大长宽比单元有限元误差分析及高精度计算

针对大长宽比单元将传统的基于单元整体尺度的有限元误差估计扩展为基于单元不同方向尺度的误差估计;根据不同方向误差应该同量级的思想,得到了误差匹配准则.该准则可以作为网格划分的判据.根据这一匹配准则,提出了一种提高计算精度的方法——单向高次插值法.数值实验表明该误差估计是正确的,误差匹配准则作为网格划分的判据是有效的,采用合理的有限元插值逼近能够有效地减小计算误差,提高计算的精度和效率.      

基于CIT技术的广域增强系统电离层延迟修正算法

提出了一种基于电离层层析成像(CIT)技术的广域增强系统(WAAS)电离层延迟 修正算法. 该算法利用模式基函数与截断奇异值分解正则化组合的方式, 实现WAAS单频用户的电离层延迟误差修正. 基于中国区域23个广域基准站 和10个用户站的仿真结果分析表明, 传统的网格算法和基于CIT技术的电离 层延迟修正算法的电离层延迟修正精度与太阳活动、昼夜变化及地磁纬度 之间均存在明显的相关性. 基于CIT技术的电离层延迟修正算法精度优于网格算法, CIT算法的平均误差与标准差相比网格算法均有较大幅度的下降.      

高超声速气动热数值计算壁面网格准则

首先对高超声速气动热数值模拟中的3种壁面法向网格准则(平均自由程(MFP)、自由来流参数网格雷诺数和壁面参数网格雷诺数准则)进行了分析.随后,提出了壁面参数预估方法,使得壁面参数网格雷诺数和MFP两种准则无需进行试算即可直接预估壁面法向网格尺度.其次,运用提出的壁面参数预估方法结合不同网格准则,在同一实验条件下确定多个壁面法向网格尺度,通过与实验数据对比研究各网格准则确定的壁面法向网格尺度是否满足热环境模拟精度要求,并对提出的壁面参数预估方法进行验证.最后,开展了高超声速二维完全气体及真实气体效应算例、三维钝双锥算例的数值模拟.研究表明,壁面参数预估方法是可靠的;自由来流参数网格雷诺数准则随来流静温变化所确定的网格尺度变化趋势与物理推理相悖;基于壁面参数网格雷诺数和MFP两种准则主要受壁面温度影响,其在同一条件下所确定的网格尺度基本一致,并满足热环境模拟的精度要求.      

基于X射线脉冲星的航天器自主导航数值分析研究木

基于X射线脉冲星的天文自主导航方法稳定、可靠、精度高.是航天器自主导航领域重要的发展方向.论文介绍了基于X射线脉冲星导航的基本原理,给出了脉冲星自主定位的滤波模型;基于扩展卡尔曼滤波导航算法,以地球同步轨道为例,分析了不同初始误差、轨道倾角、轨道偏心率以及不同脉冲星方位误差下的导航精度.仿真算例表明,在脉冲星方位误差为0.001"、脉冲到达时间测量误差为0.1μs的情况下,导航定位精度优于1 km,且对初始误差不敏感;脉冲星方位精度、脉冲计时精度、轨道面位置和轨道偏心率是影响导航精度的主要因素.     

基于X射线脉冲星的航天器自主导航数值分析研究

基于X射线脉冲星的天文自主导航方法稳定、可靠、精度高,是航天器自主导航领域重要的发展方向.论文介绍了基于X射线脉冲星导航的基本原理,给出了脉冲星自主定位的滤波模型;基于扩展卡尔曼滤波导航算法,以地球同步轨道为例,分析了不同初始误差、轨道倾角、轨道偏心率以及不同脉冲星方位误差下的导航精度.仿真算例表明,在脉冲星方位误差为0.001″、脉冲到达时间测量误差为0.1μs的情况下,导航定位精度优于1km,且对初始误差不敏感;脉冲星方位精度、脉冲计时精度、轨道面位置和轨道偏心率是影响导航精度的主要因素.      

推算错误预测的自适应位置外推算法

给出了适用于第一人称射击游戏中角色运动特征的位置外推算法.根据此类游戏角色运动方向多变、轨迹呈连续折线的特点,选取一阶外推方程和直线收敛算法用于位置推算与收敛,根据角色运动行为和距离来预测推算错误、调整收敛过程,减少游戏的网络通信量并提高推算精度.实验结果表明,该位置外推算法应用于第一人称射击游戏可以减小发送的状态更新数量和推算轨迹偏差.      

