毛乌素沙地植被水分利用效率时空变化特征及其 对水热条件的响应

水分利用效率(WUE)是研究陆地植被生态系统水-碳循环及其耦合关系的重要指标。本文基于MODIS植被总初级生产力(GPP)和蒸散发(ET)数据产品,利用趋势分析和敏感性分析方法,开展了毛乌素沙地2001—2019年植被WUE时空分布特征及对水热条件响应研究。结果表明:①毛乌素沙地2001—2019年GPP、ET均呈显著的线性增加趋势,而植被WUE呈微弱的线性减少趋势。空间上,毛乌素沙地植被WUE以减少趋势为主,占区域总面积的64.53%,集中分布在中西部和南部地区。②不同植被类型WUE中,林地、灌丛WUE呈线性增加趋势,湿地、农田、草地和荒漠呈线性减少趋势。③毛乌素沙地植被WUE对降水敏感性存在阈值效应,阈值为300 mm,即阈值范围内植被WUE随降水增加而增加,超过阈值后,植被WUE随降水增加而下降。

西北干旱区极端降水时空变化特征及驱动因素

我国西北干旱区是对全球气候变化响应的敏感区域。为探究气候变化引起的该区域极端气候事件,本文基于91个气象站点1960—2018年逐日降水数据,选用11个极端降水指数,通过趋势分析、M-K检验和交叉小波变换,探究了西北干旱区极端降水时空变化特征及其驱动因素。结果表明西北干旱区极端降水指数除持续干燥指数(CDD)表现为下降趋势外,其它10个指数均表现为上升趋势。极端降水事件发生强度、持续时间和发生频率均表现出增加趋势。除持续干燥指数外,极端降水指数与降水量呈显著正相关关系,表明西北干旱区降水增加主要由极端降水增加所致。极端降水增加区域主要位于西北干旱区西北侧,即额尔齐斯-乌伦古河流域、伊犁河流域和天山北坡。西北干旱区极端降水增加与太阳黑子周期变化关系密切,其次为东亚夏季风和ENSO。研究结论认为应加强我国西北干旱区灾害监测及防灾减灾能力建设,科学认识极端降水与大尺度气候因素关系,降低极端降水导致的气象灾害风险。

北极水声学研究的新进展和新动向

北极水声学作为水声学研究的一部分,起步要比达·芬奇所描述的声呐雏形晚很多年。第二次世界大战后北极水声学的研究开始受到发达国家（主要是美国）的重视。它的发展和研究重点带有明显的冷战烙印。冷战结束之后,随着北极持续变暖的趋势,北极及其毗邻海域的海洋水声环境受到特别的重视。环北极的8个国家组成排他性的北极理事会。我国政府于2018年1月26日发表北极政策白皮书,声明中国是近北极国家,是北极地区利益攸关方。本文介绍北极水声学研究的新进展,包括我国有关涉海单位近年来所做的科考和学术研究。指出,北极水声学的研究不局限于把传统水声学中的研究内容（如环境噪声、混响、传播等等）并行地在北极环境条件下加以重复探讨,而是要根据北极海洋环境的实际情况,进行有关领域的新研究。其中不乏传统浅海、深海水声学研究中所不具有的特色,如冰-水界面、冰下的半声道效应、冰盖下水下无人载器（UUV）的通信、定位及声呐对冰下环境的适应性研究等课题。

典型北极冰下声信道多途结构分析及实验研究

针对北极海域典型声场环境,提出了基于OASES-Bellhop耦合模型的冰下声信道多途结构快速分析方法。模型将海冰等效为具有粗糙界面的弹性分层介质,利用微扰法与Kirchhoff近似,估计海冰界面不均匀造成的散射损失,结合射线传播理论对典型北极冰下声信道多途结构进行分析与预报。数值仿真与实验结果表明,在6 km距离处,典型北极冰下声信道由于海冰与海底反射分别形成多途结构,海冰多次反射路径叠加形成的多途结构较为稳定,时延扩展在14 ms范围内,海底反射路径强度相对较弱。OASES-Bellhop模型对冰下声信道多途结构幅度和时延预测误差较小,能够较好的解释及预报实验观测到的多途结构环境特性。

干扰探测一体化信号波形设计与性能仿真

针对探测信号易被截获发现且功能单一,而传统干扰信号对目标的距离分辨能力和速度分辨能力不足,提出了一种基于双载频伪随机二相编码信号的干扰探测一体化信号波形。构建了双载频伪随机二相编码信号的信号模型,推导了其信号模糊函数与频谱函数数学表达式;为分析信号的探测能力和干扰能力,对信号的模糊函数和频率特性进行了仿真。实验仿真结果表明,双载频伪随机二相编码信号的模糊图在时延轴和多普勒频率轴上都具有较好的距离和速度分辨能力,且信号频谱特征图呈现明显的噪声特性,可为干扰探测一体化共享信号波形设计提供理论参考。

