Electron density retrieval from occulting GNSS signals using a gradient-aided inversion technique

In the coming years, opportunities for remote sensing of electron density in the Earth’s ionosphere will expand with the advent of Galileo, which will become part of the global navigation satellite system (GNSS). Methods for accurate electron density retrieval from radio occultation data continue to improve. We describe a new method of electron density retrieval using total electron content measurements obtained in low Earth orbit. This method can be applied to data from dual-frequency receivers tracking the GPS or Galileo transmitters. This simulation study demonstrates that the method significantly improves retrieval accuracy compared to the standard Abel inversion approach that assumes a spherically symmetric ionosphere. Our method incorporates horizontal gradient information available from global maps of Total Electron Content (TEC), which are available from the International GNSS Service (IGS) on a routine basis. The combination of ground and space measurements allows us to improve the accuracy of electron density profiles near the occultation tangent point in the E and F regions of the ionosphere.     

混合不确定信息航天机构可靠性综合分析方法

为更加准确全面描述和分析航天机构的概率、非概率和模糊信息混杂模型的可靠性,提出非概率-模糊-概率信息统一的可靠性模型。在凸集非概率的基础上,结合模糊随机类信息,同时考虑机构系统的响应域落在失效域和不落在失效域两种情况下的可靠性度量。统一的可靠性模型运用基于凸集非概率模型、模糊割集方法和概率可靠度方法对非概率和模糊概率信息相容处理,然后对这两种情况分别进行可靠性分析。当系统的响应域落在失效域时,将统一模型中的区间信息用基于区间均匀分布的二阶拟合可靠性分析方法(SORM)进行分析,用蒙特卡罗仿真抽样方法计算统一模型的可靠度量化值。当系统的响应域不落在失效域时,将统一模型归一到凸集模型中,用凸集模型融合模糊和概率信息,并用HL-RF求解非概率可靠性指标。最后文章通过数值案例和某卫星天线双轴驱动机构的工程案例对提出的方法进行验证。     

曲线曲面的若干几何处理基础算法研究

给出了实用的曲线曲面的离散、投影、直线－圆环求交，空间直线距离计算等算法，曲线用密切圆逼近，采用差商计算离散点参数，算法速度快，根据几何性质构造了点的投影及曲线投影跟踪算法，用拟牛顿法加大收敛范围，利用相关性减少迭代次数，采用矢量方法和局部坐标系技巧，如空间直线间距离计算，直线－圈环求交等常用算法十分简洁。     

Measuring atmospheric density using GPS–LEO tracking data

We present a method to estimate the total neutral atmospheric density from precise orbit determination of Low Earth Orbit (LEO) satellites. We derive the total atmospheric density by determining the drag force acting on the LEOs through centimeter-level reduced-dynamic precise orbit determination (POD) using onboard Global Positioning System (GPS) tracking data. The precision of the estimated drag accelerations is assessed using various metrics, including differences between estimated along-track accelerations from consecutive 30-h POD solutions which overlap by 6 h, comparison of the resulting accelerations with accelerometer measurements, and comparison against an existing atmospheric density model, DTM-2000. We apply the method to GPS tracking data from CHAMP, GRACE, SAC-C, Jason-2, TerraSAR-X and COSMIC satellites, spanning 12 years (2001–2012) and covering orbital heights from 400 km to 1300 km. Errors in the estimates, including those introduced by deficiencies in other modeled forces (such as solar radiation pressure and Earth radiation pressure), are evaluated and the signal and noise levels for each satellite are analyzed. The estimated density data from CHAMP, GRACE, SAC-C and TerraSAR-X are identified as having high signal and low noise levels. These data all have high correlations with anominal atmospheric density model and show common features in relative residuals with respect to the nominal model in related parameter space. On the contrary, the estimated density data from COSMIC and Jason-2 show errors larger than the actual signal at corresponding altitudes thus having little practical value for this study. The results demonstrate that this method is applicable to data from a variety of missions and can provide useful total neutral density measurements for atmospheric study up to altitude as high as 715 km, with precision and resolution between those derived from traditional special orbital perturbation analysis and those obtained from onboard accelerometers.     

工具型动机和自尊对高职大学英语教学的影响及对策

基于实际调查结果,对高职生在英语学习中表现出来的工具型动机和自尊这两个情感因素及其对英语教学的影响进行分析,提出相应的对策,认为充分利用两者可以提高高职大学英语教学效率。     

基于OO的CIMS信息分类编码研究

通过分析国内外现有各分类编码系统以及ＣＩＭＳ对信息集成的要求,结合面向对象（Ｏｂｊｅｃｔ－ｏｒｉｅｎｔｅｄ,ＯＯ）方法提出了一种面向ＣＩＭＳ各环节的分类编码结构模型ＦＣＣ。该模型由复杂对象Ｏ和施加于Ｏ上的操作Ｍ构成,其中Ｏ可描述为一个三元组,用于类别标识,事物特征描述和图形特征描述。类别标识项是一个可用二元组描述的复杂对象;图形特征项是一个四元组,描述了图形标识,图形符号,图形特征数据和图形文件;     

粘弹性阻尼夹层梁的振动分析

本文采用横截面位移的几何相似假设,建立更为一般的粘弹性阻尼夹层梁的振动方程.同时给出相应的自然边界条件,然后简化到DTMM方程。基于粘弹性材料本构关系的微分算子描述,时域形式的振动方程在概念上更为清晰、严格。从时频转换角度来看,剪切参量不再存在依赖频率含糊的问题.文中还讨论了几何、物理参量对阻尼频率及损耗因子的影响,并推导出阻尼梁强迫振动稳态响应的正交模态解,指出阻尼结构的自感应现象。     

高频基片复合材料的研究进展

介绍了电路组装技术对基片的性能要求、性能影响因素及基片设计原则，评述了高频基片复合材料的发展现状，并提出未来研究工作的重点。     

助烧剂对无压烧结Si3N4力学性能的影响

研究了三种助烧剂MgO、Al2O3+AlN和Y2O3+AlN对1800℃×3h工艺下无压烧结Si3N4力学性能的影响情况。发现材料力学性能主要决定于助烧剂的种类,其次取决于含量。其Y2O3+AlN虽使最终生成的β-Si3N4长径比较小,线度尺寸最短,但最利于致密化,因而使材料力学性能最佳,添加量(质量分数)为12%Y2O3+6%AlN时,陶瓷的抗弯强度、断裂韧性和维氏硬度分别达到431.6MPa、5.10MPa*m1/2和14.52GPa。     

纳米SiCp/SiO2陶瓷基复合材料的制备和性能研究

成功制备出了以非晶态SiO2为基体，以纳米SiC颗粒（n-SiCp)为增强体的n-SiCp/SiO2陶瓷基复合材料。n-SiCp的引入并示引起基体SiO2的晶化，而且对复合材料的力学性能的改善起到了重要的作用，同时，并未损害复合材料的抗热震性能。所制备的n-SiCp/SiO2复合材料的临界抗热震温差可以达到1200℃。