Faraday效应对空间目标探测雷达的影响

分析了电离层Faraday旋转效应对空间目标探测雷达的影响。从磁离子理论出发,结合电离层模型和地球磁场模型,使用了一种快速的Faraday旋转角计算模型,并与传统使用的经验计算方法进行了比较,表明该方法普适性强,更适用于中国地区。针对当前国际上典型的空间目标探测雷达工作频段,选取我国海口（低纬地区）和长春（中纬地区）为代表,分别计算了太阳活动极大年和极小年间的Faraday旋转角,给出了仿真结果并进行分析,表明Faraday旋转效应给线极化方式雷达带来明显的回波能量损失。在进行雷达设计时,需要将该效应与雷达系统性能综合考虑,采用合适的天线极化方式。     

等离子体隐身的极化损耗机理

通过等离子体对电磁波的折射效应和吸收衰减实现隐身是等离子体隐身技术的基本机理，但同时，磁化等离子体还可以通过改变电磁波的极化方式造成雷达波的极化损耗。文中研究了极化雷达波在磁化等离子体中传播时的法拉第旋转、双折射等现象，分析了这些效应使雷达波产生极化损耗的机理。结果表明磁化等离子体对雷达波的极化损耗在理论上是完全成立的，其效果也相当显著。     

基于双极化融合的外辐射源雷达实验分析

雷达极化学揭示了电磁波与目标相互作用的变极化效应,充分挖掘和利用目标极化信息,对提高雷达探测性能具有重要意义。文中对在外辐射源雷达(PBR)探测中利用双极化特征提高探测性能进行了研究。首先,对目标和城区杂波的PBR双极化特性分别进行了仿真计算与实测分析,得到了其交叉极化分量与共极化分量之间的大小关系,为PBR中双极化技术应用提供了基础;然后,根据无源相干定位技术推导了PBR 探测公式,揭示了其与目标双站距离、极化散射特性、多普勒频率之间的关系;最后,根据实验结果具体分析了双极化技术对PBR探测性能改善情况。研究表明,由于双极化探测更充分、有效地利用了目标自身极化散射特性,并用交叉极化方式有效抑制了杂波,所以采用双极化融合技术能有效提高PBR 探测性能。     

舰载微波超视距雷达天线架设高度研究

针对X波段舰载微波超视距雷达,研究了蒸发波导条件下雷达天线架设高度对其探测距离的影响.利用抛物方程模型建立了评估模型,通过仿真分析了天线架高对电磁波传播损耗及雷达作用距离的影响,研究结果表明,不同的层结条件下,虽然天线架高对电磁波传播损耗的影响不相同,但对雷达探测距离的影响却具有一般规律,即波导高度较低时,雷达作用距离随天线架高的增加而增大;当波导高度较高时,雷达探测距离随大线架高的增加先增大后减小.最后针对X波段海上雷达系统天线的架设高度给出了建议.     

全极化相控阵雷达波束扫描对目标观测极化散射矩阵的影响

全极化相控阵雷达天线的极化特性随着波束扫描角度的变化而改变,将给目标极化散射矩阵测量带来两方面的问题:一方面,天线的极化基定义在垂直电磁波传播方向的横截面内,使不同波束指向角度下的雷达观测极化基各不相同,从而导致不同波束指向下对固定姿态目标观测得到的极化散射矩阵不同;另一方面,天线的交叉极化会随着波束扫描角度增大而增大,导致相控阵雷达观测的目标极化散射矩阵元素之间会相互串扰。该文重点分析上述两个方面对目标观测极化散射矩阵的影响,以建立更精确的观测方程,为相控阵雷达极化精密测量提供数学依据。      

基于块稀疏恢复的高频雷达空间角谱和极化谱联合估计方法

高频地波雷达（high-frequency surface wave radar,HFSWR）中电离层杂波的存在,会极大地影响雷达系统的性能,降低对目标的探测能力.为了精确获得杂波参数,从而更好地抑制电离层杂波,提出了一种基于压缩感知（compressive sensing,CS）的单快拍参数估计方法,用于对电离层杂波的空域和极化域参数估计.该方法基于极化敏感阵列的块稀疏估计模型,应用块正交匹配追踪（block orthogonal matching pursuit,BOMP）算法实现距离-多普勒域的单快拍空间角度和极化参数联合估计,并进一步获得目标和杂波的空间角谱和极化谱.该方法适用于任意极化敏感阵列,在距离-多普勒域单快拍条件下,其估计性能优于传统方法,且计算复杂度极低,可以实现实时处理.仿真结果和某HFSWR系统实测数据处理结果表明了参数估计方法的有效性.     