脉冲星方位误差估计的TSKF算法

为提高脉冲星方位误差估计对方位自行速度及卫星位置误差的鲁棒性和整体运算的高效性,设计了两级卡尔曼滤波（TSKF）算法。首先,分析了方位自行速度及卫星位置误差对方位误差估计的影响,并分别结合相关算法进行了仿真验证。然后,结合方位误差估计的CV模型和两级卡尔量滤波的相关原理,写出了TSKF算法的更新方程,并分析了实现并行计算的基本流程。仿真实验的数据显示:在方位自行速度及卫星位置误差均存在的情况下,TSKF算法的方位估计精度约为0.1 mas,方位自行速度估计精度约为1.1 mas/a;与基于CV模型的估计算法相比,TSKF算法的浮点运算仅增加了0.048%。      

基于非参数边缘积分估计的局部特征描述子

为提高图像匹配性能提出了关于局部区域特征描述子的统计模型。该模型是一种基于梯度模值及方向分布的边缘积分函数模型。在离散梯度方向的边缘积分函数与梯度矢量场的模值累积方向直方图相同。采用基于核函数的非参数估计,估计了该函数,应用于尺度不变特征变换(SIFT)描述子。为了提高描述子的旋转不变性、独特性,降低运算复杂度,将特征点周围的局部区域作为圆形,由径向采样网格划分为8个子区域。在每个子区域估计边缘积分函数,特征向量由每个小块8个方向的函数值组成。实验表明,该描述子能够提高旋转变换的检测率(查全率),降低运算复杂度。      

Learning in an immersive digital theater

The Houston Museum of Natural Science, in collaboration with Rice University has an outreach program taking portable digital theaters to schools and community sites for over five years and has conducted research on student learning in this immersive environment. By using an external independent evaluator, the effectiveness of NASA-funded Education and Public Outreach (EPO) projects can be assessed. This paper documents interactive techniques and learning strategies in full-dome digital theaters. The presentation is divided into Evaluation Strategies and Results and Interactivity Strategies and Results. All learners from grades 3–12 showed statistically significant short-term increase in knowledge of basic Earth science concepts after a single 22-min show. Improvements were more significant on items that were taught using more than one modality of instruction: hearing, seeing, discussion, and immersion. Thus immersive theater can be an effective as well as engaging teaching method for Earth and Space science concepts, particularly those that are intrinsically three-dimensional and thus most effectively taught in an immersive environment. The portable system allows taking the educational experience to rural and tribal sites where the underserved students could not afford the time or expense to travel to museums.     

Scaling analysis and model estimation of solar corona index

A monthly average solar green coronal index time series for the period from January 1939 to December 2008 collected from NOAA (The National Oceanic and Atmospheric Administration) has been analysed in this paper in perspective of scaling analysis and modelling. Smoothed and de-noising have been done using suitable mother wavelet as a pre-requisite. The Finite Variance Scaling Method (FVSM), Higuchi method, rescaled range (R/S) and a generalized method have been applied to calculate the scaling exponents and fractal dimensions of the time series. Autocorrelation function (ACF) is used to find autoregressive (AR) process and Partial autocorrelation function (PACF) has been used to get the order of AR model. Finally a best fit model has been proposed using Yule-Walker Method with supporting results of goodness of fit and wavelet spectrum. The results reveal an anti-persistent, Short Range Dependent (SRD), self-similar property with signatures of non-causality, non-stationarity and nonlinearity in the data series. The model shows the best fit to the data under observation.     

Stochastic particle acceleration by the forced interaction of relativistic current sheets

We address the problem of interacting relativistic current sheets in self-consistent kinetic plasma simulations within the framework of the Particle-In-Cell model. The interaction is enforced in head-on collisions of up to 10 current sheets at relativistic bulk speeds. The simulations are motivated by the general problem of Poynting flux dissipation in ‘striped wind’ configurations presumably governing the relativistic outflows pervasive in pulsar winds and gamma-ray bursts. We identify the generation of non-thermal particles and formation of a stable power-law shape in the particle energy distributions f(γ) dγ ∝ γ^−s dγ. In 1D, a spectral index s ∼ 2 is observed and attributed to a stochastic Fermi-type acceleration mechanism. In 2D, the generic index of s ∼ 3–4 is retained as in previous simulations of individual current sheets. Whereas in 2D the high energy cut-off is constrained by the limited dissipation of magnetic energy, in 1D the process converts the bulk motion of current sheets towards directed particle momentum of an exclusive class of non-thermal particles.     

