Target detection beneath foliage using polarimetric synthetic aperture radar interferometry

Abstract

In this paper, we demonstrate how the new technology of polarimetric synthetic aperture radar (SAR) interferometry can be used to enhance the detection of targets hidden beneath foliage. The key idea is to note that for random volume scattering, the interferometric coherence is invariant to changes in wave polarization. On the other hand, in the presence of a target the coherence changes with polarization. We show that under general symmetry constraints this change is linear in the complex coherence plane. These observations can be used to devise a filter to suppress the returns from foliage clutter while maintaining the signal from hidden targets. We illustrate the algorithm by applying it to coherent L-band SAR simulations of corner reflectors hidden in a forest. The simulations are performed using a voxel-based vector wave propagation and scattering code coupled to detailed structural models of tree architecture. In this way, the spatial statistics and radar signal fluctuations closely match those observed for natural terrain. We demonstrate significant improvements in the detection of hidden targets, which suggests that this technology has great potential for future foliage penetration (FOPEN) applications.     

New polarimetric signal subspace detectors for SAR processors

Our studies aim at detecting targets embedded in a complex environment for radar applications. This article deals with new polarimetric SAR (synthetic aperture radar) processors based on subspace detectors. These algorithms use models of targets including physical and polarimetric properties of their scattering on the contrary to the isotropic point model that is commonly used. These processors are implemented by computing the corresponding subspaces that contain the relevant responses of the target. We choose for application the detection of targets under forest foliage. The obtained results both on simulated data of realistic targets and real data show the interest of these new processors.     

Effect of the illumination region of targets on waves scattering in random media with H-polarization

This paper addresses some parameters that have a significant effect on wave scattering in random media. These parameters are: target configuration, including size and curvature; random media strength, represented in the spatial coherence length; and incident wave polarization. Here, I present numerical calculations for the radar cross-section (RCS) of conducting targets and analyze the backscattering enhancement with different configurations. I postulate a concave illumination region and consider targets taking large sizes of about five wavelengths. In this aspect, waves scattering from targets are assumed to propagate in free space and a random medium with H-polarization. This polarization produces what is well known as creeping waves which in turn have an additional effect on the scattering waves that is absent in the case of E-polarization.     

Backscattering from conducting targets in continuous random media for circular polarization

The polarization of incident waves is one of the key factors in radar detection and remote sensing problems. In a previous study, attention was drawn to the anomalous increase in the radar cross-section (RCS) of a target in a random medium that occurs with H-polarization. This large increase occurs with a small-size target and is attributed to the coupling between the direct and creeping waves. In this work, we aim to probe the effect of the creeping waves on the scattering waves for circular wave polarization and compare it with the previous results. Therefore, we can control the target detection by choosing the proper polarization, which does not lead to anomalous phenomena. In doing so, we present numerical results for RCS and analyse the characteristics of the enhancement in the RCS (ERCS) behaviour of targets in random media. In this regard, we assume partially convex targets of different configurations. We consider the case in which a directly incident wave is produced by a line source distributed uniformly along the axis parallel to the conducting cylinder (target) axis. Then we can deal with this scattering problem two-dimensionally under the condition of strong continuous random media with large local scale size.     

畸形波电磁散射特性分析及其特征识别标识的研究

针对弱非线性的Longuet-Higgins模型在模拟强非线性畸形波海面时所存在的问题,采用修正的相位调制法模拟一维畸形波时间、空间波面,该方法能够实现畸形波的定时定点生成,并且其波形既能保持目标谱的频谱结构,又能较大程度地满足波浪序列的统计特性.同时,基于改进的双尺度(TSM)法及时域有限差分法建立畸形波的电磁散射模型,经过相对平均偏差和均方根偏差误差分析后,基于TSM法研究分析了畸形波及其背景海面波的归一化雷达散射截面(NRCS)的计算结果.实验表明,合成孔径雷达成像中畸形波的NRCS比背景波要小,即畸形波的合成孔径雷达图像成像比背景波要灰暗,因此可以将NRCS作为畸形波的特征识别标识.通过分析研究不同极化方式、入射角、入射频率条件下畸形波与背景波面的电磁散射特性实验数据得出:当二者的NRCS差值大于-11.8 dB及以上时,即认为产生畸形波,这为实际的工程应用提供了参照标准.     

