实测海尖峰特性分析及抑制方法

在高分辨率、低擦地角、高海况以及HH极化工作方式下,雷达回波强度会明显增强,容易产生海尖峰,使得海杂波具有高幅值、非平稳和非高斯等特点,严重影响雷达对海面微弱动目标的检测.为此,该文提出一种海尖峰抑制方法,首先,给出了海尖峰判别和筛选方法,在此基础上,分析了海尖峰的幅值特性、时间相关性、多普勒谱和分数阶功率谱特性;其次,通过剔除背景中的海尖峰并选取最小平均功率水平的背景杂波作为待检测数据,能够抑制海杂波,改善SCR;最后,实测数据的实验结果验证了该方法的有效性.      

基于实测数据的分时全极化雷达海杂波分布研究

首先基于IPIX分时全极化雷达的海杂波实测数据,分析了瑞利分布、韦布尔分布、对数正态分布和K分布对实测数据的拟合效果,发现对数正态分布拟合水平极化接收通道的杂波数据效果较好,K分布拟合垂直极化接收通道的杂波数据效果较好,但是仍然存在海杂波"拖尾"区域拟合不理想的情况.为此,假设海杂波中的布拉格散射体的回波服从K分布,离散海尖峰的回波服从对数正态分布,海杂波整体服从K分布和对数正态分布的叠加混合分布,提出了一种具有闭式表达式的混合分布,以改善海杂波分布"拖尾"部分的拟合效果.实测数据分析表明,本文提出的混合分布对海杂波幅度分布的拟合效果优于对数正态分布和K分布.     

Measurement and classification of low-grazing-angle radar seaspikes

High-resolution dual-polarization X-band images of the ocean surface were obtained at a grazing angle of about 3°. Area extensive imaging allowed us to study the backscatter properties of sea spikes and to compare radar measurements with visual surface features evident from video recordings. The vertically polarized radar images consist of distributed scatter whose amplitude and Doppler velocity are modulated by larger scale gravity waves consistent with Bragg scattering and composite surface theory (CST). The horizontally polarized radar images are dominated by spatially discrete scattering centers (or sea spikes) moving at velocities comparable to the phase velocities of gravity waves beyond the spectral peak. These sea spikes also exist in the corresponding V-pol radar images, but are less prominent due to the dominant Bragg backscatter. Sea spikes are characterized by polarization ratios H/V that often exceed unity, typically by about 5 dB. Comparison of the larger spikes with simultaneous co-registered video recording of the surface indicates that approximately 30% of observed sea spikes are associated with actively breaking waves (whitecaps) while the remainder are identified with “steep” wave features. By classifying the larger sea spikes according to their corresponding surface features, we find hat the Doppler velocities for sea spikes due to whitecaps are noticeably faster (about 50%) than other sea spikes, though the distributions for both overlap significantly. We also find little measurable difference in the polarization ratios of the two classes of sea spikes as observed on the open ocean     

L波段小擦地角海杂波幅度均值与风速关系建模

在对比分析国际上现有模型中风速对海杂波幅度均值的影响基础上, 提出了一种新的两分量高斯模型建立海杂波幅度均值与风速的关系.结合我国黄海L波段VV极化小擦地角海杂波实测数据, 对该模型进行了验证与精度分析, 并研究了该模型参数随擦地角的变化关系, 给出了模型拟合参数.从数据分析的结果来看, 该模型较好地捕获了海杂波幅度随风速的变化趋势, 可为雷达系统设计和雷达海杂波预测提供技术支撑.       