一流本科教育背景下高校本科生导师制的审视与实践

本科教育的回归,意味着高校人才培养质量提升进入加速阶段。建设一流本科教育,高校本科教育的理念、模式、方式和方法都会发生变革。在实践中,高校本科生导师制面临着导师指导意愿不强、导师高级职称偏少、导师类型相对单一、导师指导机制松散、导师激励手段有限等困境,阻碍和限制了高校本科生导师制人才培养作用和功能的发挥。本科生导师制作为提高学生学业水平、强化学生职业规划的常态化和有效化手段,在一流本科教育建设背景下需要在信念整合、师资强化、类型丰富、机制健全和激励完善等方面进行实践改革和探索。

多机有源无源协同探测及精度分析

针对有源探测易被截获发现,无源侦察精度较低问题,研究采用多机有源无源协同工作方式降低被发现概率,提高探测精度。在探测初始阶段,先利用多机时差算法对目标进行粗定位。在探测跟踪阶段,再使用无源粗定位结果牵引有源探测平台实施目标快速精定位,提高定位精度。仿真结果表明,文中所设计的多机有源无源协同探测方式,对目标定位精度优于单纯的多机无源定位和有源探测方式,且具有作用距离远和良好的隐蔽性优点。

砾石土心墙摊铺CA-BINN动态路径规划模型研究

现有砾石土心墙摊铺作业路径多采用静态规划,未能考虑推土机、自卸汽车等复杂作业环境障碍物的动态变化。针对上述问题,本文提出了一种考虑复杂动态作业环境的砾石土心墙摊铺CA-BINN动态路径规划模型。该模型基于元胞自动机(cellular automata,CA)建模理论,将实时感知的复杂作业环境数据抽象为动态障碍物、摊铺厚度等元胞状态以实时重构动态施工环境;并以元胞状态信息作为生物激励神经网络(biological inspired neural network,BINN)算法的外部输入,同时重构CA框架下BINN算法的神经活性值计算分流方程,实现多料堆整体摊铺作业动态路径规划。其中,单料堆摊铺作业采用以摊铺平整度、无效路径比等指标为目标函数,并结合现场三刀法(三次推移料堆)施工工艺提出的三刀法CA规则实现动态路径规划。工程应用结果表明:所提模型不仅能够在复杂动态作业环境下表现出高安全性、高适应性,而且相较于静态规划模型,路径长度、转折次数和无效路径比分别降低1.9%、42.9%和48%,摊铺平整度提高7%;相较于人工作业,摊铺平整度提高28%,无效路径比降低47%,有效改善了摊铺质量和摊铺效率。

提孜那甫河流域地表太阳辐射估算及其影响因素分析

地表太阳辐射是地球表层主要能量来源,对地表能量平衡、能量交换以及生态水文过程等具有决定性意义。山区地形复杂,其地表太阳辐射时空差异较大且较难估算。采用适用于山区的地表太阳辐射模型对西北昆仑山提孜那甫河流域地表太阳辐射时空分布进行了估算,分析了该流域季节太阳辐射空间分布规律并探讨了地形和云2个重要因素对太阳辐射空间分布的影响。结果表明:(1)地形因子中周围地形阻挡即地形开阔度(Sky view factor,SVF)与年总太阳辐射的关系最为显著,太阳辐射随SVF增加而增加。(2)年总太阳辐射随着高程增加首先减少,再而随之增加。探究SVF随高程的变化,发现其与太阳辐射随高程的变化趋势较为一致,因此在山区复杂地形下地表太阳辐射估算中仅利用高程对其校正存在明显不足,需综合考虑地形效应。(3)研究计算了季节云出现频率空间分布与太阳辐射空间分布的相关系数,结果表明夏季太阳辐射受云影响较其他季节显著。定量分析了地形因子以及云对地表太阳辐射空间分布影响的贡献率,周围地形阻挡SVF对地表太阳辐射空间分布的影响最大,高程和云次之。因此综合考虑地形和云对太阳辐射的影响在山区太阳辐射模拟中是非常必要的,研究可为山区地表太阳辐射模拟提供理论依据,并为山区生态水文过程研究提供方法支撑。

基于多普勒雷达的降水-蒸发预报模型研究及应用

黏土心墙堆石坝在雨季难以有效施工,为解决这一问题,大多工程采用在降雨集中时段暂停施工的方式,从而导致设备及人员闲置、作业效率低、施工成本高等问题。为此,提出以多普勒雷达为基础的降水-蒸发预报模型,定量分析雨季有效施工时间,并建立了精细化的雨季施工体系。模型能够较准确预判坝址区小区域的降雨,指导现场充分利用窗口期开展施工,提高了施工效率,避免了雨季停工及人员设备长时间闲置等情况的出现,确保了雨季填筑强度满足工期要求。该模型在两河口水电站施工中得到有效应用。