二维海面上舰船目标电磁散射及合成孔径雷达成像技术研究

研究了二维海面上三维金属目标的电磁散射计算以及合成 孔径雷达(SAR)成像技术. 基于物理光学法、几何光学法和射线弹跳法计算了海 面与目标镜面反射及相互耦合作用; 并用等效电流法计算了目标的棱边绕射作用, 该方法考虑了阴影效应,同时为了消除人为截断引起的边缘衍射, 采用锥形入射波入射. 应用蒙特卡罗法生成的Pierson-Moskowitz谱粗糙面模拟实际海洋面, 计算海面上立方体及舰船的双站雷达散射截面, 通过与数值算法的结果相比较, 验证了该算法的正确性. 运用该解析法快速获取不同频率、 不同角度入射波照射下海面与目标的复合后向散射场数组, 结合SAR成像技术, 得到海面上立方体以及不同姿态舰船目标的SAR成像结果. 该研究成果在实际海洋遥感、海面上军事目标的探测与识别 等领域中具有重要的应用价值.     

Backscattering enhancement for partially convex targets of large sizes in continuous random media for E-wave incidence

The shape of the target constitutes an important factor in the radar detection problem. In a previous study, the enhancement in the radar cross section (ERCS) has proved to be affected largely by the target parameters as well as the effects of the double passage and the spatial coherence length of incident waves around the target. However, the target size was limited to about less than one wavelength. Here, we estimate numerically the RCS of targets taking large sizes of more than three wavelengths, and analyse the characteristics of RCS. Moreover, we investigate the ERCS phenomenon of such targets under different circumstances of random medium and target configuration. In this regard, we assume partially convex targets in continuous random media and also horizontal incident wave polarization (E-wave incidence).     

Dual-beam coaxial amplification system of stimulated Brillouin scattering based on polarization control technology

In the field of Brillouin lidar, it has very important significance to find one method that can amplify the Brillouin scattering signal in real time. One new-type Brillouin lidar detection system based on Nd:YAG pulsed laser and polarization control device is designed in this paper. The key point of this detection system is to have two pulsed coherent lights with same frequency, same polarization and same initial phase, of which one beam is taken as the detection wave for generating stimulated Brillouin scattering signal and the other beam is taken as pumping wave for real time and effective amplification of stimulated Brillouin scattering signal. This detection system mainly includes two pulsed lasers and one electro-optical polarization controller. The laser is mainly used to obtain the pulsed lights with same frequency and same phase, and the polarization controller is mainly used to change the polarization state of two coaxial beams to make them change into same polarization state from orthogonal polarization state thus to enable the pumping wave to amplify the backward stimulated Brillouin scattering signal. It is shown from the experimental results that the adoption of this new system can realize the effect of pumping amplification and can increase the signal to noise ratio to a certain extent.     

Improved GO/PO method and its application to wideband SAR image of conducting objects over rough surface

To simulate the multiple scattering effect of target in synthetic aperture radar (SAR) image, the hybrid method GO/PO method, which combines the geometrical optics (GO) and physical optics (PO), is employed to simulate the scattering field of target. For ray tracing is time-consuming, the Open Graphics Library (OpenGL) is usually employed to accelerate the process of ray tracing. Furthermore, the GO/PO method is improved for the simulation in low pixel situation. For the improved GO/PO method, the pixels are arranged corresponding to the rectangular wave beams one by one, and the GO/PO result is the sum of the contribution values of all the rectangular wave beams. To get high-resolution SAR image, the wideband echo signal is simulated which includes information of many electromagnetic (EM) waves with different frequencies. Finally, the improved GO/PO method is used to simulate the SAR image of targets above rough surface. And the effects of reflected rays and the size of pixel matrix on the SAR image are also discussed.     

Method for radar cross section measurements in millimeter and submillimeter wave regions

We present a new method for electromagnetic modeling radar scattering processes. In this method we use a quasi-optical waveguide of the type of "hollow dielectric waveguide" as a fundamental component of radar cross section (RCS) instrumentation systems. Such waveguide structure produces required target-illuminating quasiplane wave and suppresses unwanted waves as well as transmits the legitimate signal from test object, mounted inside this waveguide, to the reception zone. Presented method is especially effective one in millimeter and submillimeter wave regions, in particular, for investigation RCS of targets with small reflectivity.     

