基于分数阶傅里叶变换的水下目标速度估计

在高斯白噪声背景下,匹配滤波器作为线性调频信号的最优检测器,在水声信号处理中被广泛应用。 当发射信号为线性调频信号时,由水下目标径向速度引起的多普勒频移会造成回波和样本之间失配, 使匹配滤波器的检测性能下降,增加了目标速度估计的难度。 利用分数阶傅里叶变换对于线性调频信号的聚焦特性,提出了应用分数阶傅里叶变换的水下运动目标线性调频回波检测算法,完成对目标速度的估计, 推导目标运动速度与分数阶傅里叶变换阶数之间的关系,并对测量结果进行误差分析。 仿真测试表明,该算法可有效地估计混响背景下的目标径向速度,且具有良好的估计性能。     

Gravitational Waves and Gravitational-wave Sourcestwo

The recent discovery of gravitational-wave burst GW150914 marks the coming of a new era of gravitational-wave astronomy, which provides a new window to study the physics of strong gravitational field, extremely massive stars, extremely high energy processes, and extremely early universe. In this article, we introduce the basic characters of gravitational waves in the Einstein's general relativity, their observational effects and main generation mechanisms, including the rotation of neutron stars, evolution of binary systems, and spontaneous generation in the inflation universe. Different sources produce the gravitational waves at quite different frequencies, which can be detected by different methods. In the lowest frequency range (f < 10^?15 Hz), the detection is mainly dependent of the observation of B-mode polarization of cosmic microwave background radiation. In the middle frequency range (10^?9 < f < 10^?6 Hz), the gravitational waves are detected by analyzing the timing residuals of millisecond pulsars. And in the high frequency range (10 ? 4 < f < 10⁴ Hz), they can be detected by the space-based and ground-based laser interferometers. In particular, we focus on the main features, detection methods, detection status, and the future prospects for several important sources, including the continuous sources (e.g., the spinning neutron stars, and stable binary systems), the burst sources (e.g., the supernovae, and the merge of binary system), and the stochastic backgrounds generated by the astrophysical and cosmological process. In addition, we forecast the potential breakthroughs in gravitational-wave astronomy in the near future, and the Chinese projects which might involve in these discoveries.     

Quantifying the vertical distribution pattern of land-use conversion in the loess hilly region of northern Shaanxi Province 1995–2015

Journal of Geographical Sciences - The mountainous and hilly region plays an important role in ecological safety and production in China. However, recent studies have poorly characterized the...     

Multi-dimensional expansion of urban space through the lens of land use: The case study of Nanjing City,China

Journal of Geographical Sciences - The study of multi-dimensional expansion of urban space (MEUS) addresses the laws of urban spatial expansion from all directions and angles. Using Nanjing as an...     

赣北风沙-粉尘堆积体系元素地球化学变化特征及其对黄土物源示踪意义

粉尘的搬运与堆积过程,理论上是不同粒径的岩屑与矿屑颗粒被风力分选并依次沉降的过程,而元素在不同粒级的岩屑与矿屑的富集程度有所差异;因此,对岩屑与矿屑颗粒分选,在一定程度上是对地球化学元素的分选。因此,基于元素地球化学对黄土粉尘示踪研究中,单一节点或(和)剖面均不具有对粉尘源区的严格代表性,通过比较单一节点或剖面的元素地球化学而进行物源示踪的方法往往存在较大的不确定性。本文通过对中亚热带赣北鄱阳湖地区芙蓉-周溪一带的风沙-粉尘堆积体系常量和微量元素的多维度统计分析,发现地球化学元素及其元素对均存在系统性的分选与分异特点。稳定元素(如Al、Ti、Zr、Nb、Lu、Hf、Ta)及其元素对随距物源区的远近呈现线性、指数或对数等的单向变化特点,其变化幅度在沉积体系中可达31%~42%;在二维和三维散点图中其元素对的投影轨迹,是一个有方向的线状或者带状区域。这表明,风沙-粉尘堆积体系内元素地球化学从上风向到下风向是单调变化的,传统上基于元素地球化学的黄土粉尘物源示踪,通过判断投影区域的远近而进行示踪分析,在理论上有一定缺陷。初步建议,未来的元素地球化学示踪应避免单一剖面元素示踪的方法,而应根据粉尘堆积体系的分布情况系统采样,通过揭示粉尘堆积体系元素地球化学的系统特征来开展基于地球化学元素的黄土粉尘物源示踪研究。     

山区农户双重风险感知对搬迁意愿和搬迁行为的影响

以受汶川“5.12”地震和“7.10”特大山洪泥石流灾害严重影响的汶川县原草坡乡为例,从农户风险感知现状出发,考察风险感知对搬迁意愿和搬迁行为的不同影响。研究结果表明：农户对自然灾害风险感知越强、对搬迁安置风险感知越弱,其搬迁意愿越强;较高的自然灾害风险感知会提高农户搬离灾害隐患点或风险区的概率,较强的搬迁安置风险感知会降低农户全家搬迁的可能性;搬迁安置风险感知对未搬迁和部分搬迁农户的影响程度超过自然灾害风险感知的影响。基于汶川县原草坡乡的实证研究,提出避灾搬迁安置决策中的“双重风险感知”假说。     

