Spin motion determination of the Envisat satellite through laser ranging measurements from a single pass measured by a single station

The Satellite Laser Ranging (SLR) technology is used to accurately determine the position of space objects equipped with so-called retro-reflectors or retro-reflector arrays (RRA). This type of measurement allows to measure the range to the spacecraft with high precision, which leads to determination of very accurate orbits for these targets. Non-active spacecraft, which are not attitude controlled any longer, tend to start to spin or tumble under influence of the external and internal torques and forces.If the return signal is measured for a non-spherical non-active rotating object, the signal in the range residuals with respect to the reference orbit is more complex. For rotating objects the return signal shows an oscillating pattern or patterns caused by the RRA moving around the satellite’s centre of mass. This behaviour is projected onto the radial component measured by the SLR.In our work, we demonstrate how the SLR ranging technique from one sensor to a satellite equipped with a RRA can be used to precisely determine its spin motion during one passage. Multiple SLR measurements of one target over time allow to accurately monitor spin motion changes which can be further used for attitude predictions. We show our solutions of the spin motion determined for the non-active ESA satellite Envisat obtained from measurements acquired during years 2013–2015 by the Zimmerwald SLR station, Switzerland. All the necessary parameters are defined for our own so-called point-like model which describes the motion of a point in space around the satellite centre of mass.     

小天体动能撞击防御中动量传递因子敏感参数分析

动量传递因子β是评估动能撞击效果的重要参量。根据动能撞击过程中动量传递因子的理论模型,分析了撞击器特性参数和小天体结构特性参数对动量传递因子取值的影响,并对不同动能撞击方案以及不同材料特性小天体的成坑效应和动量传递因子进行分析。研究表明：标度律参数μ对β影响较大,μ是地面实验拟合得到的系数,与材料强度特性相关;当小天体为单体岩石结构时,撞击器速度及密度、小天体密度及表面强度对β影响较大,而撞击器半径和小天体引力对β影响较小;当小天体为碎石堆结构时,β对撞击器特性参数和小天体特性参数不敏感,且数值较小。对三种不同动能撞击方案的成坑效应与动量传递因子形成规律进行研究,发现撞击器初始动能对β影响较大。当小天体为单体岩石结构时,其对应的动量传递因子取值较大,而当小天体为碎石堆结构时,其对应的β取值较小且基本不变。对相同动能撞击方案下不同材料特性小天体（C型、S型和X型小行星）产生的撞击效应进行分析,发现在引力主导时,βC>βS>βX,而在强度主导时β取值较小且基本相同。     

平动点周期轨道间小推力转移的Gauss伪谱法

针对三体问题共线平动点附近周期轨道间的小推力转移问题，构造了一种新的形状函数，在此基础上提出了一种基于Gauss伪谱法的优化设计方法。首先，建立小推力轨道转移动力学模型，参考初始轨道和目标轨道的类型，构造一种新的形状函数以近似小推力转移轨道。为满足不同的约束要求，提出了振幅和相位按多项式变化的假设，推导了小推力转移轨道的近似解析解；然后利用Gauss伪谱法将小推力轨道转移的最优控制问题转化为非线性规划问题，并对推导的近似解析解进行解算和处理，为Gauss伪谱法求解非线性规划问题提供较为有效的控制变量的初始猜测值；最后以地月系统L1点附近Halo轨道间的小推力转移问题为例进行了仿真分析。仿真结果表明，小推力转移轨道近似解析解具备有效性和普适性，使得Gauss伪谱法的迭代效率提高55%以上，同时也表明Gauss伪谱法可有效解决平动点周期轨道间的小推力转移轨道优化设计问题。     

Shape optimization of a three-dimensional membrane-structured solar sail using an angular momentum unloading strategy

A shape of the satellite’s solar sail membrane is essential for unloading angular momentum in the three-axis stabilized attitude control system because the three-dimensional solar sail can receive solar radiation pressure from arbitrary directions. In this paper, the objective is the shape optimization of a three-dimensional membrane-structured solar sail using the angular momentum unloading strategy. We modelled and simulated the solar radiation pressure torque, for unloading angular momentum. Using the simulation system, since the unloading angular momentum rate is maximized, the shape of the three-dimensional solar sail was optimized using a Genetic algorithm and Sequential Quadratic Programming. The unloading velocity in the optimized shaped solar sail was greatly improved with respect to a conventional flat or pyramid solar sail.     

