基于扩张状态观测器的无拖曳系统参数辨识

分析了卫星无拖曳控制系统的在轨参数辨识问题,由于无拖曳系统的不稳定性质,需要设计控制器使其稳定,在此基础上进行闭环辨识.根据自抗扰控制原理,设计了扩张状态观测器以估计系统不同控制回路的扰动和状态,基于状态和扰动估计值设计控制器使系统稳定.提出了基于扩张状态观测器（ESO）的多输入多输出系统闭环参数辨识方法.为提高实际应用中的辨识效果,引入积分型滤波器对观测状态中的噪声进行抑制.将这种方法应用于类似LISA Pathfinder的单轴无拖曳模型,对系统动力学参数进行估计,通过数值仿真实验验证了该辨识方法的有效性和实用性.      

The determination for ideal release point of test masses in drag-free satellites for the detection of gravitational waves

Gravitational waves are ripples in space–time predicted by Albert Einstein's general relativity and provide a new way to understand the universe. Space-borne detectors of gravitational waves, extending to very large scales, can effectively detect the middle and low-frequency gravitational wave source with the frequency band of 0.1 mHz–1 Hz. The test masses are used to make an inertial reference point in the detection of gravitational waves. Currently, there are few studies concerning the ideal release position for the test masses in the detection of gravitational waves. In this study, we give a general solution for test mass release points to minimize the relative motion between the test mass and the satellite mass center. Moreover, we discuss the situation when the release point equation is not satisfied, and the ideal release point of the along-track. Finally, we report on simulations that verify the accuracy of the theoretical derivation.     

Estimating low resolution gravity fields at short time intervals to reduce temporal aliasing errors

The Gravity Recovery and Climate Experiment (GRACE) satellite mission has been estimating temporal changes in the Earth’s gravitational field since its launch in 2002. While it is not yet fully resolved what the limiting source of error is for GRACE, studies on future missions have shown that temporal aliasing errors due to undersampling signals of interest (such as hydrological variations) and errors in atmospheric, ocean, and tide models will be a limiting source of error for missions taking advantage of improved technologies (flying drag-free with a laser interferometer). This paper explores the option of reducing the effects of temporal aliasing errors by directly estimating low degree and order gravity fields at short time intervals, ultimately resulting in data products with improved spatial resolution. Three potential architectures are considered: a single pair of polar orbiting satellites, two pairs of polar orbiting satellites, and a polar orbiting pair of satellites coupled with a lower inclined pair of satellites. Results show that improvements in spatial resolution are obtained when one estimates a low resolution gravity field every two days for the case of a single pair of satellites, and every day for the case of two polar pairs of satellites. However, the spatial resolution for these cases is still lower than that provided by simply destriping and smoothing the solutions via standard GRACE post-processing techniques. Alternately, estimating daily gravity fields for the case of a polar pair of satellites coupled with a lower inclined pair results in solutions with superior spatial resolution than that offered by simply destriping and smoothing the solutions.     

Validation of the in-flight calibration procedures for the MICROSCOPE space mission

The MICROSCOPE space mission aims to test the Equivalence Principle with an accuracy of 10^-15${10}^{-15}$. The drag-free micro-satellite will orbit around the Earth and embark a differential electrostatic accelerometer including two cylindrical test masses submitted to the same gravitational field and made of different materials. The experience consists in testing the equality of the electrostatic acceleration applied to the masses to maintain them relatively motionless. The accuracy of the measurements exploited for the test of the Equivalence Principle is limited by our a priori knowledge of several physical parameters of the instrument. These parameters are partially estimated on-ground, but with an insufficient accuracy, and an in-orbit calibration is therefore required to correct the measurements. The calibration procedures have been defined and their analytical performances have been evaluated. In addition, a simulator software including the dynamics model of the instrument, the satellite drag-free system and the perturbing environment has been developed to numerically validate the analytical results. After an overall presentation of the MICROSCOPE mission, this paper will describe the calibration procedures and focus on the simulator. Such an in-flight calibration is mandatory for similar space missions taking advantage of a drag-free system.     

双星跟飞立体成像的构形保持控制

针对强干扰及输出饱和条件下微小双星立体成像的构形保持问题，提出一种基于观测器的抗干扰复合控制策略.根据立体成像双星跟飞运动机理，建立双星相对运动动力学模型；设计了一种自适应干扰观测器，可同时实现系统状态和干扰信息的在线估计，并采用Lyapunov稳定性理论和线性矩阵不等式技术给出观测器存在条件.采用极点配置方法改善观测器系统的动态性能，引入指数衰减因子提高控制器的收敛速度.考虑执行机构的输出饱和特性，提出一种加权PD+LQR反馈与干扰前馈补偿的复合控制策略，能够抑制未知干扰的影响，保证系统的动态和稳态性能，具备双星构形保持控制能力.仿真结果验证了所提算法的有效性.      