海杂波统计特性分析

分析了海杂波背景下的目标检测的困难,研究了后向散射系数在海杂波的各个散射区中的性质,列举分析了几种常用的海杂波后向散射系数理论模型,重点分析了海杂波幅度特性和海杂波相关特性,不但有助于设计和实现雷达滤波器,而且有助于提高抑制杂波的能力以及保证雷达应有的探测性能。     

X波段雷达实测海杂波的特性分析

舰载雷达和海岸警戒雷达的目标检测和跟踪性能与海杂波密切相关。对X波段雷达实测海杂波进行了功率谱密度和相关分析,得到了一些有益的结果。与VV极化相比,HH极化的海尖峰特性更明显;与HH极化相比,VV极化海杂波的功率谱密度具有更窄的带宽,且衰减得更快。而HH和VV极化海杂波具有相似的相关性,但HH极化的相关系数要比VV极化的要小。得到的结果对雷达目标检测与识别具有一定的借鉴意义。     

Multipath scattering in ultrawide-band radar sea spikes

This paper presents sea scatter data collected with an ultrawide-band (UWB) polarimetric radar system that indicates that multipath scattering plays an important role in the generation of sea spikes. The radar system used in this study produces short pulses with a bandwidth of approximately 3 GHz centered at 9 GHz for a range resolution of approximately 4 cm. Pulse-to-pulse switching allows collection of the microwave echoes produced by all four combinations of linear transmit and receive polarizations [vertical-transmit vertical-receive (VV), horizontal-transmit horizontal-receive (HH), horizontal-transmit vertical receive (HV), and vertical-transmit horizontal-receive (VH)] each of which is collected by a sampling oscilloscope utilizing equivalent time sampling. In June 1996, upwind sea scatter data at grazing angles of 10°, 20°, and 30° were collected while the system was deployed on a research pier on the Outer Banks of North Carolina. An analysis of the strongest echoes (sea spikes) from this data set is presented and discussed. First, the cumulative distribution functions are presented. Second, an increase of approximately 5 dB is shown to occur in the polarization ratio (HH/VV) of the strongest echoes as the grazing angle decreases from 30° to 10°. Third, differences in the spatial and spectral characteristics of the VV and HH spikes are described. Through comparisons with laboratory results and a simple scattering model, these observations are explained by the presence of a multibounce scattering mechanism. The use of the model to extract wave height from the sea-spike frequency response is also explored     

机载L波段雷达海杂波幅度分布特性分析

机载雷达主要在海杂波背景下探测舰船目标，探测性能受海杂波影响大，海杂波分布特性直接影响检测器设计和系统探测性能.本文对L波段机载雷达回波特性、海杂波分布特性进行了理论分析建模，从目标检测的角度提出了海杂波预处理流程，并采用拟合的方法，基于实测数据对Rayleigh分布、Lognormal分布、复合K分布、Weibull分布等四种典型的杂波分布进行了验证，所用5级高海情和3级低海情数据样本分析表明，四种分布中Lognormal分布显著占优.上述方法与结论可用于辅助雷达最优检测器设计，具备较高的工程应用价值.     

Evaporation duct effects on sea clutter

Existing sea clutter models are in general agreement on the trends and magnitudes of sea reflectivity at low (1°-10°) grazing angles. However, at extremely low grazing angles (<1°), models, theory, and measurements show considerable differences. The effects of the oceanic evaporation duct on grazing angle are investigated. Significant changes in grazing angle due to the evaporation duct are found and the illumination of the sea surface by the radar is shown to extend well beyond the standard (4/3 earth) radar horizon range. A series of sea clutter model comparisons shows that much of the difference between models can be attributed to evaporation duct effects     

基于高斯短时分数阶Fourier变换 的海面微动目标检测方法

海上目标随海面颠簸导致姿态变化,引起回波功率调制效应,导致回波多普勒体现时变和非平稳特性.为此,本文将微多普勒理论应用于海杂波中弱目标检测,提出一种基于高斯短时分数阶Fourier变换(GSTFRFT)的海面微动目标检测方法.首先,建立海面目标的平动和三维转动回波模型;然后,基于海尖峰判别方法对回波信号进行数据筛选,改善信杂比,并采用GSTFRFT对微动信号进行增强处理,利用海面目标与海杂波的微动特征差异设计恒虚警检测方法;最后,通过GSTFRFT域滤波,提取信号的微动特征并得到瞬时频率.实测雷达数据仿真结果验证了算法的有效性,具有在强海杂波中检测微弱目标的能力.     

