先,将大幅晃动的液体燃料等效为运动脉动球模型,建立刚-液耦合航天器动力学方程;其次,针对充液航天器姿态稳定问题,提出一种具有固定时间收敛的分数阶非奇异快速终端滑模算法;同时,设计扰动观测器来估计系统的综合扰动,并且利用径向基函数神经网络来估计液体大幅晃动引起的扰动;设计Lyapunov函数分析并证明系统固定时间稳定性;最后,通过对比的仿真结果验证本文提出控制策略的可行性和有效性。

基于干扰观测器的多航天器固定时间姿态协同跟踪控制

针对受扰动下的多航天器系统姿态一致性跟踪问题,提出一种基于干扰观测器与固定时间理论的分布式姿态协同控制策略。多航天器系统在空间中易受太阳光压、重力梯度与地磁力等外部干扰影响而导致控制精度下降,此外航天器内部惯量不确定性带来的影响也不可忽视,因此设计一种基于二阶积分滑模的干扰观测器对复合干扰进行快速精确估计,且该观测器无需任何干扰先验信息。利用观测器获得的估计值,结合领导跟随法,提出一种分布式固定时间姿态协同控制器,实现系统固定时间收敛,收敛时间可通过控制参数进行调节。利用Lyapunov函数证明了系统的固定时间稳定性。仿真与对比试验结果表明,干扰观测器能在较快时间内实现对复合干扰准确估计,协同控制器能在固定时间内实现对期望姿态鲁棒跟踪,与自适应有限时间方法相比,该控制策略精度更高、收敛速度更快和燃油消耗更少。

应用变速控制力矩陀螺的航天器姿态自适应控制

为提高敏捷挠性航天器在轨连续机动的快速性和高稳定性,应用变速控制力矩陀螺(variable speed control moment gyroscopes,VSCMGs)作为姿态控制执行机构,提出了一种将观测器与自适应控制结合的姿态控制律与VSCMGs复合操纵律。考虑到机动过程中挠性模态及精确惯量不可知,采用模态观测器和转动惯量估计器对不可测的状态或参数进行辨识,辨识结果用于精确估计前馈补偿力矩,利用Lyapunov分析方法证明了闭环控制系统的稳定性。鉴于VSCMGs实际使用的力矩分配能力、避奇异能力、轮速平衡能力与末态框架角定位能力,分别设计了加权伪逆操纵律与3种对应的零运动。基于雅可比矩阵条件数提出了末态框架角的优选方法,给出了VSCMGs零运动在机动过程不同阶段的部署方案。结果表明:通过连续姿态机动数值仿真验证了所提算法的有效性;VSCMGs在连续机动过程中平滑切换模式,在不同的机动阶段实现了相应功能。模态观测值和惯量估计值在多次机动后收敛至真值附近,经过参数辨识后的控制器使航天器在机动末端更快更稳地达到指向精度要求。

考虑输入饱和的空间飞行器姿态神经鲁棒自适应滑模控制

针对空间飞行器姿态跟踪控制过程中出现的执行器饱和、模型不确定性和未知外部扰动问题,提出了一种鲁棒自适应径向基函数神经网络(RBFNN)增强滑模控制方法。首先,基于四元数法建立空间飞行器姿态运动学和动力学模型,并针对未知外部扰动建立非线性扰动观测器。其次,为了解决执行器饱和问题,引入高斯误差函数对控制器幅值进行约束,并以PID滑模控制为框架,对控制器进行设计。在控制器设计过程中,利用新型切换函数将鲁棒自适应控制和RBFNN进行结合,对模型不确定性进行逼近,并利用梯度下降法解决RBFNN权值优化问题。随后,基于Lyapunov理论证明闭环系统的有界性,并分析了闭环系统的收敛域。最后,通过仿真分析验证了所设计控制器的有效性和鲁棒性。

基于扰动观测器的大挠性航天器姿态预见控制

针对航天器的大型挠性附件振动会严重影响其姿态控制的问题,设计了基于扰动观测器的信息融合姿态预见控制器。将挠性附件振动引起的不确定项视作对姿态的复合扰动,设计扰动观测器对该复合扰动进行实时估计。在此基础上,通过融合期望轨迹和系统动态方程等信息,依据信息融合理论,推导了大挠性航天器的姿态预见控制律,用以改善航天器姿态系统的控制性能。所设计的姿态控制算法具有设计过程简单及容易实现等优点。仿真结果表明,所设计的控制策略对挠性附件振动具有较好的抑制作用,航天器姿态角能快速达到期望指令,具有良好的控制特性。

