【作者】 李健；

【导师】 陈泽宗；

【作者基本信息】 武汉大学 ， 电路与系统， 2021， 博士

【摘要】 海洋环境监测对于海洋防灾减灾,保护专属经济区利益、维护国家主权和海洋权益、海洋环境保护和生态研究、海洋资源开发、居民的生产生活等都具有重要意义。高频地波雷达是一种海洋环境监测的有效工具,吸引了国内外学者的广泛关注。近年来,国内外也开始了对高频雷达组网、岸基双基地高频雷达、GIOS(Ground-Ionosphere-OceanSpace)高频雷达、天地波高频雷达、空基高频雷达等一系列新形式、新体制高频雷达的研究。通过剖析国内外雷达发展趋势,本文也提出了一种名为岸空双基地高频雷达的新体制雷达,来对大范围的海洋环境进行监测。目前,国内外缺少对于岸空双基地高频雷达海面散射机理的研究,本文结合国家自然科学基金项目“岸空双基地高频雷达海洋环境探测新理论与新方法研究”,围绕岸空双基地高频雷达的散射机理进行了一系列的研究。在海面一阶散射机理的研究基础上,对高频雷达海洋表面参数反演和海杂波抑制等当前高频雷达热点问题进行了深入研究,并提出了改进算法。具体工作内容如下:(1)构建了岸空双基地高频雷达几何模型,研究了该新体制下的电磁波海面散射机制,推导了岸空双基地高频雷达的一阶散射截面方程和二阶散射截面方程。岸空双基地高频雷达将发射机放置于海洋沿岸,采用掠入射发射窄波束的垂直极化电磁波照射海面,处于机载平台或者空载平台的接收机接收处理海洋回波信号,来测量海洋上风、浪、流信息。然而,关于岸空双基地高频雷达的散射机理方面还缺少相关研究。本文通过建立了岸空双基地高频雷达几何模型,当垂直极化的电磁波照射到轻度粗糙海面时,将海面视为良导体,得到海面电场分布,利用微扰法和Rice边界条件得到表面场的一阶电场和二阶电场,根据基尔霍夫近似理论得到远区场电场分布,推导出了岸空双基地雷达的一阶散射截面方程和二阶电磁散射截面方程,结合水动力耦合对二阶散射截面积的作用,得到了岸空双基地雷达的二阶散射截面方程。为了分析岸空双基地雷达的回波特性,采用PM谱对岸空双基地高频雷达在多种方位角和散射角、多种工作频率、多种风速、风向情况下的一阶回波和二阶回波进行了仿真分析,并对回波谱上奇异值点进行了理论分析,为后续的海态信息提取奠定了基础。将岸空双基地高频雷达散射截面方程退化为岸基单基地和岸基双基地高频雷达情况,与国内外学者所导出的散射截面方程进行对比,验证了所导出的散射截面方程的正确性。(2)在岸空双基地雷达散射截面积的研究基础上,探究了岸空双基地高频雷达的系统参数选择和雷达布站方式对探测距离的影响。通过建立海洋回波信号和背景噪声信号模型,计算电磁波在发射路径和接收路径上的能量衰减,提出了一种岸空双基地雷达探测距离的估计方法。仿真了多种参数下岸空双基地雷达的回波距离多普勒谱,并对探测距离进行了估计。为岸空双基地高频雷达研制过程中的系统参数选择和雷达布站提供了理论指导。(3)对高频雷达风速反演方法进行了研究。为了扩大高频雷达对于风速的探测距离,基于之前对于高频雷达一阶散射机理的研究,导出了海面风速与一阶峰能量之间的理论关系,分析了利用单频率一阶海洋回波反演风速的不足,提出了一种利用多频率一阶海洋回波来反演风速的方法,首先通过对单个频率的原始数据进行数字波束形成,利用雷达回波多普勒谱的正负一阶峰的能量比值反演出风向与雷达波束之间的夹角,从而获得去除了方向函数的一阶峰能量,得到一阶Bragg峰能量与风速之间的关系,最后利用多频数据融合来提高风速反演精度。通过长达20天实验数据结果验证了算法的性能,表明基于多频率一阶海洋回波的方法能提高对于风速的探测距离和风速反演精度。(4)高频雷达中一阶海洋回波对于高频雷达目标检测而言是海杂波,当目标信号和海杂波在多普勒谱上混叠在一起时,目标信号会被海杂波所屏蔽,本文针对这种情况下的海杂波抑制问题进行了研究。首先对一阶海杂波进行了建模和仿真,介绍了奇异值分解、循环对消法等传统的海杂波抑制方法,总结了这几种算法的特性。通过综合利用目标和海杂波的空时维度特征,提出了一种在基于空时维正交投影的海杂波抑制方法。首先从临近距离元的多个通道和慢时间维采样数据中得到协方差矩阵,然后通过特征分解获得海杂波子空间。再将待检测距离元的原信号投影到海杂波的子空间,获得原信号在海杂波空间的分量,最后在原信号中减去原信号在海杂波空间的分量,来达到抑制海杂波的目的。仿真和实测数据结果显示,空时维正交投影算法能有效抑制高频地波雷达一阶海杂波,在抑制海杂波的同时可以将目标信号保留,增强了高频地波雷达的目标检测能力。更多还原