一种利用地物散射特性进行后续类别调整的极化SAR影像分类方法

针对常规极化SAR影像监督分类在分类时由于极化信息利用不完全,导致分类结果不可信的缺陷,在分析地物极化散射特性的基础上,提出一种结合复Wishart分类器思想的极化SAR监督分类方法,在监督分类的基础上,利用几乎包含所有极化信息的相干矩阵进行后续分类,利用地物所有极化散射信息完成类别的划分。首先,利用Cloude&Pottier极化特征组合对影像进行监督分类,获得初始类别划分结果;为纠正监督分类时仅利用特征矢量的空间分布特性进行类别划分导致的错误,对监督分类结果精度评价及研究区地物散射相似性分析,综合确定初始分类结果中分类效果较差、待后续调整类别的像素集;然后以各类别的相干矩阵均值为初始聚类中心,利用核模糊C均值算法,结合相干矩阵的复Wishart分布特性,对待修正的像素集进行后续类别迭代调整,获得精细分类结果。采用国产X-SAR获取的海南陵水地区的全极化SAR影像进行分类试验,结果表明：提出的对极化SAR影像监督分类后利用地物散射特性相似性对错分像素重新进行聚类调整类别的方法,与仅进行监督分类相比,分类结果总体精度更高,也更满足地物的散射特性。     

电磁波极化的应用

极化是电磁波的一个重要参量,是电动力学、电磁场与电磁波课程教学中的重点内容之一.系统介绍电磁波极化这一概念在通信、雷达和抗干扰等方面的应用,即利用电磁波极化实现无线电信号的最佳发射和接收,利用极化提高通信容量,探讨了电磁波极化在雷达目标识别、检测、成像以及抗干扰中的应用.     

Polarized optical coherence imaging in turbid media by use of a Zeeman laser

A method that uses a Zeeman laser in conjunction with a Glan-Thompson analyzer to image an object in a turbid medium is proposed. A heterodyne signal is generated only when the scattering photons are partially polarized, and the spatial coherence is not seriously degraded after the signal propagates in the turbid medium. A system combining polarization discrimination with optical coherence detection to image the object in a scattering medium is successfully demonstrated. The medium is a solution of polystyrene microspheres measuring 1.072 mum in diameter suspended in distilled water contained in a 10-mm-thick quartz cuvette. The advantages of this optical system, including better selectivity of the weak partially polarized scattering photons and better imaging ability in higher-scattering media, are discussed.     

基于海面散射模型的全极化合成孔径雷达海洋环境探测关键技术参数设计仿真研究

技术参数设计研究是提升全极化SAR海洋环境探测能力的有效途径. 本文通过分析噪声等效后向散射系数和辐射分辨率与海洋环境探测的地球物理联系,提出一种全极化SAR关键技术参数设计方法. 采用全极化海面散射模型计算不同海洋环境条件下的雷达后向散射系数,据此确定全极化SAR对海探测的噪声等效后向散射系数,再将其作为合成孔径雷达方程输入参数,以辐射分辨率和信噪比的函数关系为约束条件,开展信噪比、辐射分辨率和系统功率孔径积等技术参数设计研究. 论文通过全极化海面散射模型的仿真计算,发现对海探测的噪声等效后向散射系数设计为-35.0 dB,可满足全极化SAR不同海洋环境条件的探测需求. 通过研究辐射分辨率与信噪比的函数关系,发现对海探测的最优信噪比为8.0 dB. C波段机载全极化SAR设计结果表明,由于兼顾了海洋环境探测需求,本文设计方法使得全极化SAR技术参数同时满足了海洋环境应用需求和系统设计要求. 关键词： 海面散射模型 全极化合成孔径雷达 海洋环境探测 技术参数设计     

