Tightly combined GPS/Galileo RTK for short and long baselines: Model and performance analysis

To ensure the compatibility and interoperability with modernized GPS, Galileo satellites are capable of broadcasting navigation signals on carrier phase frequencies that overlap with GPS, i.e., GPS/Galileo L1-E1/L5-E5a. Moreover, the GPS/Galileo L2-E5b signals have different frequencies with wavelength differences smaller than 4.2?mm. Such overlapping and narrowly spaced signals between GPS and Galileo bring the opportunity to use the tightly combined double-differenced (DD) model for precise real-time kinematic (RTK) positioning, resulting in improved performance of ambiguity resolution and positioning with respect to the classical standard or loosely combined DD model. In this paper, we focus on the model and performance assessment of tightly combined GPS/Galileo L1-E1/L2-E5b/L5-E5a RTK for short and long baselines. We first investigate the tightly combined GPS/Galileo DD observational model for both short and long baselines with simultaneously considering the GPS/Galileo overlapping and non-overlapping frequencies. Particularly, we introduce a reparameterization approach to solve the rank deficiency that caused by the correlation between the DISB parameters and the DD ionospheric parameters for both overlapping and non-overlapping frequencies. Then we present performance assessment for the tightly combined GPS/Galileo RTK model with real-time estimation of the differential inter-system bias (DISB) parameters for short and long baselines in terms of ratio value, ambiguity dilution of precision (ADOP), ambiguity conditional number, decorrelation number, search count, empirical success rate, time-to-first-fix (TTFF), and positioning accuracy. Results from both static and kinematic experiments demonstrated that compared to the loosely combined model, the tightly combined model can deliver improved performance of ambiguity resolution and precise positioning with different satellite visibility. For the car-driven short baseline experiment with 10° elevation cut-off angle, the tightly combined model can not only significantly increase the ratio value by approximately 27.5% (from 16.0 to 20.4), but also reduce the ambiguity ADOP, the conditional number, and the search count in LAMBDA by approximately 22.2% (from 0.027 to 0.021 cycles), 14.9% (from 199.2 to 169.6), and 25.4% (from 150.1 to 112.0), respectively. Comparable decorrelation number, empirical success rate, and positioning accuracy are also obtained. For the car-driven long baseline experiment, it is also observed that the ambiguity resolution performance in terms of the ratio value, the decorrelation number, the condition number, and the search count are significantly improved by approximately 18.5% (from 2.7 to 3.2), 22.0% (from 0.186 to 0.227), 55.9% (from 937.6 to 413.7), and 10.3% (from 43.8 to 39.3), respectively. Moreover, comparable ADOP, empirical success rate, and positioning accuracy are obtained as well. Additionally, the TTFF can be reduced (from 54.1 to 51.8 epochs with 10° elevation cut-off angle) as well from the results of static experiments.     

适用于参数可调结构的非定常气动力降阶建模方法

基于计算流体力学(CFD)的非定常气动力降阶模型(ROM)可以极大提高气动弹性分析效率,然而现有的ROM只能针对固定参数结构,即只适合于固定模态振型,这使得现有ROM在气动弹性优化设计和不确定性分析等结构变参问题中应用受限。针对该问题,在文献[20]基础上提出了一种新的适用于任意模态振型的非定常气动力建模方法。首先将待设计/分析的结构进行参数化抽样和模态分析,之后通过主成分分析(PCA)得到若干基振型,再将这些基振型线性叠加即可拟合抽样空间内任何参数下结构的前若干阶振型。当结构参数改动时,仅仅是叠加系数发生变化。算例表明,仅用很少的基振型就能达到理想的拟合精度。经典的气动力降阶方法可用于基振型坐标下的气动力降阶,进一步变换可得到适用于不同结构的ROM,这意味着,结构参数可以在抽样空间内任意调节改动,而ROM却是通用的。该方法能广泛用于气动弹性优化设计和不确定性分析工作,可提高颤振分析精度和效率。     