Finding the suitable drag-free acceleration noise level for future low-low satellite-to-satellite tracking geodesy missions

This paper evaluates the impact of residual acceleration noise on the estimation of the Earth’s time-varying gravity field for future low-low satellite-to-satellite tracking missions. The goal is to determine the maximum level of residual acceleration noise that does not adversely affect the estimation error. The Gravity Recovery And Climate Experiment (GRACE) has provided monthly average gravity field solutions in spherical harmonic coefficients for more than a decade. It provides information about land and ocean mass variations with a spatial resolution of ～350?km and with an accuracy within 2?cm throughout the entire Earth. GRACE Follow-on was launched in May 2018 to advance the work of GRACE and to test a new laser ranging interferometer, which measures the range between the two satellites with higher precision than the K-Band ranging system used in GRACE. Moreover, there have been simulation studies that show, an additional pair of satellites in an inclined orbit increases the sampling frequency and reduces temporal aliasing errors. Given the fact that future missions will likely continue to use the low-low satellite-to-satellite tracking formation with laser ranging interferometry, it is expected that the residual acceleration noise will become one of the largest error contributor for the time-variable gravity field solution. We evaluate three different levels of residual acceleration noise based on demonstrated drag-free systems to find a suitable drag-free performance target for upcoming geodesy missions. We analyze both a single collinear polar pair and the optimal double collinear pair of drag-free satellites and assume the use of a laser ranging interferometer. A partitioned best linear unbiased estimator that was developed, incorporating several novel features from the ground up is used to compute the solutions in terms of spherical harmonics. It was found that the suitable residual acceleration noise level is around 2?×?10^?12?ms^?2?Hz^?1/2. Decreasing the acceleration noise below this level did not result in more accurate gravity field solutions for the chosen mission architecture.     

卫星编队飞行的相对轨道自主确定研究

为了研究卫星编队飞行相对轨道的自主确定,基于相对轨道根数建立编队卫星间的相对运动方程,利用测量所得到的星间距离和方位信息作为观测量。不同于目前广泛采用的扩展卡尔曼滤波算法,设计Unscented Kalman Filter(UKF)算法实现卫星编队飞行的相对轨道自主确定。仿真结果表明这种相对轨道自主确定方案能获得较高的定轨精度。     

Pose measurement and motion estimation of non-cooperative satellite based on spatial circle feature

Malfunctioned satellites have seriously threatened orbital safety, and the capture of these satellites is of great significance. The pose measurement and the motion estimation of the tumbling satellite is the premise of capture. In this paper, the docking ring of the satellite is identified, which is equivalent to a spatial circle. Combined with the nozzle feature, the pose duality of the spatial circle can be eliminated. And the measurement accuracy is improved by minimizing the reprojection error of the docking ring and the nozzle. Due to the symmetry of the docking ring, the measured pose has only five degrees of freedom, losing the degree of freedom of rotation around the normal vector. In the motion estimation algorithm, the observability of the tumbling motion is firstly analyzed, then an error-state Kalman filter with inertia ratio constraints is designed. To improve the convergence speed and stability of the filter, a rough estimation algorithm of filter initial value based on linear term extraction and particle swarm optimization is proposed. The effectiveness of the pose measurement and motion estimation method is verified by simulations.     

基于双滤波器的高精度目标跟踪估计方法

针对连续推力的合作航天器，采用双重无迹卡尔曼滤波(DUKF)算法估计其状态和加速度。通过状态滤波器和参数滤波器的配合，提升滤波精度，完成运动状态和参数的估计，从而实现合作目标的运动轨迹跟踪。与合作航天器相比，非合作航天器存在大小未知、发生时刻未知的机动，无法获得加速度，且信息获取和运动状态的估计难度大。针对非合作航天器，基于简化的相对运动方程，结合天基平台获得目标的观测信息，采用两个扩展卡尔曼滤波(EKF)及基于半正焦弦的机动检测策略实现多未知脉冲机动的运动状态的估计。仿真结果表明：相比于无迹卡尔曼滤波（UKF），DUKF在对合作航天器的状态和加速度估计方面具有更快的收敛速度和更高的滤波精度；对于存在未知机动的非合作航天器，通过对比验证机动检测策略与滤波器切换策略相结合的方法的有效性，该方法能够检测到多次机动并且减少误判。     