Development of a gas chromatography compatible Sample Processing System (SPS) for the in-situ analysis of refractory organic matter in martian soil: preliminary results

In the frame of the 2009 Mars Science Laboratory (MSL) mission a new sample preparation system (SPS) compatible with gas chromatography–mass spectrometry (GC–MS) has been developed for the in situ analysis of complex organic molecules in the Martian soil. The goal is to detect, if they exist, some of the key compounds that play an important role in life on Earth including carboxylic acids, amino acids and nucleobases.     

Concept options for the aerial survey of Titan

Various aerial platforms intended for long endurance survey of the Titan surface are presented. A few novel concepts are introduced, including a heated methane balloon and a balloon with a tethered wind turbine. All the concept options are predicted to have lower scientific payload fractions than the Huygens probe. It is concluded that the selection of the best aerial platform option depends on more accurate mass estimates and a clear decision on whether, or not, in situ surface composition measurements are required in conjunction with aerial remote sensing.     

Heterogeneous solid/gas chemistry of organic compounds related to comets,meteorites, Titan,and Mars: Laboratory and in lower Earth orbit experiments

To understand the evolution of organic molecules involved in extraterrestrial environments and with exobiological implications, many experimental programs in the laboratory are devoted to photochemical studies in the gaseous phase as well as in the solid state. The validity of such studies and their applications to extraterrestrial environments can be questioned as long as experiments conducted in space conditions, with the full solar spectrum, especially in the short wavelength domain, have not been implemented. The experiments that are described here will be carried out on a FOTON capsule, using the BIOPAN facility, and on the International Space Station, using the EXPOSE facility. Vented and sealed exposition cells will be used, which will allow us to study the chemical evolution in the gaseous phase as well as heterogeneous processes, such as the degradation of solid compounds and the release of gaseous fragments.     

基于LuGre模型的电液加载系统摩擦补偿

为了提高电液加载系统控制精度,针对摩擦问题提出了基于LuGre摩擦模型的前馈补偿方法.建立了用于摩擦仿真分析和补偿器设计的电液加载系统数学模型;通过实验获取并分析了相关的摩擦数据;基于实验数据进行LuGre模型参数辨识,把LuGre模型和辨识结果引入电液加载系统数学模型,并进行仿真结果与实际摩擦数据的对照,证明了LuGre摩擦模型的准确性.设计前馈补偿器,进行了实验对比,实验结果表明前馈补偿器可将摩擦产生的控制误差有效地降至未补偿时的30%左右.      

The ESA scientific programme

The ESA scientific programme has, so far, provided several significant astrophysics experiments and further important missions are scheduled for execution during the next decade. These missions are briefly summarised together with several astrophysics investigations presently under study.     

CoRoT and Kepler results: Solar-like oscillators

The space-borne observatories CoRoT (Convection Rotation and planetary Transits) and Kepler have provided photometric time series data of unprecedented precision for large numbers of stars. These data have revolutionized the fields of transiting exoplanets and asteroseismology. In this review some important asteroseismic results obtained using data from the CoRoT and Kepler space missions concerning stars that show solar-like oscillations are discussed. These results comprise, among others, measurements of the location of the base of the convection zone and helium second-ionization zone in main-sequence stars, the presence (or not) of core-helium burning in red-giant stars, as well as differential rotation in these stars.     

Return to Europa: Overview of the Jupiter Europa orbiter mission

Missions to explore Europa have been imagined ever since the Voyager mission first suggested that Europa was geologically very young. Subsequently, the Galileo spacecraft supplied fascinating new insights into this satellite of Jupiter. Now, an international team is proposing a return to the Jupiter system and Europa with the Europa Jupiter System Mission (EJSM). Currently, NASA and ESA are designing two orbiters that would explore the Jovian system and then each would settle into orbit around one of Jupiter’s icy satellites, Europa and Ganymede. In addition, the Japanese Aerospace eXploration Agency (JAXA) is considering a Jupiter magnetospheric orbiter and the Russian Space Agency is investigating a Europa lander.