基于窄带雷达RCS序列的空间目标尺寸估计

从窄带雷达横截面积(RCS)序列中提取目标尺寸特征对空间目标的分类识别具有重要价值. 本文在研究椭球体RCS序列特性的基础上, 提出了一种空间目标形状估计方法, 并进一步研究了空间目标尺寸估计问题, 提出了一种新的尺寸估计方法. 实测数据的验证表明, 该方法能有效估计卫星、碎片等空间目标的形状和二维尺寸.      

Spin axis orientation of Ajisai determined from Graz 2 kHz SLR data

The Graz 2 kHz Satellite Laser Ranging (SLR) measurements allow determination of the spin axis orientation of the geodetic satellite Ajisai. The high repetition rate of the laser makes it possible to determine the epoch time when the laser is pointing directly between two corner cube reflector (CCR) rings of the satellite. Identification of many such events during a few (up to 3) consecutive passes allows to state the satellite orientation in the celestial coordinate system. Six years of 2 kHz SLR data (October 2003–October 2009) delivered 331 orientation values which clearly show precession of the axis along a cone centered at 14^h56^m2.8^s in right ascension and 88.512° in declination (J2000.0 celestial reference frame) and with an half-aperture angle θ of 1.405°. The spin axis precesses with a period of 117 days, which is equal to the period of the right ascension of the ascending node of Ajisai’s orbit. We present a model of the axis precession which allows prediction of the satellite orientation – necessary for the envisaged laser time transfer via Ajisai mirrors.     

基于朝向约束和重识别特征的目标轨迹关联方法

基于检测关联和深度学习的目标轨迹关联方法是计算机视觉领域的研究热点之一，但现有方法设计中缺乏有效的时空约束，且目标表观特征泛化能力不足，在目标朝向差异明显的情况下会发生识别错误，在目标轨迹关联时会导致频繁的ID切换和错误关联。针对该问题，提出了一种基于朝向约束和重识别特征的目标轨迹关联方法。将行人朝向判别引入行人重识别中，提出了一种具有朝向约束力的行人重识别网络模型，提升了目标特征的表示能力。结合目标朝向、卡尔曼滤波得到的位置信息、重叠面积等时空特征，提出一种基于朝向约束的分层轨迹关联模型，得到单相机内的目标轨迹。在跨相机场景中，通过引入一种简单有效的双向竞争匹配机制，实现了目标轨迹的有效关联。实验结果表明: 所提方法在MOT数据集上度量指标优于多种方法，能够减少频繁的ID交换，有效解决了相似目标相向而行时的错误关联；帧率达到19.6帧/s，能够满足近实时场景下的使用要求。      

一种用于SGCMG奇异规避的CVP轨迹规划

采用单框架控制力矩陀螺(SGCMG)作为执行机构的小型敏捷卫星在姿态机动过程中存在着奇异问题.本文从SGCMG姿态控制系统整体出发，将奇异问题转化为状态约束的动态控制问题，基于控制变量参数化(CVP)方法，设计了一种用于SGCMG奇异规避的轨迹规划.该算法在实现小型敏捷卫星大角度姿态机动过程无奇异的基础上，将SGCMG框架角转速的最优轨迹通过CVP方法进行分段线性规划.这种规划策略对框架伺服系统的算法设计无复杂要求，仅需要简单的加减速控制，从而节约了星上资源.在轨迹规划实现过程中，考虑了工程实际中的约束条件，可以按照姿态机动任务要求规划出一条综合考虑能量资源和目标精度的最优轨迹.仿真结果表明：该算法实现了姿态参数轨迹和星体角速度轨迹的平缓变化，目标误差在1×10-3量级，星体在机动过程中运行稳定，SGCMG不会出现奇异现象.     

万向磁悬浮动量轮研究

目前大部分空间飞行器三轴姿态控制采用的是飞轮系统 ,其精度较高 ,而采用磁悬浮轴承支承的动量轮比普通的滚珠轴承动量轮有更高的姿控性能。在此基础上发展的动量矩方向可改变的万向磁悬浮动量轮可以单独实现三轴较高精度的主动姿态控制 ,具有重要的实用价值。同时 ,万向磁悬浮动量轮的研究也为集成能源与姿态控制系统的研究提供了技术支持。文意介绍了万向磁悬浮动量轮在国际上研究和发展的情况 ,概括了其中的关键技术 ,并给出了一种初步的结构设计实例。     

On the Lagrange and Hill stability of motion in the three-body problem

In the three-body problem, we consider the Lagrange and Hill stability including the Lagrange stability for the manifold of symmetric motions that exists in the case where two of three bodies have equal masses. To analyze the stability, in addition to integrals of energy and angular momentum we use the Lagrange–Jacobi equality. We prove theorems on the Lagrange and Hill stability. The theorem on the Hill stability has effective application in the case where the mass of a body is much less than masses of two other bodies. In this case, as it is known, the model of the restricted three-body problem is usually applied.     