含卷浪Pierson-Moscowitz谱海面电磁散射研究

海尖峰的存在会导致雷达虚警概率的上升和多目标环境中检测性能下降,因此研究海尖峰现象意义重大.海尖峰现象的一个重要特点是海面的水平极化散射强度接近甚至大于垂直极化散射强度,卷浪被认为是产生海尖峰的一个原因.首先建立了卷浪和Pierson-Moscowitz谱海面的共同模型,利用矩量法研究了卷浪模型的水平和垂直后向电磁散射特征,包括入射频率、入射角、风速和风向对电磁散射特征的影响.发现在小擦地角情况和较大风速下超级现象(水平散射强度大于垂直极化散射强度)比较明显,从而推论出在小擦地角入射下产生海尖峰现象的概率较大.同时对时变卷浪在小擦地角入射时的海杂波幅值分布特性和多普勒谱进行了分析.     

极化合成孔径雷达海面成像模拟及分析

海浪极化SAR成像模拟对海面散射的研究有重要的意义.基于海浪谱理论,在考虑大、小尺度海浪影响的基础上,构建了双尺度的随机粗糙海面.根据该海面模型,利用Bragg散射模型,小扰动模型与速度聚束理论,模拟了不同状况下海浪的极化SAR图像.并针对海浪参数对极化图像的影响进行了分析,结果符合成像统计特性,得出了一些有益于海浪信...     

A new technique to characterize foliage attenuation using passive radar in the L-band

The goal of the experiment proposed in this paper is to give rapidly and with a limited equipment the attenuation level in the L-band for various elevation angles, between 20 and 70 degrees. The original principle is to use the L-band signal transmitted from an airport radar. The signal backscattered by a plane flying over the forest next to the airport is received on many antennas: some are over the canopy; others are on the ground under the foliage. The direct path signal transmitted by the airport radar is received by the antennas located above the forest. This signal is used to synchronize the temporal signals by detecting the waveform of the transmitting pulses. The signal backscattered by the plane is received by two H and V polar antennas located over the forest and by two other antennas placed under the foliage. The signals received by these antennas are digitized and processed to extract the plots of the opportunistic targets that approach the airport. The magnitudes of each plane echo are measured on each channel, and a comparison of the level of signal is made between the antenna above and under the forest. The ratio of magnitude between the two measurements on each polarization component gives the absorption factor of the foliage at the place of experiment. The position of the plane is given by an ADS-B receiver. For each elevation position of the antennas, the pattern of the chosen target will describe all the angles of arrival. This experiment has been deployed on two forested sites near an airport in South-East Asia.     

Multi-target real-time ranging with chaotic laser radar

We demonstrate the feasibility of multi-target real-time ranging with a chaotic laser radar. The used chaotic laser is emitted by a semiconductor laser with optical feedback. We design a proof-of-concept experiment based on the correlation detection and realize the range measurements of two targets simultaneously. The range resolution of 9 cm between two targets is achieved, which is limited by the bandwidth of the used real-time oscilloscope. A preliminary experiment of chaotic laser coherence is carried out to verify the high resolution of the chaotic lidar.     

Backscattering enhancement analysis for targets in continuous random media based on wave polarization

Owing to the double passage effect, the phenomenon of backscattering enhancement arises in which the radar cross-section (RCS) in a random medium is twice that in free space. In a previous study, it was proved that the enhancement in radar cross-section (ERCS) deviates from two and has large and anomalous fluctuations, sometimes as a result of the wave polarization and other parameters, especially for targets in strong random media. Linear, including horizontal and vertical, polarizations were considered. In this paper, a numerical analysis is presented to show that the fluctuations can be reduced and make ERCS dependent almost only on the double passage effect under certain conditions. Therefore, we will have a better detection technique of targets of large sizes in continuous random media. In doing that, the linear and circular polarizations of incident waves are considered. We assume the case where a directly incident wave is produced by a line source in the far field distributed uniformly along the axis parallel to the conducting cylinder (target) axis.     

On radar accuracy using beam wave in random media

The accuracy of radar detection is measured in this paper through comparing the laser radar cross-section (LRCS) to RCS data for plane-wave incidence in free space. This is to study the performance of backscattering enhancement from conducting targets with finite size. LRCS is calculated using beam wave incidence propagating in a random medium. Targets are of large size with relatively complex cross-sections. E-wave polarization is assumed for incident waves in continuous random media with different spatial coherency.