基于门控循环神经网络的海杂波幅度预测

海杂波是雷达在海洋表面采集到的海面电磁散射回波.受海洋环境要素（风速、风向、浪高、浪向等）和雷达参数的影响,其幅度随时间具有随机起伏性,海杂波的幅度预测精度的提高有助于增加目标检测准确度.本文结合海杂波非高斯非线性的特点,提出了基于门控循环神经网络的海杂波幅度预测方法.通过对IPIX雷达和P波段雷达海杂波实测数据的预测分析,结果表明,本文方法相对已有传统方法具有更高的预测精度.     

Numerical simulation of low-grazing-angle ocean microwavebackscatter and its relation to sea spikes

This paper presents the results of numerically simulating microwave backscatter from a deep-water breaking wave profile. Enhanced microwave backscatter from the crests of breaking waves has been hypothesized as the source of bright short-lived microwave radar echoes that are observed at low-grazing angles (LGAs). The characteristics of these “sea spikes” are distinctly different from the Bragg-scatter echoes that dominate measurements made at moderate grazing angles. Of particular interest is the high contrast that sea spikes present against ocean background backscatter when observed with horizontally polarized transmit/receive configurations [horizontal (HH) versus vertical (VV)]. This HH/VV contrast disparity has been attributed to polarization-selective cancellation of the direct reflection from the wave crest by the surface reflection. This hypothesis is reinforced first by showing evidence that VV polarization is suppressed in the intensity range that would normally be populated by the brightest scatterers. Histograms of unaveraged Doppler-centroid measurements show further that the depleted VV backscatter population is responding to scatterers that are moving much more slowly than the HH scatterers. The Doppler-centroid histograms provide a sharper delination between the two scattering populations than do the unconditionally averaged Doppler spectra that are more commonly reported. Finally, our numerical simulations show evidence of an interference mechanism that selectively suppresses VV backscatter. In our simulations, the polarization selectivity comes from the phase dependence of the backscatter from the wave crest. A Brewster phenomenon at the surface reflection point is not necessary     

Compound representation of high resolution sea clutter

A proposed form of compound distribution to describe the non-Rayleigh distribution and correlation properties of high resolution radar sea clutter is shown to be a good fit to experimental data. From this model the K distribution is derived, and a possible physical mechanism is discussed.     

非高斯海杂波背景最优相干检测性能分析

非高斯海杂波背景下雷达目标检测一直是研究的重点。海杂波分布参数形式的多样化使得检测器对杂波平均功率的恒虚警特性无法保证,因此文中对描述非高斯海杂波的K分布、广义Pareto分布和逆高斯纹理复合高斯分布的参数形式进行了规范化,得到了规范化后的三种杂波分布对应的最优相干检测器。理论分析和实验验证了三个自适应检测器分别对杂波平均功率、目标多普勒频率和散斑协方差矩阵具有恒虚警特性。仿真实验表明:散斑协方差矩阵估计方法和辅助数据的数量均影响着自适应检测器的检测概率和虚警概率。     

海杂波背景下的低空小目标检测

海面上低空小目标通常具有RCS（雷达散射截面）小、速度快的特点,并且容易湮没在海杂波背景中,很难被雷达系统检测到。文中首先分析了海杂波数据的统计特性;然后在此基础上,利用海杂波和目标在相关特性上的差异,提出了一种熵加权非相参积累方法,该方法能够抑制海杂波,并且积累目标能量;为了进一步降低检测门限,采用了TBD（检测前跟踪）技术。某S波段雷达搜索模式下低分辨实测数据的处理结果表明,熵加权非相参积累和TBD方法能够有效降低海杂波背景下低空小目标信号的最小可检测信杂比。     

Uncovering nonlinear dynamics-the case study of sea clutter

Nonlinear dynamics are basic to the characterization of many physical phenomena encountered in practice. Typically, we are given a time series of some observable(s) and the requirement is to uncover the underlying dynamics responsible for generating the time series. This problem becomes particularly challenging when the process and measurement equations of the dynamics are both nonlinear and noisy. Such a problem is exemplified by the case study of sea clutter which refers to radar backscatter from an ocean surface. After setting the stage for this case study, the paper presents tutorial reviews of: (1) the classical models of sea clutter based on the compound K distribution and (2) the application of chaos theory to sea clutter. Experimental results are presented that cast doubts on chaos as a possible nonlinear dynamical mechanism for the generation of sea clutter. Most importantly, experimental results show that on timescales smaller than a few seconds, sea clutter is very well described as a complex autoregressive process of order four or five. On larger timescales, gravity or swell waves cause this process to be modulated in both amplitude and frequency. It is shown that the amount of frequency modulation is correlated with the nonlinearity of the clutter signal. The dynamical model is an important step forward from the classical statistical approaches, but it is in its early stages of development     

