超高精度时间频率同步及其应用

文章介绍了时间频率同步的主要概念及方法。重点介绍了在清华大学与中国计量科学研究院之间往返80 km的商用光纤链路上进行时间频率传输与同步的方案,实验得到7×10-15/s,5×10-19/天的频率传输稳定度和50 fs的时间同步稳定度。针对不同网络结构,文章作者提出了多种光纤同步方案,并着重介绍了时间频率同步在科学研究领域中的一些重要应用。     

A Kinetic Transition from Low to High Fragility in Cu-Zr Liquids

Researchers have reported that Cu-Zr liquids are kinetically strong at the best glass-forming compositions. Here we systematically study the temperature dependence of viscosity and diffusion of Cu-Zr liquids using molecular dynamics simulations, and the results illustrate that the better glass formers are actually more fragile close to the glass transition. There is a kinetic transition from low to high fragility when the optimal glass-forming liquids are quenched into glass states. This transition is associated with the more rapid decrease of the excess entropy of the liquids above and close to the glass transition temperature, Tg, compared to other compositions. Accompanied by the transition to high fragility, peaks in the thermal expansivity and specific heat are observed at the optimal compositions. Furthermore, the Stokes Einstein relation is examined over a wide composition range for Cu-Zr alloys, and the results indicate that glass-forming ability closely correlates with dynamical heterogeneity.     

Electrical Resistivity of Silane Multiply Shock-Compressed to 106 GPa

Since Wigner et al. proposed that hydrogen would become metallic under sufficient pressure compres- sions in 1935,scientists have paid their attention on making metallic hydrogen at high pressures, and con- siderable progresses were made in theoretical and ex- perimental researches. Nellis et al. observed that the electrical resistivity of fluid hydrogen declined by several orders of magnitude when liquid hydrogen was multiply shocked to 140 GPa, and concluded that fluid hydrogen underwent metallization phase tran- sition from semiconductor to metal in their experi- ments. Although further researches should be carried out to distinguish the highly conductive state and the metallic state of fluid hydrogen, researchers have made great efforts to find new technical approaches to de- crease the threshold pressure for hydrogen metalliza- tion. For this purpose, hydrogen-rich compounds at- tract much attention. Some researchers believed that non-hydrogen elements in those compounds may re- duce, to some extent, the activation energy of met- allization by the effect of chemical pre-compression. Silane, a typical hydrogen-rich compound of group IV hydrides, has been the subject of most of the theoretical and experimental research so far, and it was also expected to be a potential candidate for a high-To superconductor at high pressure research.[61 Compared to hydrocarbons,[71 the chemical bonds in the silane molecule are theoretically more sensitive to pressure and temperature. At sufficiently high pres- sure and temperature, the fluid silane possibly be- comes some metallic alloy consisting of hydrogen and silicon elements. Theoretical calculations showed thatthe metallic transition for the silane system may oc- cur even below 100 GPa, while there are also some other later articles that claimed that silane would re- main an insulator up to around 200 GPa and became metallic and supconducting at 220 GPa with a theo- retical Tc of 16 K. Recently, Eremets et al. have re- ported that silane can transform to metal at 50 GPa,     

基于分布式光纤拉曼测温系统的核电一回路泄漏点监测技术

为了能对核能发电站内反应堆冷却系统管道的泄漏情况进行实时监测及高精度定位,利用拉曼散射效应和光时域反射技术构建了分布式光纤测温系统,结合特定规则的传感光纤布设方式,实现管道外壁温度场的全覆盖式实时在线监测.通过实验验证,当泄露发生导致二维平面内某区域的温度异常达到预先设置的阈值时,监测系统会依据布线规则对该点进行位置解码并标记,从而实现二维平面的精确定位.实验得到的定位精度为0.5m,并显著降低了误报可能性,可极大的提高管道外表面温度异常点的定位精度.     

