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521.
最差用户位置算法研究与仿真分析 总被引:2,自引:1,他引:2
在计算Galileo卫星导航系统空间信号误差(SISE)、空间信号精度(SI-SA)和空间信号监测精度(SISMA)时需要用到的一个关键参数——最差用户位置(WUL)。WUL表示空间信号的轨道误差在卫星覆盖域内所导致的伪距误差最大的位置,WUL的确定可以帮助系统向用户提供正确可靠的SISA、SISMA和完好性标志(IF)等完好性信息,对提高系统的可靠性和可用性具有非常重要的意义。WUL的算法主要有两种:临界圆法和网格搜索法。详细介绍两种算法的基本原理与计算流程,比较两种算法的结果,并基于全球和区域完好性监测的概念对二者进行分析评价。 相似文献
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新型升力再入飞船返回舱气动外形选型研究 总被引:2,自引:0,他引:2
未来低成本天地往返运输系统的气动外形设计的总体要求是升阻比高、机动性强、稳定性好、过载低、空间大、可重复使用等。结合国内技术水平和国外经验教训,提出了我国未来新型升力再入飞船返回舱的总体气动需求,采用平面斜切二次曲线构造截面的模线设计方法,设计了四种满足长度、截面尺寸和容积等总体需求的返回舱布局方案。利用基于牛顿理论的气动力工程方法对气动特性进行预测,利用气动力与六自由度弹道的耦合计算研究返回舱无控再入飞行特性。研究表明,所设计的返回舱外形高超声速稳定配平升阻比大于0.9,阻力系数大于0.5,法向过载低于2.5,具有较好的静动稳定性,具备可部分重复使用的潜力。 相似文献
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The deformation work rate can be expressed by the time rate of pair functional potentials which describe the energy of mate-rials in terms of atomic bonds and atom embedding interactions. According to Cauchy-Born rule, the relations between the microscopic deformations of atomic bonds and electron gas and macroscopic deformation are established. Further, atomic bonds are grouped according to their directions, and atomic bonds in the same direction are simplified as a spring-bundle component. Atom embedding interactions in unit reference volume are simplified as a cubage component. Consequently, a material model composed of spring-bundle components and a cubage component is established. Since the essence of damage is the decrease and loss of atomic bonding forces, the damage effect can be reflected by the response functions of these two kinds of compo-nents. Formulating the mechanical responses of two kinds of components, the corresponding elasto-damage constitutive equa-tions are derived. Considering that slip is the main plastic deformation mechanism of polycrystalline metals, the slip systems of crystal are extended to polycrystalline, and the slip components are proposed to describe the plastic deformation. Based on the decomposition of deformation gradient and combining the plastic response with the elasto-damage one, the elasto-plastic dam-age constitutive equations are derived. As a result, a material model formulated with spring-bundle components, a cubage component and slip components is established. Different from phenomenological constitutive theories, the mechanical property of materials depends on the property of components rather than that directly obtained on the representative volume element. The effect of finite deformation is taken into account in this model. Parameter calibration procedure and the basic characteris-tics of this model are discussed. 相似文献
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A 6-degree of freedom (6-DOF) aircraft wing position and pose automatic adjustment method is presented to improve ARJ21 wing-fuselage connection precision and efficiency. Wing position and pose are adjusted by three pillars which are driven by six high-precision servo motors. During the adjustment process, wing is tracked and positioned by laser tracker. Wing initial posi-tion and pose are calibrated by using the measurement coordinates of assembly reference points. Wing target position and pose are calculated according to wing initial, fuselage position and pose, and relative position and pose requirements between wing and fuselage for the connection. Combining Newton-Euler method with quaternion position and pose analyzing method, the inverse kinematics of servo motors, together with the adjustment system dynamics is obtained. Wing quintic polynomial trajec-tory planning algorithm based on quaternion is proposed; the initial, target position and pose need to be solved and the inter-mediate moving path is uncertain. Simulation results show that the adjustment method has good dynamic characteristics and satisfies engineering requirements. Preliminary engineering application indicates that ARJ21 wing adjustment efficiency and precision are improved by using the proposed method. 相似文献
527.
基于卫星公用平台的卫星设计、制造理念,以卫星公用平台目录作为重要的采办工具,在当今许多国家的卫星采办项目中得以应用。本文在整理美国卫星公用平台目录建设、管理与应用领域的进展情况基础上,特别是研究分析了美国国家航空航天局下属航天器快速发展办公室(RSDO)的运作情况,探讨了我国航天领域卫星公用平台型谱管理与应用的前景和方向,以及专业咨询机构在采办过程中的作用,为进一步深入实践航天产品工程理论提供了新方法、新模式。 相似文献
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Hong-xia Zheng Wei-ming Tian Hong-ji Yan Hua-dong Jiang Shan-shan Liu Lei Yue Fang Han Li-jun Wei Xiong-biao Chen Yu Li 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
This study investigated intracellular oxidative stress and its underlying mechanisms in a rotary cell culture system used to achieve a simulated microgravity (SMG) environment. Experiments were conducted with human breast cancer cell lines MCF-7 (an estrogen receptor (ER) α positive cell line) and MDA-MB-231 (an ERα negative cell line) encapsulated in alginate/collagen carriers. After 48 h, SMG led to oxidative stress and DNA damage in the MDA-MB-231 cells but a significant increase in mitochondrial activity and minimal DNA damage in the MCF-7 cells. The activity of superoxide dismutase (SOD) significantly increased in the MCF-7 cells and decreased in MDA-MB-231 cells in the SMG environment compared with a standard gravity control. Moreover, SMG promoted expression of ERα and protein kinase C (PKC) epsilon in MCF-7 cells treated with PKC inhibitor Gö6983. Overall, exposure to SMG increased mitochondrial activity in ERα positive cells but induced cellular oxidative damage in ERα negative cells. Thus, ERα may play an important role in protecting cells from oxidative stress damage under simulated microgravity. 相似文献
529.
德国Walter集团是世界著名的金属切削刀具和刀具数据管理软件系统供应商。它成立于1919年,总部位于德国图宾根市,目前其分支机构及销售网络遍及26个国家和地区。从1926年Walter开始研制硬质合金刀具至今,一直在这一领 相似文献
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