基于Hankel矩阵填充的稀疏孔径ISAR成像

在稀疏孔径(SA)逆合成孔径雷达(ISAR)成像中,传统压缩感知(CS)方法使用稀疏信号处理来处理数据缺失下的成像问题。这类方法存在模型不匹配这一固有问题,在一定程度上会限制成像质量。提出了一种利用Hankel矩阵填充(HMC)的基于结构化稀疏ISAR成像方法。该方法是一种典型的无网格方法,可以有效地提高稀疏孔径ISAR成像性能。首先,建立ISAR稀疏孔径成像信号模型,根据每个距离单元的回波构造Hankel矩阵;其次,通过证明所构造Hankel矩阵的低秩性质,作为方位稀疏成像的先验信息约束;最后,通过逐步迭代求解基于增广拉格朗日乘子(ALM)的矩阵填充(MC)来实现重构方位维成像。提出的基于低秩约束的方法,可以避免过完备基的假设,有效地克服了CS方法的离网格效应。基于实测数据的实验分析,进一步验证了所提算法的有效性。     

考虑高耗时约束的全电推进卫星多学科优化

为提高卫星系统整体性能与优化设计效率，本文采用多学科设计优化（MDO）方法进行全电推进卫星总体参数优化。主要考虑轨道转移、位置保持、空间环境、供配电、结构及质量六个学科，以整星质量最小为优化目标，考虑轨道转移时间等约束条件，建立了全电推进卫星MDO模型。提出一种基于增广拉格朗日乘子法的高效全局优化方法（ALM-EGO）以快速求解卫星MDO问题。标准数值算例对比研究表明，对于处理高耗时约束优化问题，ALM-EGO方法在全局收敛性与优化效率方面具有一定的优势。最后，采用ALM-EGO求解全电推进卫星MDO问题，优化后的全电推进卫星设计方案满足各类工程设计约束，实现整星减重161.09 kg，从而验证了本文所建立模型的合理性和ALM-EGO方法的有效性。     

Motion equations of hemispherical resonator and analysis of frequency split caused by slight mass non-uniformity

The mass non-uniformity of hemispherical resonator is one of reasons for frequency split, and frequency split can cause gyroscope to drift. Therefore, it is of great significance to analyze the relationship between mass non-uniformity and frequency split, which can provide a theoretical basis for mass balance of imperfect resonator. The starting point of error mechanism analysis for gyroscope is the motion equations of resonator. Firstly, based on the Kirchhoff-Love hypothesis in the elastic thin shell theory, the geometric deformation equations of resonator are deduced. Secondly, the deformation energy equation of resonator is derived according to the vibration mode and relationship between the stress and strain of hemispherical thin shell. Thirdly, the kinetic energy equation of resonator is deduced by the Coriolis theorem. Finally, the motion equations of resonator are established by the Lagrange mechanics principle. The theoretical values of precession factor and natural frequency are calculated by the motion equations, which are substantially consistent with the ones by the finite element method and practical measurement, the errors are within a reasonable range. Simultaneously, the varying trend of natural frequency with respect to the geometrical and physical parameters of resonator by the motion equations is consistent with that by the finite element analysis. The above conclusions prove the correctness and rationality of motion equations. Similarly, the motion equations of resonator with mass non-uniformity are established by the same modeling method in case of ignoring the input angular rate and damping, and the state equations with respect to the velocity and displacement of vibration system are derived, then two natural frequencies are solved by the characteristic equation. It is concluded that one of reasons for frequency split is the 4th harmonic of mass non-uniformity, and thus much attention should be paid to minimizing the 4th harmonic of mass non-uniformity in the course of mass balancing for imperfect resonator.     

A fast Chebyshev polynomial method for calculating asteroid gravitational fields using space partitioning and cosine sampling

This paper presents a computationally fast method for solving gravitational accelerations near irregularly-shaped asteroids. This method is based on analytical three-dimensional Chebyshev polynomial approximation of the polyhedral gravity. For the purpose of improving the approximation accuracy, space partitioning schemes based on practical flight zones is used to avoid interpolation the whole space around the target asteroid. Specifically, a minimum ellipsoid close to the asteroid surface is defined to select the space for surrounding trajectories with safe distance and a cone connected to the surface is defined to select the space for descent trajectories. Moreover, interpolation points are sampled in a cosine sampling fashion according to the Chebyshev-Gauss-Lobatto nodes and a radial adaption technique. The performance of different space partitioning schemes is analyzed. The effectiveness of the proposed method is validated through simulations of solving gravitational accelerations at the test points near different shaped asteroids 1996 HW1, 433 Eros, 25143 Itokawa and 101955 Bennu.     