基于核密度估计的城市动态密集场景激光雷达定位

城市环境中的精确定位是自动驾驶领域的重点和难点,现有的激光雷达定位算法虽然能够在多数情况下保持较高的精度,但在一些比较复杂的城市动态场景中仍存在问题。针对这类场景中遮挡导致的全球定位系统定位精度下降,以及运动目标和环境变化导致的有效点云特征减少的问题,提出一个新的概率定位框架;该框架使用核密度估计的方法对改进后的多层次随机采样一致性算法和直方图滤波算法进行融合,以有效克服多层次随机采样一致性算法在部分场景中的定位波动问题,以及直方图滤波算法在位姿误差较大时的效率低下和局部最优问题。结果表明:所提框架在保证定位精度的前提下,提升了对动态密集场景的适用性,能够在现有算法容易出错的场景中实现更加稳定精确的定位,并能够容忍更大的初始位姿误差。     

精确稀疏扩展信息滤波的地图优化研究

研究了移动机器人同时定位与地图创建（SLAM）精确稀疏扩展信息滤波（ESEIF）的地图优化算法,即利用信息熵度量变量不确定性的特性来对地图特征点进行分类,选择不同类型的特征点处理ESEIF的不同更新过程,同时优化活动地图,使SLAM更新在恒定时间内实现,且提高了机器人和地图的估计精度.仿真实验证明：在特征点多的大环境下,特征点优化后的算法实时性强,估计精确度更高.     

与CRP相结合的SAFT在混凝土损伤检测成像中的研究

将传统合成孔径聚焦技术(Synthetic aperture focusing technique,SAFT)与共反射点(Common reflection point,CRP)信号叠加法相结合应用于混凝土损伤检测中,可提高在强噪声环境中拾取缺陷回波信号的能力。本文采用二维有限元仿真验证了该思想的合理性,通过混凝土实验检验了该方法在实际应用中的可行性和有效性。与传统SAFT成像结果相比,应用CRP信号叠加法的SAFT提高了成像横向分辨率和损伤定位精度,为工程应用提供了理论和实验参考。     

基于UKF的车辆GPS/蜂窝网无缝定位算法

当车辆运行在存在遮挡和多路径的复杂环境下时,车载GPS接收机由于无法接收到足够数量的GPS定位卫星将导致车辆无法准确定位。GPS/蜂窝网无缝定位技术将蜂窝基站视作伪卫星,将蜂窝基站观测值与GPS观测值结合进行融合解算有效解决了该问题。针对目前融合解算算法所采用的EKF解算算法存在高阶项截断误差和Jacobian矩阵难计算的缺点,本文将不敏卡尔曼滤波(UKF)引入到基于GPS和蜂窝网的无缝定位算法中。UKF算法是一种基于UT变换的非线性的滤波方法,避免了因进行强行线性化带来的误差影响。通过在高斯信道、静止和运动场景下,对两种混合定位算法性能进行对比分析,实验结果表明:将UKF算法用于GPS和蜂窝网络的混合定位中时,在不同的定位环境下,其定位均方根误差(RMSE)相较EKF算法降低了20%~70%。     

一个水稻苗期黄化叶突变体基因的定位

通过筛选甲基磺酸乙酯(Ethyl Methane Sulfonate,EMS)诱变的籼稻Kasalath突变体库,得到一个黄化叶突变体Osyl1(Oryza sativa yellow leaf 1),在二叶期前该突变体叶色黄化,后期突变体叶色逐渐恢复到野生型水平.遗传分析显示Osyl1突变性状受一对隐性基因控制,经图位克隆技术将Os YL1基因定位在水稻第3染色体上的STS(Sequence Tagged Site)标记S5362和S5845之间,物理距离约为483 kb,在该区间内无已知黄化叶相关基因.对Os YL1基因的定位,为该基因的克隆打下基础.     

基于LANDMARC的最近邻居改进算法

对基于动态主动式射频识别标定的定位识别(LANDMARC)室内定位系统算法进行了研究,针对现有LANDMARC室内定位系统算法最近邻居数量固定导致优良的最近邻居丢失、不良最近邻居引入的问题,提出了一种改进的最近邻居算法(K-nearest neighbor algorithm,KNN).该算法利用待测标签的预估位置,划定最近邻居的选择区域,然后根据选择区域内参考标签与预估位置能级差选择最近邻居.MATLAB仿真结果表明,在同等条件下的新算法定位精度提高了9.8%,最大误差降低了21.8%.