异构多星编队的脉冲构型保持控制方法

近地轨道的双星编队通常设计具有自稳定性的编队构型参数初值，通过保持编队构型参数形成长期稳定的相对周期运动。针对编队中卫星数量增多产生的相对运动耦合问题，提出了基于Hill坐标和三角函数公式的多星相对运动分析方法。基于SAR载荷测量基线定义，结合多星编队构型参数的相对运动特性，提出了编队构型参数的设计方法，能够实现多星编队的最大有效基线组合。通过分析J2项摄动和大气阻力摄动的长期影响，研究了异构多星编队的相对运动衍化规律，提出了主从形式的脉冲偏置控制，能够有效保持针对异构多星编队设计的编队构型。通过面质比异构的四星编队控制仿真，验证了脉冲偏置控制形式下异构多星编队构型保持控制方法的有效性。     

一种基于双目视觉的卫星相对位姿测量方法

卫星位姿测量技术是与航天飞行器相关的空间操控活动中一项核心的支撑技术.考虑到太空环境中光源单一，且卫星表面一般为反光材质，提出了一种基于双目视觉的卫星相对位姿测量方法.在卫星面板标志点可见时，本文方法利用卫星对接环外环和卫星面板标志点测量卫星相对位姿.在卫星距离对接目标较近，无法观测到卫星面板标志点时，本文方法利用ORB特征点匹配测量位姿.为了增强鲁棒性，本方法还利用光流追踪法和卡尔曼滤波器优化位姿测量的结果.仿真实验结果表明，本方法能够在光源单一背景下对任意种类的卫星在对接过程中进行准确的位姿测量.     

编队飞行卫星群描述及摄动分析

定义了一组新的编队飞行卫星群相对运动描述参数 ,以及基于此的构型设计方法 ;并分析了地球非球形摄动对这组参数的影响 ,给出了为消除摄动长期影响所需的速度脉冲量 ,并对部分参数给出了减少摄动影响的设计方法     

一种提高卫星相对轨道运动全物理仿真逼真度的新方法

现有的卫星控制系统全物理仿真很少对卫星绕地球的轨道运动进行模拟,即使在卫星间相对轨道运动的全物理仿真中也没有考虑地心引力差和惯性力项的存在,因此其逼真度受到了影响.提出一种在共面圆轨道近距离卫星相对运动全物理仿真中引入地心引力差和惯性力项的方法,提高了物理仿真实验的逼真度.     

基于扩展卡尔曼滤波的舰机相对位姿估测

通过将基于扩展卡尔曼滤波的长序列图像分析方法与单目视觉技术相结合,把无人机自主着舰视觉导引中舰机间相对位姿的估测,转化为机载摄像机对着舰靶标平面3D位姿的实时估测问题.首先根据透视投影理论,建立了以摄像机的透镜中心为原点且Z轴与光轴重合的摄像机坐标系和世界坐标系,然后利用机载摄像机连续拍摄的靶标图像序列,选择描述相对运动的3个欧拉角、平移向量及它们的速度作为状态变量;由靶标角点的提取和帧间匹配,建立了反映着舰靶标上特征点的图像坐标和状态变量之间关系的观测方程,带入扩展卡尔曼滤波器,估测出舰机的相对运动参数.计算机数据仿真和基于DSP平台的半实物仿真试验验证了算法的有效性和鲁棒性.      

基于卫星自旋的纯引力轨道万有引力干扰抑制

一个称为“内卫星”的验证质量块位于外卫星的内部空腔中,不与外卫星接触而自由飞行,由于外卫星的屏蔽,其不受大气阻力、太阳光压等干扰作用,沿着纯引力轨道飞行。外卫星会对内卫星产生万有引力作用,是内卫星纯引力轨道的主要残余干扰。根据卫星相对运动动力学方程,建立了万有引力干扰对内卫星纯引力轨道影响的分析模型;基于将外卫星绕轨道面法向旋转以调制万有引力的策略,建立了外卫星自旋对万有引力干扰影响的抑制模型。以内编队纯引力飞行系统为例,对比计算了外卫星有无自旋时万有引力干扰对内卫星纯引力轨道的影响。基于模型的分析表明,外卫星自旋能够显著抑制万有引力干扰对内卫星纯引力轨道的长期影响;实例计算表明,万有引力干扰的天长期影响能够降低5~7个数量级。     

Modeling and identification of an electro-mechanical system: The LISA grabbing positioning and release mechanism case