充液航天器姿轨耦合预设性终端滑模控制

主要研究充液航天器在存在参数不确定和外部扰动的情况下,姿轨耦合一体化运动与控制问题。将液体晃动等效为一阶弹簧质量模型,基于对偶四元数建立了充液航天器姿轨耦合动力学模型,按照动量与动量矩守恒定律推导充液航天器姿轨耦合动力学方程与运动学方程;考虑外界干扰和模型耦合特性,提出了一种基于指数形式的预设性能终端滑模控制策略,采用干扰观测器对综合扰动进行估计;并利用Lyapunov理论证明闭环系统的稳定性;最后,数值仿真结果表明,在设计的控制器作用下,充液航天器可以实现精确的姿轨控制,验证了控制器的有效性。

充液挠性航天器时变滑模控制及振动抑制

针对存在外界未知干扰、参数不确定问题的刚-液-柔多体耦合航天器姿态控制进行了研究.将液体燃料的晃动等效为球摆模型,挠性附件假设为欧拉-伯努利梁,建立了多体耦合航天器动力学方程.首先,设计了积分滑模干扰观测器,使其能够在有限的时间范围内对控制系统的集总扰动实现准确估计;其次,以此干扰观测器为基础,设计了一种时变滑模控制方法,该控制运用双曲正切函数;最后,结合光滑整形技术,设计出零振动指令光滑器,从而抑制液体晃动和挠性附件振动.数值仿真结果表明:本文所设计的控制方案具有可行性和有效性,多模态充液挠性航天器在姿态机动过程中所引起的残余振动可以得到有效抑制.

基于观测器的航天器执行机构失效故障重构

针对刚体航天器在轨运行时存在执行机构失效故障和外部干扰问题,提出了一种基于观测器的故障检测与重构方法。该方法运用迭代学习技术,利用上一步的执行机构故障观测信息来估计当前的故障信息,并最终实现故障的精确重构。此外基于Lyapunov方法从理论上证明了此故障诊断方法能够实时、在线地实现对常值、时变甚至随机型的执行机构失效故障的重构,并且对外干扰力矩具有一定的鲁棒性。最后,仿真结果验证了该方法的有效性。

基于模态观测器的挠性航天器姿态控制

针对带有挠性附件航天器的姿态跟踪问题,提出了基于挠性模态观测器的滑模控制律。采用混合坐标法建立挠性航天器动力学模型,构造挠性模态观测器观测挠性模态位移及其变化率。选择一类滑模面,用Lya-punov方法得出基于挠性模态观测器的滑模控制律,并给出了稳定性证明。分别在变速率姿态跟踪,恒速率姿态跟踪和零速率姿态跟踪的情况下进行了仿真。仿真结果显示,与一般的滑模控制律相比,提出的控制律能够有效提高姿态控制的稳态精度,减小挠性模态振动对姿态控制的影响。

基于矢量观测确定飞行器姿态的算法综述

随着无陀螺飞行器的出现 ,关于矢量定姿算法的研究已引起重视 .这方面的理论基础比较薄弱 ,有待解决的问题还很多 ,为给今后的研究工作提供有用的参考 ,按照确定性算法与状态估计法的分类原则 ,介绍近几十年来国外研究者在矢量定姿算法研究方面所取得的主要研究成果 ,阐述典型算法的基本思想及其发展状况 ,并进行比较分析 .结合航天科技发展现状及趋势 ,特别指出了无陀螺矢量定姿法研究的重要现实意义 :不仅为促进小型 (无陀螺 )飞行器的发展奠定了理论基础 。

航天器姿态机动及稳定的自抗扰控制

针对航天器动力学参数不确定性以及系统存在外部持续干扰的问题,提出了一种自抗扰姿态控制器的设计方法.在为期望姿态安排过渡过程的基础上,设计了扩张状态观测器,对参数不确定性和外部干扰进行估计,并实时补偿.为抑制跟踪误差,设计了非线性状态误差反馈律.仿真结果表明,该控制器不仅能很好地估计并补偿系统受到的持续干扰,而且对航天器动力学参数的不确定性具有较强的鲁棒性,满足航天器姿态快速机动和高稳定度的控制要求,性能指标明显优于PD控制.