The feasibility of using an L1 positioned dust cloud as a method of space-based geoengineering

In this paper a method of geoengineering is proposed involving clouds of dust placed in the vicinity of the L₁ point as an alternative to the use of thin film reflectors. The aim of this scheme is to reduce the manufacturing requirement for space-based geoengineering. It has been concluded that the mass requirement for a cloud placed at the classical L₁ point, to create an average solar insolation reduction of 1.7%, is 7.60 × 10¹⁰ kg yr⁻¹ whilst a cloud placed at a displaced equilibrium point created by the inclusion of the effect of solar radiation pressure is 1.87 × 10¹⁰ kg yr⁻¹. These mass ejection rates are considerably less than the mass required in other unprocessed dust cloud methods proposed and are comparable to thin film reflector geoengineering requirements. Importantly, unprocessed dust sourced in-situ is seen as an attractive scheme compared to highly engineered thin film reflectors. It is envisaged that the required mass of dust can be extracted from captured near Earth asteroids, whilst stabilised in the required position using the impulse provided by solar collectors or mass drivers used to eject material from the asteroid surface.     

基于配点法的轨道在线优化方法研究

针对传统轨道优化方法无法应用于轨道在线优化的问题,提出了三阶辛普森配点法结合乘子法求解轨道在线优化问题的方法.首先建立变轨的最优化模型;然后,用三阶辛普森配点法将该最优控制问题转化为受约束的非线性规划问题;最后,采用增广拉格朗日乘子法对转化得来的受约束的非线性规划问题进行求解.仿真结果表明,所提出的方法可以得到高精度的轨道优化结果,并且对状态量和控制量的初值选取不敏感,且仿真具有实时性,计算速度快,可以达到在线进行轨道优化的要求.     

空间红外遥感相机制冷机微振动对MTF影响分析

空间红外遥感相机内部活动部件微振动载荷会引起相机光学元件产生刚体位移和面形变化,从而影响相机的在轨光学成像品质。文章基于某空间红外遥感相机内部扰振源的测试结果,建立了适用于微振动分析的相机精细有限元模型,经分析计算,确定了基于Zernike多项式曲面拟合和调制传递函数(MTF)分析的方法和流程;最后结合某空间红外遥感相机进行MTF影响分析,结果显示制冷机微振动对该相机MTF影响在工程可接受的范围内。     

地月第二拉格朗日点卫星激光测距技术研究

介绍了月球/中继星激光测距的科学意义、研究状况和发展趋势;研究了地月第二拉格朗日点(L2点)纯反射式激光测距技术和任务设计,主要包含单体大孔径激光角锥反射器的设计与研制,以及基于云南天文台1.2 m望远镜的月球/中继星激光测距系统研究。研究结果表明:采用单脉冲能量3 J和10 ns脉宽脉冲激光器,预期系统能接收到的单脉冲回波光电子数约为0.74,单光子测距精度优于1 m。     

基于拉格朗日力学的伞-弹系统动力学模型

为了回避牛顿力学体系在多体动力学建模过程中存在的复杂约束力求解问题,以拉格朗日力学体系为基础,从分析力学的角度出发,建立了铅垂平面内降落伞-弹系统五自由度动力学模型。详细介绍了拉格朗日力学体系动力学建模过程,推导了铅垂平面内伞-弹系统动能公式,解决了伞-弹系统广义力求解问题,给出了伞-弹所受空气动力的广义力表示。算例将建立的伞-弹系统模型和牛顿力学体系下简化的伞-弹模型进行对比,两种模型计算结果吻合,验证了本文模型的正确性。降落伞-弹系统动力学模型验证了拉格朗日力学在常规飞行器动力学建模中的可行性,为使用ADAMS等基于拉格朗日力学思想建模的商业软件进行伞-弹系统动力学求解提供理论依据,可以用于指导伞-弹系统的分析和设计。     

含时滞结构线性时变系统伺服机构设计的PGOPO法

采用线性、连续、有界算子－分段广义正交多项式算子对含时滞结构的线性时变系统的线性伺服机构设计问题进行了研究,得到了一种形式简洁、计算精度较Ｗａｌｓｈ函数、块脉冲函数法更高的逼近方法。文中所列举的两个例子充分说明了分段广义正交多项式算子法在时滞系统伺服机构设计中的适用性和有效性。