In the frame of space missions, mechanisms often constitute critical systems whose functionality and performance need to be tested on ground before the mission launch. The LISA scientific space mission will detect gravitational waves by measuring the relative displacement of pairs of free-floating test masses set into geodesic motion onboard of three spacecrafts. Inside each satellite, the injection of the test masses from the caged configuration into the geodesic trajectory will be performed by the grabbing positioning and release mechanism. To provide a successful injection, the test masses must be dynamically released with a minimal residual velocity against adhesion with the holding device. A parameter that determines the test mass residual velocity is the quickness of the retraction of the holding device. The need arises then to characterize the dynamic response of the release mechanism in order to predict its behaviour in the in-flight conditions. Once a validated model of the mechanism is available, the compliance of the system to the tight requirement on the maximum allowed residual velocity of the test mass may be verified. Starting from an electro-mechanical model of the mechanism dynamics, this paper presents the results of the experimental identification of its relevant parameters.     

Modeling spatial density in low earth orbits using wavelets and random search

The events that occurred after 2007 such as Chinese anti-satellite test, explosion of Briz-M upper stage, break up of cosmos-2421 and collision of cosmos-2251 with Iridium-33 satellites have completely changed the spatial density patterns in low earth orbits. This has increased the risk of collision between active satellites and debris created by them. Aftermath, the risk assessment of possible collisions called as conjunction analysis of working satellites from day to day has become more crucial. Spatial density models are useful in understanding the long-term likelihood of a collision in a particular region of space and helpful in pre-launch orbit planning. In this paper we present an algorithmic procedure for automatically estimating exact model parameters corresponding to the peak location and number of peaks using wavelets that will speed up the parameter estimation process for the models with peaks.     

Perception of gravity by plants.

Physical principles can be used to predict some features about the gravity perception system in plants. The nature of the system has made it rather elusive, so this approach represents an additional source of information to help find it. For a gravitational stimulus to be detected, two masses must move relative to each other in a manner which causes a significant amount of work to be done on a receptor. Relative to cellular dimensions, the masses must be large, be dense and move noticeable distances. The main sources of noise are thermal motion and flexing of the plant tissue. Some new models for the function of amyloplasts as statoliths are presented.     

一种巡视器惯性/视觉组合导航新方法

在以运动参数误差为状态量、视觉导航与惯导导航相对运动参数差为观测量 的传统惯性/视觉组合导航方法中, 为解决相对运动参数同时与前后两个时 刻状态相关的问题, 采用将前一时刻位置和姿态误差增广到状态量中的方法, 并且假设增广的状态量为常值, 导致状态模型中引入了较大的误差. 基于 真实位置、姿态建立观测量误差模型, 导致观测量同时与前后两个时刻的状 态相关. 本文以惯导误差方程为状态模型, 采用四元数差形式的相对运动 参数差作为观测量, 基于上一时刻组合导航位置、姿态估计值建立观测量误 差模型, 实现了状态的增广, 并使得量测信息仅与当前时刻的位置误差和平 台失准角相关, 克服了状态模型误差较大的问题. 月面仿真和地面模拟实验 均表明, 该方法能够达到较高的位置和姿态估计精度.      

Symmetry,reduction and relative equilibria of a rigid body in the J2 problem

The J₂ problem is an important problem in celestial mechanics, orbital dynamics and orbital design of spacecraft, as non-spherical mass distribution of the celestial body is taken into account. In this paper, the J₂ problem is generalized to the motion of a rigid body in a J₂ gravitational field. The relative equilibria are studied by using geometric mechanics. A Poisson reduction process is carried out by means of the symmetry. Non-canonical Hamiltonian structure and equations of motion of the reduced system are obtained. The basic geometrical properties of the relative equilibria are given through some analyses on the equilibrium conditions. Then we restrict to the zeroth and second-order approximations of the gravitational potential. Under these approximations, the existence and detailed properties of the relative equilibria are investigated. The orbit–rotation coupling of the rigid body is discussed. It is found that under the second-order approximation, there exists a classical type of relative equilibria except when the rigid body is near the surface of the central body and the central body is very elongated. Another non-classical type of relative equilibria can exist when the central body is elongated enough and has a low average density. The non-classical type of relative equilibria in our paper is distinct from the non-Lagrangian relative equilibria in the spherically-simplified Full Two Body Problem, which cannot exist under the second-order approximation. Our results also extend the previous results on the classical type of relative equilibria in the spherically-simplified Full Two Body Problem by taking into account the oblateness of the primary body. The results on relative equilibria are useful for studies on the motion of many natural satellites, whose motion are close to the relative equilibria.