Analysis of the spatiotemporal riding modes of dockless shared bicycles based on tensor decomposition

ABSTRACT

Studies on the riding modes of shared bicycles have aimed to heighten the understanding of cycling characteristics. This paper analyzes the spatiotemporal riding modes of shared bicycles based on tensor decomposition in Beijing, China. Two third-order tensors are constructed for the origin and destination points of shared bicycles in the day, hour, and space dimensions. Three factor matrices explicitly reveal two modes, three modes, and six modes in the day dimension, hour dimension, and space dimension, respectively. The relationships among the different modes in the three dimensions are demonstrated in an interaction table. Further, the density for different types of points of interest (POIs) are calculated to further analyze the potential riding purpose for different riding modes. Notably, the main POI types for the areas of O2 and D2 modes are consistent with the areas of D3 and O3 modes, which reflects the tidal characteristics of the commuting activities of shared bicycles. The main functional areas are inferred according to the riding modes and POIs, which enables verification of the correctness of the obtained riding modes to some extent. By method comparison, tensor decomposition shows the advantage of being able to reveal the spatiotemporal modes among multiple dimensions.     

北京地区气溶胶粒径尺度谱与PM2.5浓度转换模型研究

大气细颗粒物（PM_2.5）质量浓度是重要的空气质量指标之一。为了促进区域PM_2.5浓度监测的研究,同时拓展利用CE318太阳光度计等光学传感器反演的大气气溶胶产品的应用领域,本文首先基于北京地区2014年—2017年大气气溶胶反演的粒径尺度谱分布产品,计算表征PM_2.5的粒子体积,并结合同一时间北京地区35个空气质量站点提供的PM_2.5质量浓度参考值计算转换系数,对样本区间进行划分以构建转换模型。其次,利用各CE318站点数据所得转换系数及其相对精度,对研究区PM_2.5质量浓度估算误差空间分布,以及转换系数偏差对估值误差贡献情况进行评价。研究结果表明,由CE318站PM_2.5粒子体积与其临近空气质量站PM_2.5质量浓度联合建立的转换系数是一种与气溶胶理化属性密切相关的参数指标,可将时间和空间维度上PM_2.5体积与PM_2.5质量浓度之间的关系映射到由理化属性主导的维度上,可用于对估值模型进行细化和分类,构建分段转换函数模型,使得各分段区间内具有较高的模型拟合精度。基于转换系数的北京地区PM_2.5浓度估值的相对误差多年均值介于12.8 %—28.7 %,而转换系数相对偏差对PM_2.5质量浓度估值相对误差的影响显著,二者之间具有“r”型结构。当转换系数相对偏差介于-16.3 %至24.5 %时,该偏差的出现概率约为66.5 %,使得PM_2.5质量浓度估值误差在20 %以内,表明采用此种方法对相应站点的PM_2.5质量浓度进行估值具有相当的精度和稳定性。以上研究结果可为地面观测站稀少区域利用卫星光学遥感开展空气质量大范围监测应用提供理论前提和技术支持。     

基于MALDI-TOF MS快速鉴定弧菌的数据库的扩展和应用

近年来由海洋细菌导致的感染越来越常见,弧菌是海洋细菌中最常见的细菌之一。目前,对弧菌缺乏快速有效的鉴定。基质辅助激光解吸电离飞行时间质谱仪（Matrix-assisted laser desorption ionization–time of flight mass spectrometry,MALDI-TOF MS）已成为快速鉴别细菌强有力的工具。然而,它的准确性取决于数据库中的菌株数量。本研究中,我们利用MALDI-TOF MS对弧菌数据库进行了扩展和应用。我们将74株弧菌纳入了新的数据库,构建了基于RpoB序列的进化树（邻接法和最大似然比法）,并针对扩展后数据库和扩展前数据库进行了聚类分析。我们利用30株弧菌,对扩展后数据库进行了评价,并进行了主成分分析。本研究中,我们创建了一个新的数据库,促进了弧菌的快速准确鉴定。MSP(main spectrum profile)和PCA（Principal Component Analysis）分析为追踪弧菌感染的来源和发生率提供了理论支持。此外,特征峰和差异峰的发现对弧菌的鉴定也有帮助。     

南海西部风驱离岸急流次中尺度锋面的动力学分析

本文利用卫星观测资料和500 m分辨率数值模拟结果,结合理论分析,对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明,南海西部（WSCS）存在侧向尺度为O（1-10）km的次中尺度锋面,在地转和非地转运动的共同作用下,次中尺度密度锋面具有一阶Rossby（Ro）和Richardson（Ri）数。锋面诊断结果显示,沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送,有效地在跨锋面方向将表层冷水平流输送至暖水侧,导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡（PV）,表明该密度锋面易受次中尺度对称不稳定（SI）的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度,其最大值可达100 m·d^-1。能量评估结果表明,次中尺度湍流的两个主要能量源,即地转剪切项（GSP）和垂向浮力通量（BFLUX）在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下,大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此,次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量,可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。