【Abstract】 Marine surveillance is important for disaster prevention,resource management,national sovereignty,environmental protection,marine resource management,and human activities.As an effective tool for marine surveillance,high frequency(HF)ground wave radar has attracted much attention from scholars around the world.In recent years,a series of HF radar with new configurations have been developed,e.g.,shore-based bistatic HF radars,GIOS(GroundIonosphere-Ocean-Space)HF radars,hybrid sky-surface wave HF radar,and air-based HF radar.Through the analysis of HF radar development trends,a new HF radar system named as shoreto-air bistatic HF radar has been proposed to monitor the large-scale ocean environment.Howere,there is a lack of research on the scattering mechanism for shore-to-air bistatic HF radar with sea surface.Combining the National Natural Science Foundation named "Research on new theories and new methods of shore-to-air bistatic high-frequency radar for ocean observation" undertaked by Wuhan University,a series of researches have been conducted on the scattering mechanism of HF radar in this paper.Based on the research on the first-order scattering mechanism for HF radar with the sea surface,the current hot issues for HF radar such as the ocean surface parameters inversion and sea clutter suppression have been studied.The specific work is as follows:(1)Beginning with the establishment of the geometry of shore-to-air bistatic HF radar,the first-order and second-order cross section for shore-to-air bistatic HF radar are derived.The transmitter of shore-to-air bistatic radar installed on the coast emits vertically polarized and narrow-beam electromagnetic waves to illuminate the ocean patch in a grazing incidence;the electromagnetic waves are scattered to a radar receiver deployed at an air platform(airplane or airship)owing to the rough sea surface;then the power spectra are estimated from radar echoed to extract the sea state information.However,there is a lack of research on the scattering mechanism for shore-to-air bistatic HF radar with sea surface.To solve this problem,beginning with the establishment of the geometry of shore-to-air bistatic HF radar,the electric field intensity is obtained for the perfectly conducting rough ocean surface.The first-order and the second-order electric field are derived based on the the perturbation method and boundary conditions.The electric field distribution near the observation point based on Kirchhoff theory is obtained.The first-order cross section and second-order electromagnetic cross section for shore-to-air bistatic HF radar are derived.The second-order cross section for shore-to-air bistatic HF radar is obtained in conjunction with the contribution of hydrodynamic coupling.To analyze the echo characteristics of the shore-to-air bistatic radar,the echo spectra in varies azimuth and scattering angles,varies operating frequencies,wind speeds and wind directions are simulated,and the singularities that occur in the simulated Doppler spectra are theoretically analyzed,which lays the foundation for the subsequent sea state information extraction.Finally,the cross section for shore-to-air bistatic HF radar is reduced into the situation of shore-based monostatic HF radar and shore-based bistatic HF radar to compare with the cross section derived by scholars to verify the correctness of the derived cross section.(2)Based on the cross section of shore-to-air bistatic HF radar,the selection of the system parameters for radar and the configurations of radar deployment is explored.By establishing sea echo signal and background noise models and calculating the attenuation of electromagnetic waves on the transmitting path and receiving path,a method of the detection range estimation for shore-to-air bistatic HF radar is proposed.The Range-doppler spectrum of shore-to-air bistatic HF radar is simulated and the detection range is estimated in varies parameters.It provides theoretical guidance for the selection of system parameters and the deployment of radar stations in the development of shore-to-air bistatic HF radar.(3)Next,the method of wind speed extraction using HF radar is studied.To expand the detection range for wind speed extraction using HF radar,the theoretical relationship between wind speed and first-order peak energy is derived based on the previous study on the cross section of HF radar,and a method using first-order sea echo for multi-frequency HF radar to extract the wind speed is proposed.A digital beamforming is performed on the raw data from single frequency,and the energy ratio of the positive and negative first-order peaks of the Doppler spectrum is obtained to estimate the wind direction.The relationship between the energy of first-order Bragg peak and wind speed is obtained and the accuracy of wind speed measurement is improved through the multi-frequency data fusion.The experimental data results verify the performance of the algorithm,which show that the method based on the multifrequency first-order sea echo can improve the detection range and the accuracy of wind speed measurement.(4)The first-order sea echo in HF radar echo is sea clutter for target detection,which would significantly degrade the target detection and tracking performance of HF radar.The conventional sea clutter suppression methods tend to remove the sea clutter and target signals when they are mixed in the Doppler spectrum.Beginning with the establishment of the model of first-order sea clutter,the conventional sea clutter suppression methods such as singular value decomposition and cyclic cancellation method are introduced and analyzed.Based on the features of the target signal and sea clutter in the spatial-temproal domain,a new method for sea clutter suppression using orthogonal projection is proposed.The covariance matrix is obtained using the data from the adjacent range cell,and the sea clutter subspace is obtained through eigendecomposition.Then project the original signal onto the subspace of sea clutter to obtain the component of the original signal in the sea clutter subspace.Finally subtract the component of the original signal in the sea clutter space from the original signal to achieve the suppression of sea clutter.The simulation and experimental results show that the orthogonal projection algorithm in spatial-temporal domain can effectively suppress the first-order sea clutter of the HF surface wave radar,and the target detection ability of the HF radar is enhanced in the sea clutter area.更多还原