The response of the high-latitude thermosphere to geomagnetic substorms

Enhanced ionization and electric fields associated with geomagnetic substorms lead to time-varying momentum (ion drag) and heat (Joule dissipation) sources in the high latitude thermosphere. These momentum and heat sources, in turn, cause changes in the structure of the wind, pressure and temperature fields, and, in addition, provide a mechanism for the generation of atmospheric gravity waves. We have developed a model which has been used to simulate, on a spatial scale of tens of kilometers, the response of the high latitude thermosphere to these momentum and heat sources, which constitute a major factor in the coupling between the magnetosphere, the ionosphere and the upper atmosphere. Simulations reported herein indicate that, because the coupling between ions and neutrals is highly dependent on the neutral-ion collision frequency, the response of the neutral atmosphere varies, not only as a function of total ionization, but also as a function of the details of the electron density profile.     

非合作目标安全走廊设计及飞越逼近轨迹优化

为提高空间非合作目标近距离逼近轨迹的安全性，同时对接近时间及所消耗燃料进行优化，针对空间失效自旋非合作目标近距离接近问题，给出了失效卫星动态安全走廊，并以飞越逼近方式抵达走廊入口，进一步提出了飞越逼近轨迹优化方法。首先，在建立失效卫星自旋模型的基础上，规划了安全区与禁飞区，提出了2种安全走廊的选择依据。其次，采用飞越逼近作为近距离接近方式，以节约燃料和缩短逼近时间为目标对两脉冲机动模型进行优化，选择3种优化算法得到接近轨迹。仿真结果表明:安全走廊的选择与卫星失效自旋的形式、外形以及接口位置有关；在飞越逼近两脉冲机动模型的优化问题中，采用Fgoalattain算法进行优化处理更具优越性。      

Delta-electron emission in fast heavy ion atom collisions.

Biological damages such as mutations, chromosomal aberrations etc. are a consequence of biochemical changes mostly in the DNA. With ionizing radiation, these chemical changes are due to primary ionization events and secondary ionization effects caused by the primarily produced electrons. Differences in the biological response of densely ionizing radiation, like heavy charged particles, in comparison to sparsely ionizing radiation, such as X- or gamma-rays, are mainly due to the differences in the production of the so called delta-electrons. Therefore, the emission process of electrons i.e. the cross section for the primary ionization event as well as the energy and angular distribution of the emitted electrons should be understood in detail. The delta-electron emission processes occuring in fast heavy ion atom collisions are explained qualitatively. The different spectral structures of electron emission arising from either the target or the projectile are explained in terms of simple models of the kinetics of momentum transfer induced by the COULOMB forces. In collisions of very heavy ions with matter, high nuclear COULOMB forces are created. These forces lead to a strong polarization of the electronic states of the participated electrons. The effects of this polarization are discussed.     

基于SD-LCMV算法的FDA平台外干扰抑制

针对频率分集阵列（FDA）接收机在抑制与目标位置接近的平台外干扰过程中,最小方差无失真响应（MVDR）波束形成器在阵元数较大、导向矢量失配情况下出现的主瓣畸变问题展开分析。在采用重叠正弦频率分集阵列接收结构代替一维均匀线性频率分集阵列（ULA-FDA）接收的基础上,通过可变加载约束的最速下降线性约束最小方差（SD-LCMV）准则求解导向矢量失配时的权矢量,实现对阵列方向图主瓣的有效纠偏和保形。仿真验证了采用正弦频控函数的重叠子阵FDA阵列具有最佳的主瓣宽度和低旁瓣特性。当存在2°的指向误差时,正弦频控函数的重叠子阵FDA阵列经导向矢量修正得到的阵列方向图在有效抑制干扰的同时能够实现主瓣的纠偏和保形。      