大气波导环境下雷达参数的敏感性分析

大气波导传播可导致电磁波的传播路径和有效覆盖范围与正常折射条件下明显不同,这会影响雷达的探测性能和部署.文中借助于抛物形波方程,研究了雷达系统参数的设置对3种典型波导环境下电磁波单程传播损耗的影响,并对雷达探测性能做出评估,给出了建议,为以后雷达回波资料反演大气折射率廓线作必要的准备.     

涡旋电磁波雷达锥体目标旋转多普勒探测

涡旋电磁波具有独特螺旋状波前结构,其受目标横向微动调制产生的旋转多普勒效应,有望为雷达目标探测技术的发展提供一个新途径。在涡旋电磁波照射下,利用锥体微动对回波瞬时频率的周期性调制特性,可以有效反演出锥体目标的微动参数和几何特征。该文重点研究了雷达前视条件下的锥体目标参数估计。首先,基于涡旋电磁波雷达目标旋转多普勒探测原理,推导了涡旋电磁波锥体目标回波数学方程,建立了锥体目标回波旋转多普勒模型。其次,提出了前视条件下的锥体目标参数估计方法,利用锥顶散射点和锥底散射点两维的旋转多普勒信息,可以对锥体目标微动参数和几何特征参数进行有效估计,仿真结果验证了该文所提方法的有效性及抗噪声鲁棒性。      

一种基于频域解模糊的星载P波段全极化SAR法拉第旋转效应校正方法

星载P波段合成孔径雷达（Synthetic Aperture Radar, SAR）极化测量性能易受电离层法拉第旋转（Faraday Rotation, FR）效应的影响，随着系统带宽增加，以及中心频率对应的法拉第旋转角（FR Angle, FRA）增大，FR效应色散特征突显，将导致距离向成像恶化以及额外的极化测量误差.针对色散FR效应，提出了一种基于频域解模糊的星载P波段全极化SAR法拉第旋转效应校正方法，通过模糊度一致化以及频域拟合等关键步骤，解决了P波段SAR系统FRA估计值模糊问题，最后在频域实现色散FR效应的校正.利用机载P波段全极化SAR数据开展了仿真验证实验，与传统的图像域校正方法相比，有效抑制了色散的FR效应，显著改善了交叉极化通道图像之间的相关性.     

超视距雷达目标的距离测量与跟踪

(一)引言超视距雷达是一种新体制的短波雷达,可以探测地平线以下的低空和中空运动目标,单跳距离能达3000公里,也可以用于通讯或导航。这种雷达由于探测距离远、探测高度低,所以具有一般微波雷达所没有的优点。超视距雷达发射和接收的电磁波,必须通过电离层反射进行传播,所以,雷达工作频率的确定和目标探测,均受到电离层特性的制约。目前尚存在着有待于进一步研究解     

Polarization characteristics of VHF radio waves reflected by the Es-layer

Interference in VHF-TVs caused by the radio waves reflected from the ionosphere is a very serious problem in the area around Japan. The polarization rotation of the waves reflected by an E_s-layer are clarified by the measurement of a VHF television signal. Two types of polarization rotation are observed. In one case, the more common of the two, the polarization rotation varies randomly in a narrow range around an angle which rotate from the transmitted horizontal polarization in a short fading period. In the other case, which has a long fading period, the polarization rotation varies slowly in a wide range. It is shown that these polarization characteristics are caused by the Faraday effect in the ionosphere below the E_s-layer and in the E_s-layer itself     

Application of synthetic-aperture radars for measuring the Faraday rotation angle

The influence of the ionosphere on polarization characteristics of the electromagnetic radiation of a spacebome synthetic-aperture radar is analyzed. The value of the Faraday rotation angle of a linearly polarized wave radiated by this radar into the ionosphere is estimated for a comparatively long-wave band. A technique is proposed for determining the Faraday rotation angle by measuring the reflectivity of the Earth’s surface with a polarimetric radar. This technique is independent of the mapped geometry and orientation of reflected elements of the Earth’s surface.     

The variability of the ionospheric total electron content and its effect on satellite microwave communications

Rotation of linearly polarized waves (Faraday rotation) passing through the ionosphere causes depolarization in frequency reuse satellite communication systems. As the ionospheric total electron content (TEC) is not constant, dynamic compensation for this effect may be required. This paper investigates the magnitude of the TEC variations, the time scales of the variability and the predictability of the excursions. Analysis of long-term TEC measurements shows that the variations are statistical with yearly and seasonal trends strongly coupled to long term solar and geophysical effects. Short-term variations are difficult to predict. Data indicate that reliable Faraday rotation estimates can be made for 99·9 per cent of the time, provided that long-term solar, seasonal and geographical factors are considered. Occasional bursts of solar activity limit the accuracy of long-term predictions.     