低空雷达导引头海面目标检测性能分析

该文以雷达导引头低空检测海面目标为背景,综合分析了多径环境和海杂波环境对雷达导引头目标检测的影响。通过对目标回波、多径散射的镜反射、漫反射以及海杂波进行建模,结合具体场景,仿真分析了镜反射、漫反射以及海杂波对雷达导引头接收信号的影响,进而分析了多径散射和海杂波对雷达导引头检测不同大小目标时检测性能的影响效果。仿真结果表明:雷达导引头检测RCS低于1 m2的小目标时,海杂波是影响雷达导引头检测性能的主要因素;雷达导引头检测RCS大于10 m2的目标时,多径效应是影响雷达导引头检测性能的主要因素;雷达导引头检测RCS大于104 m2的目标时,雷达导引头的检测性能不受海杂波和多径效应影响。     

On the fractal dimension of sea surface backscattered signal at low grazing angle

Fractal analysis of sea surface backscattering signal (sea clutter in radar terminology) represents a novel technique for the study of sea surface roughness. When Kirchhoff's assumption is satisfied, the fractal dimension of the signal is linearly related to the fractal dimension of the sea surface. Moreover, such a relationship is independent of transmitted frequency, polarization, time, space, sea wave propagation direction, incident angle (within the constraint of Kirchoff's assumption) and significant wave height. Nevertheless, for a low grazing angle, the Kirchhoff approximation does not hold and the behavior of the sea clutter fractal dimension cannot be theoretically predicted. The purpose of this paper is to investigate the fractal dimension of the sea clutter at low grazing angle, in order to extend the theoretical results. Moreover, the effects of the presence of a target on the sea surface are analyzed by means of the fractal dimension. Such an analysis is performed by using live recorded clutter data. In detail, the fractal dimension's dependence on space, time, sea wave propagation direction, sea wave height, transmitted polarization and presence of targets is investigated. A discussion on the use of the sea clutter fractal dimension for sea surface monitoring is addressed.     

复杂海杂波背景下分数低阶匹配滤波检测方法

针对在复杂海杂波背景下,雷达目标检测中动目标检测(Moving Target Detection,MTD)技术的检测性能显著下降的问题,以及局部最优检测器(Locally Optimum Detector,LOD)仅适用于低信杂比背景下弱目标检测的问题,基于分数低阶统计量理论,提出了一种分数低阶匹配滤波检测方法.该方法通过幂变换抑制杂波的非高斯特性,通过应用杂波分数低阶协方差矩阵特征值分解的方法白化相关杂波,在此基础上应用匹配滤波进行目标积累,以提高信杂比.通过仿真和实测数据,对所提出方法的检测性能进行了验证,并且与MTD和LOD进行了比较.结果表明,本文所提出方法能较好地解决非高斯相关杂波背景下的目标检测问题,检测性能明显优于MTD和LOD方法.     

高频雷达海杂波的多重分形建模方法研究

将多重分形理论引入高频雷达海杂波建模,提出了一种新的高频雷达海杂波的时域多重分形模型。对比分析了实测数据海杂波、分数布朗运动(FBM)模型和多重分形模型的质量指数函数和奇异指数,结果表明:该多重分形模型与实测数据具有相似的多重分形性。此外,通过对韦布尔分布、对数正态分布和瑞利分布三种最常用海杂波的概率密度函数的比较分析,并利用修正的柯尔莫哥洛夫-斯米尔洛夫(K-S)统计检验,得出了高频雷达海杂波的多重分形模型具有先验的统计特性的结论,从而进一步证明了该多重分形模型的合理性和有效性。