Spatial and temporal trends of climate change in Xinjiang, China

Temperature and precipitation time series datasets from 1961 to 2005 at 65 meteorological stations were used to reveal the spatial and temporal trends of climate change in Xinjiang, China. Annual and seasonal mean air temperature and total precipitation were analyzed using Mann-Kendall （MK） test, inverse distance weighted （IDW） interpolation, and R/S methods. The results indicate that： （1） both temperature and precipitation increased in the past 45 years, but the increase in temperature is more obvious than that of precipitation; （2） for temperature increase, the higher the latitude and the higher the elevation the faster the increase, though the latitude has greater influence on the increase. Northern Xinjiang shows a faster warming than southern Xinjiang, especially in summer; （3） increase of precipitation occurs mainly in winter in northern Xinjiang and in summer in southern Xinjiang. lli, which has the most precipitation in Xinjiang, shows a weak increase of precipitation; （4） although both temperature and precipitation increased in general, the increase is different inside Xinjiang; （5） Hurst index （H） analysis indicates that climate change will continue the current trends.

Towed line array sonar platform noise suppression based on spatial matrix filtering technology

The spatial matrix filter was designed and used for solving the problem to detect a weak target who was influenced by the strong nearby platform noise interference of the towed line array sonar. The MFP technology and the DOA estimation technology were combined together by using the sound propagation characteristics of both target and interference. The spatial matrix filter with platform noise zero response constraint was designed by the near-field platform noise normal modes copy vectors and the far-field plane wave bearing vectors together. The optimal solution of the optimization problem for designing the spatial matrix filter was deduced directly, and it was simplified by the generalized singular value decomposition. The total response error to the plane wave bearing vectors and the total response to the platform noise copy vectors were given. The phenomena that strong interferences existed in the bearing course and blind areas existed after filtering were analyzed by the correlation between the plat- form noise copy vectors and the plane wave bearing vectors. It could be found from simulations that it has less blind area and higher detection ability by using the spatial matrix filtering technology.

Iterative inverse beamforming algorithm and its application in multiple targets detection of passive sonar

With the increasing of detection ability of passive sonar,the weak signal detection problem in multiple interferences becomes more and more important.In the time/bearing record（TBR） display of sonar detection,when there exist traces of multiple interferences,the identification of weak signal is difficult or impossible.The adaptive noise cancellation technique provides the theoretical basis for suppressing strong interferences.But the solution for finding the steady-state optimum filter matrix is quite difficult due to the real time calculation of inverse matrix of input data correlation matrix.The iterative inverse beamforming（IBF） algorithm for solving the optimum filter vector,which is expressed by inverse matrix of the ocean environment data,is derived in this paper,by which,the optimum filter can be eventually expressed as a sum of series simple matrices of constructed from sensor data.Based on the algorithm proposed in this paper,some examples of at sea experiment are provided.The strong interferences are cancelled and the weak signal is emerged,even it didn＇t appear in the conventional beamforming（CBF） processing.

A simplified ZOOM FFT algorithm in digital sonar

DEMON and LOFAR have been proved the powerful means in underwater acoustic signal processing, especially in weak signal detection and target noise classification. Sometimes one need to know the fine structure of frequency spectrum of received signal. It is necessary to take a very long data to get high frequency resolution. This is not always possible due to the hardware and software limitation. Zoom FFT is one of the trade-off consideration for solving high frequency resolution problem, if we are only focus on some special frequency bins. Previous discussions mainly bifurcate into two different representations, the Complex Modulation and Cascade FFT. The former one traditionally needs some kind of special treatments, such as the complex modulation, Lowpass filtering, down-sampling. While the latter achieves the same result by two cascade FFT, with necessary modifications in phase and amplitude, thus is feasible for real-time implementation. Based on some theoretical analysis, a relationship between the complex moduIation and cascade FFT has been described in this paper. In addition, the selection of parameters such as windows function, sample rate, overlap factor have been discussed. Finally, the algorithm is presented and some simulation results are illustrated

A new algorithm of background equalization in digtal sonar

Background equalization technique has been extensively studied in the field of fre quency spectrum analysis. It has been proved an effective method to improve line spectrum extraction. In a multi-beam digital sonar system, the time/bearing display is the most im portant interface between signal processing module and console module. It has been proved that the system gain is often lost in the illterface between these two modules. Background equalization technique can improv the overall performance of the time/bearing display win dow. The result of post processing of multi-beam data is filtered each time before displayed in screen. It is shown that the algorithm of median value filtering combined with OTA (Order Truncate Average) method is an efficient technique to equalize the non-uniform, non-stationary background data. A variable length of window is adopted to match the different width of main lobe of beam patterns. The results of system simulation show that the algorithm described in this paper is efficient in the case that the background of multi-beam data is not uniform