小型薄片零件高速铣削加工试验研究

针对小型薄片结构零件切削过程中的变形问题,以薄片"一"字电极高速铣削加工试验为例,从编程方式、切削参数选用、工序安排、走刀路线等方面展开研究。控制其变形量达到最小,以满足产品设计要求。     

飞行参数记录系统直流模拟量拉低原因分析

针对某架飞机飞行参数记录系统多个直流模拟参数拉低故障，对该型飞行参数记录系统直流模拟量采集转换原理进行分析，查找故障原因，制定相应的排故方法，并提出预防措施。     

中子星性质的模型依赖性

采用相对论平均场理论（RMFT）下的GL85,GL92,TM1和TM2等核物质参数组,研究了这些参数组对中子星的粒子数密度分布、状态方程及质量-半径关系的影响.结果表明,在不同的参数组下,超子出现的密度、顺序和种类均不同;状态方程的软硬情况也较大不同;中子星的引力质量和相应的半径有明显不同.计算结果也表明,采用GL85,TM1和TM2等参数组得到的中子星质量和半径均在脉冲星的观测数据范围内,而采用GL92参数组得到的中子星质量小于中子星质量下限1.4M⊙（M⊙=1.99×1030kg为太阳质量）.     

Physical properties and long-term evolution of the debris clouds produced by two catastrophic collisions in Earth orbit

The population of cataloged orbital debris increased by approximately 40% in just a couple of years, from January 2007 to February 2009. This was due to two collisions in space, which involved the catastrophic destruction of three intact satellites (Fengyun 1C, Cosmos 2251 and Iridium 33) in high inclination orbits. Both events occurred in the altitude range already most affected by previous launch activity and breakup events, thus boosting the cataloged population in low Earth orbit by more than 60%.     

旋翼翼型动态失速模型参数识别及应用

为了拓展Leishman-Beddoes(L-B)动态失速模型的应用范围,以适应特定翼型的动态失速分析,在详细分析L-B动态失速模型特点的基础上,提出一种模型参数的识别方法。以SC-1095翼型为例,采用其静态升阻特性数据,对L-B动态失速模型中的参数进行了识别,并据此对该翼型的动态失速升阻特性进行了数值计算,计算结果与试验值吻合良好。     

伴随方法用于叶轮机优化设计的回顾与展望

3维反问题方法停滞不前,叶轮机设计越加倚重基于CFD分析的计算机优化。然而,随着设计要求越发精细、设计参数越多,优化计算量越显著增加,一般优化方法越难以满足工程设计实践要求。因而,计算量与设计变量数目近乎无关的伴随方法一经引入便受到航空领域深刻关注。以伴随方法发展过程中顺次突破的关键问题为主线,从流动领域引入伴随方法到外流领域的研究与应用,再到内流领域的探索尝试,回顾了国内外关于伴随方法的研究成果;在此基础上,面向深化在叶轮机设计领域的应用,对伴随方法进一步发展做出了展望。     

基于LFT的鲁棒变增益控制的降保守性设计

研究了线性变参数系统的鲁棒变增益控制,介绍了基于LFT的鲁棒变增益控制的思想;针对目前基于LFT的鲁棒变增益控制的保守性,采用全块标量矩阵,通过减少对全块标量矩阵的结构约束;提出了一种保守性更小的稳定性判据,并且利用消元定理,给出了基于LFT的鲁棒变增益的输出反馈控制器存在的条件;同时给出了控制器的构建方式,并且给出了...     

双曲面自动铺带机床的构型改进

结合预浸带在铺叠过程中的特点,分析了铺带成型压力对铺带工艺过程和最终成型构件的影响.对双曲面的特点进行了研究,阐述了通用5轴铺带机在铺放此类曲面时所遇问题及对策.为了更好地解决传统铺带机由于缺少俯仰自由度所造成的铺带头压力方向与模具表面不能垂直的问题,提出了6轴自动铺带机构型,并分析了改进机构在铺放双曲面时的可行性以及...