Tri‐orthogonal polarization diversity reception for non‐geosynchronous satellite orbit ionospheric channels

Electromagnetic signals propagating through the ionosphere are subject to path delay and the depolarizing effect of Faraday rotation. These are both dependent on global position and link geometry, which constantly vary for satellites in non‐geosynchronous orbits. These effects introduce performance error and reduce range resolution of remote sensing polarimetric measurements. Communication with ground receivers may be severely degraded by these effects. In this paper, a tri‐orthogonal approach at the receiver is introduced to enhance performance of conventional polarization diverse receive schemes. Performance is measured through a capacity metric. The work presented forms part of a large‐field‐of‐view, non‐geosynchronous satellite model exploiting tri‐orthogonal receive polarimetry as a means to enhancing link performance in a field‐of‐view. Copyright © 2016 John Wiley & Sons, Ltd.     

用IGS发布的TEC数据提高法拉第旋转校正精度

电磁波穿过电离层时引入的法拉第旋转是导致全极化微波辐射计观测亮温产生交叉极化的重要原因.分析得知IRI（international reference ionosphere）模型产生的TEC（total electron content）数据在部分低纬地区无法满足全极化辐射计对法拉第旋转的校正精度要求.为提高校正精度,分析了沿观测路径积分法和应用IGS（International GPS Service）发布的TEC数据校正两种方法对校正精度的影响.结果表明,沿观测路径积分法不能有效改善校正精度,而应用IGS数据校正可大幅提高低纬地区法拉第旋转校正精度,满足全极化微波辐射计对极化旋转角的校正精度要求.     

星载极化干涉SAR系统电离层去相干研究

中低频段(UHF/VHF、P、L)星载极化干涉SAR系统运行于电离层之上,会受到相位扰动、法拉第旋转等电离层效应的影响,从而降低系统的成像质量以及参数反演的准确度.本文从电离层造成的成像散焦和极化平面旋转角度出发,重点分析和推导了电离层不规则体去相干以及法拉第旋转效应对相干最优的影响,并通过仿真实验模拟了典型系统条件以...     

On the detection of Faraday rotation in linearly polarized L-band SAR backscatter signatures

The potentially measurable effects of Faraday rotation on linearly polarized backscatter measurements from space are addressed. Single-polarized, dual-polarized, and quad-polarized backscatter measurements subject to Faraday rotation are first modeled. Then, the impacts are assessed using L-band polarimetric synthetic aperture radar (SAR) data. Due to Faraday rotation, the received signal will include other polarization characteristics of the surface, which may be detectable under certain conditions. Model results are used to suggest data characteristics that will reveal the presence of Faraday rotation in a given single-polarized, dual-polarized, or quad-polarized L-band SAR dataset, provided the user can identify scatterers within the scene whose general behavior is known or can compare the data to another, similar dataset with zero Faraday rotation. The data characteristics found to be most sensitive to a small amount of Faraday rotation (i.e., a one-way rotation <20/spl deg/) are the cross-pol backscatter [/spl sigma//spl deg/(HV)] and the like-to-cross-pol correlation [e.g., /spl rho/(HHHV/sup */)]. For a diverse, but representative, set of natural terrain, the level of distortion across a range of backscatter measures is shown to be acceptable (i.e., minimal) for one-way Faraday rotations of less than 5/spl deg/, and 3/spl deg/ if the radiometric uncertainty in the HV backscatter is specified to be less than 0.5 dB.     

高频地波雷达协同检测新方法

电离层杂波、海杂波以及射频干扰的存在造成高频地波雷达目标检测上的困难。针对高频地波雷达的极化特性和目标在距离-多普勒谱中的扩张特性,先设计一种改进的二维恒虚警检测算法,后将该方法与极化处理方法融合设计一种新的协同检测算法,该检测算法实现了能量域和极化域的协同检测,突破传统极化检测方法计算复杂等问题。高频地波雷达实测数据中的仿真和检测结果表明:在电离层杂波背景中,协同检测算法性能优于恒虚警方法。     

Sense reversal of circularly polarized waves on earth-space links

On VHF earth-space links near the geomagnetic equator, the sense of rotation of circularly polarized waves may, under some conditions, be reversed when penetrating the ionosphere. More generally, circular polarization is deformed to elliptical polarization. The effect may cause large, unexpected fades on links with circularly polarized terminals.