Investigation of the spatial correlations of flow noise in vector hydrophone towed linear array

Following the wall pressure spectrum of the turbulent boundary layer developed by Corcos, a method in the frequency-wavenumber space was presented to analyze the flow noise in the vector hydrophone towed linear array. The general forms of the acoustic pressure and particle velocity in the flow noise field were obtained, and the spatial correlations of the flow noise were calculated. The numerical analysis results based on wavenumber integration show that： （1） The spatial correlations of flow noise drops rapidly with increasing axial separation between the elements, so the flow noise received by different vector hydrophones usually sampled in a half-wavelength rate can be considered as independent; （2） The flow noise is highly correlated in the radial direction at low frequency, and only those of high frequency componet can be neglected.

The time-delay estimation method of resolving left-right target ambiguity for twin-line array and its realization

The directivity function of twin-line array sonar can be used to identify left/right target. But the performance is limited by the distance between two lines and also exists the blind area for target identification. A method, which is based on the time-delay estimation between two line arrays, is proposed to identify left/right target. The method is robust with the variation of distance between two line array, the blind area is considerable decreased. The theoretical analysis results and realization method is described. An algorithm base on interpolation method for precisely estimating time-delay of two line array are derived, therefore the left-right ambiguity problem is solved.

A new method of passive ranging for underwater target:distance information extraction based on wave guide invariant

The study of wave guide invariant in underwater acoustics is one of attracted topics in recent 30 years. The interferences of direct wave and reflect wave from sea surface and sea bottom of underwater target radiated noise inherent the information of target distance. Extraction of these distance information will provide a possible new way in passive ranging for underwater target. The theoretical analysis and the results of at sea experiments show that the LOFAR （Low Frequency Analysis Record） figure inherently contains the range and moving information of passive acoustic sources, even in the situation that the receiver is only one single hydrophone. The theoretical analysis of extraction of target distance information by using wave guide invariant is presented in this paper. It is shown that, based on the interference striation pattern of target, the hydrophone array system is possible to extract the distance information with quite high array gain. Although the mathematical constrain conditions in forming interference striation pattern are different for individual array element, but it is proved that the differences of time delays between array elements can be used in compensation of beamforming. The theoretical analysis, system simulation and some results of at sea experiment show a new way in passive ranging and target recognition.

Multipath structure of the typical under-ice sound channel in the Arctic:theory and experiment

Considering the typical acoustic environment of the Arctic, this paper proposes a method based on OASES-Bellhop coupled model to rapidly analyze the multipath structure of the under-ice sound channel. Firstly the proposed model refers to ice plate as stratified elastic media with some roughness. Secondly,it uses the perturbation method and Kirchhoff approximation theory to solve the scattering loss due to the sea ice inhomogeneity. Finally, the model predicts the multipath structure of the under-ice channel through Ray theory. The results of the numerical simulation and experiment indicate that the typical Arctic sound channel presents multipath structures due to the sea ice and seabed in the range of 6 km, respectively.The sea ice reflection paths are stable,with a short multipath spread within 14 ms. The seabed reflection paths have relatively weak strength. The proposed OASES-Bellhop coupled model successfully predicts the amplitude and delay of the multipath structure with small error,which indicates the proposed model is able to analyze and predict the multipath structure of the observed acoustic environment in experiment.

Precise bearing and automatic target tracking in digital sonar

Precise bearing and Automatic Target Thacking (ATT) for moving vessels is an important thesis in passsive sonar design. Thaditionally in the analogue sonar, DOA (Direction Of Arrival) of the target is determined by the single beam steerillg directioll, which rotate some kind of compensator to steer the target. The compensator is typically made of many electronic and mechanical components. The bearing accuracy substantially depellds on the accuracy of tapped delay line and many mechanical factors. With the application of digital technique in sonar, the precise bearing and ATT technique have attracted much interests for sonar designer. For a digital sonar the multi-beam system has discrete beam direction, which distributed in space, each beam has its fixed steering direction. It is important to use them to estimate precisely DOA of target and then to track the target automatically when the target is moving. An algorithm, which can precisely estimate the value of DOA of the target, is derived in this paper. The relationship among bearing accuracy and the array aperture, signal frequency band is expressed. The expression of time-delay estimation and bearing accuracy is presented. DOA of the target can be precisely calculated by the parabolic interpolation in the neighbourhood of the main lobe of the directivity function. It is easy to implement ATT technique in real time by DSP chips. The results of system simulation show that the algorithm derived in this paper has very good performance in target tracking.