基于Frenet和改进人工势场的在轨规避路径自主规划

在轨道间机动的航天器规避空间目标，需兼顾沿转移轨道飞行的绝对运动和规避空间目标的相对运动，路径自主规划难度较大且目前国内外公开研究成果较少。针对上述问题，提出了一种将Frenet坐标系与改进人工势场相结合的在轨规避路径自主规划方法。首先，构建Frenet坐标系表述空间规避运动，解决了路径规划中航天器与既定转移轨道相对位置不易表述的难题，实现了空间规避运动的简便表示；其次，改进人工势场函数、调整势场作用区域，避免了传统人工势场法存在过早轨迹偏离以及局部震荡现象，实现了对空间目标的自主规避；最后，考虑规避安全、轨道保持、制动时效以及燃料消耗因素构建全局优化函数，能够满足不同任务的需求与偏好，实现了沿转移轨道飞行的最小偏移与快速恢复。算法比对与算例求解表明:所提方法应用优势明显，路径平滑、偏移量小，满足航天器规避空间目标的路径规划需求。      

Spin parameters of nanosatellite BLITS determined from Graz 2 kHz SLR data

The nanosatellite BLITS (Ball Lens In The Space) is the first object designed as a passive, spherical retroreflector of the Luneburg type, dedicated for Satellite Laser Ranging (SLR). The 2 kHz SLR station Graz measures spin parameters of this satellite, providing information about the rotational dynamics of the body. The measurements obtained during the period from September 26, 2009 to November 24, 2010 show a significant change of the spin configuration. The spin axis was dynamically precessing since the launch and currently is sinus-like behaving between coordinates RA 120°…150°, Dec 30°…60° (J2000 inertial reference frame). The angle between the symmetry axis and the spin axis of BLITS is not constant, but is decreasing since the launch, while its spin period is rather stable with a mean value of 5.613 s (clockwise rotation). The satellite was dynamically changing its attitude during the first three months after deployment; after this time the spin parameters are relatively stable.     

22 Years of AJISAI spin period determination from standard SLR and kHz SLR data

Satellite Laser Ranging (SLR) is a powerful and efficient technique to measure spin parameters of satellites equipped with corner cube reflectors. We obtained spin period determination of the satellite AJISAI from SLR data only: 17246 pass-by-pass estimates from standard 1–15 Hz SLR data (14/Aug/1986–30/Dec/2008) and 1444 pass-by-pass estimates (9/Oct/2003–30/Dec/2008) from data of the first 2 kHz SLR system from Graz, Austria. A continuous history of the slowing down of AJISAI spin is derived from frequency analysis, and corrected for the apparent effects. The apparent corrections, elaborated here, allowed very accurate determination of AJISAI initial spin period: 1.4855 ± 0.0007 [s]. The paper identifies also non-gravitational effects as a source of the periodical changes in the rate of slowing down of the satellite.     

Removing orbital debris with lasers

Orbital debris in low Earth orbit (LEO) are now sufficiently dense that the use of LEO space is threatened by runaway collision cascading. A problem predicted more than thirty years ago, the threat from debris larger than about 1 cm demands serious attention. A promising proposed solution uses a high power pulsed laser system on the Earth to make plasma jets on the objects, slowing them slightly, and causing them to re-enter and burn up in the atmosphere. In this paper, we reassess this approach in light of recent advances in low-cost, light-weight modular design for large mirrors, calculations of laser-induced orbit changes and in design of repetitive, multi-kilojoules lasers, that build on inertial fusion research. These advances now suggest that laser orbital debris removal (LODR) is the most cost-effective way to mitigate the debris problem. No other solutions have been proposed that address the whole problem of large and small debris. A LODR system will have multiple uses beyond debris removal. International cooperation will be essential for building and operating such a system.     

微重力池沸腾中的气泡和传热行为数值模拟

基于标准动量传输方程、连续性方程以及能量方程,建立了单个气泡的二维数值模型,考察了微重力过冷核态池内沸腾中的气泡动力学和传热行为.在动量传输方程中耦合了表面张力和Marangoni力,连续性方程和能量方程中耦合了相变效应,考虑了加热面上过热液体层的影响,并引入相场函数捕捉气液界面的动态变化.结果表明:气泡在生长过程中,形状由最初的半球形变为椭球形,最后变为脱附时的梨形;气泡在加热面上自由迁移,呈非轴对称结构,并且气泡上部的温度场呈现为蘑菇云状;气泡的脱附直径正比于g-0.488,脱附周期正比于g-1.113,加热面上的平均热通量正比于g0.229.