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.     

全极化微波辐射计10．7GHz极化通道法拉第旋转校正分析

全极化微波辐射计海面风场反演使用的10.7GHz电磁波在穿过电离层时其极化平面会发生法拉第旋转.根据目前微波辐射计法拉第旋转校正使用的3种方法,分析了其应用于10.7GHz极化通道校正的可行性.对基于电离层和地磁场数据沿观测路径积分计算法拉第旋转角的校正方法进行详细研究,建立了观测路径单位矢量与地磁场矢量的夹角随高度变化的几何关系,给出了积分计算的具体流程.最后根据计算结果对法拉第旋转角的变化规律进行了分析,为星载全极化辐射计的升降交点选择提供参考.     

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.     

电离层Faraday效应对卫星通信的影响

分析电离层Ｆａｒａｄａ旋转效应对线极化卫星通信信号产生的影响，计算出了一睦城市的旋转角和极化角，为线极化信号的良好匹配提供了依据。实践证明，这些计算结果是准确的。     

星载P波段SAR电离层效应的双频校正方法

对于高分辨星载P波段SAR系统,电离层效应对P波段SAR会带来一系列较为严重误差,包括时间延迟、法拉第旋转和色散等,这些误差与电波频率和电离层TEC值关系密切,并使得图像质量下降。如何测量与校正电离层效应误差是星载P波段SAR面临的重要问题。根据SAR系统自身的宽带特点,借鉴GPS系统采用的电离层双频测量校正方法,提出了一种基于双频SAR距离像相关的延迟误差测量方法,计算机仿真结果表明,该方法有效提高了双频回波信号的电离层延迟误差测量精度,适于低频段星载SAR系统的电离层效应测量与校正应用。     

电离层Faraday效应对远程预警雷达性能的影响

在对近地空间目标进行预警跟踪时,雷达发射的电磁波会在电离层中传播,电离层作为磁化等离子体,会导致电磁波产生Faraday旋转,从而对雷达探测性能造成影响。通过数值计算的方式,仿真了Faraday旋转效应导致的雷达极化失配损耗,并对仿真结果进行了分析。通过仿真结果可以看出:太阳活动高年,Faraday旋转角比太阳活动低年大;雷达工作频率越高,受Faraday旋转影响越小;目标仰角较低时,线极化失配损耗较小;椭圆极化方式下,极化失配损耗随椭圆轴比增大而增大;用圆极化方式可以最大程度地减小由Faraday旋转导致的雷达探测性能降低。     

中国区域电离层特性对卫星导航系统的影响

利用GPS数据对中国区域电离层环境(电离层赤道异常、电离层暴、电离层闪烁)对卫星导航系统的完好性、可用性影响进行分析。结果表明,中国区域电离层异常将影响电离层延迟误差的空间相关性,从而影响电离层完好性门限的建立。电离层暴破坏了电离层的空间相关性,在影响系统完好性门限建立的同时,还影响到系统对扰动的检测能力。电离层闪烁影响接收机测量的精度、可用的卫星数和定位精度,影响系统的可用性;同时,闪烁相关的不均匀体也会影响电离层TEC的准确提取,从而影响系统电离层修正模型的实现。电离层这些不同影响效应之间往往又相互影响,使得中国区域的电离层环境对卫星导航系统的影响具有复杂性。     

圆波导极化器原理及设计

卫星通信和雷达系统广泛采用圆极化天线,它的优点是可以抑制大气内水滴和电离层法拉第旋转效应对天线工作的干扰,并且可以接收任意取向的极化波.圆极化器是圆极化天线系统中实现极化变换的关键部件.如图1所示,它把喇叭接收到的圆极化信号变换为线极化信号,通过收发正交器送到接收机,或把发射机发出的线极化信号变换为圆极化信号,通过喇叭发向目标.极化器的质量直接影响到天线系统极化性能的优劣.     

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。     

返回散射回波去电离层相位污染方法比较与分析

高频信号利用电离层的反射作用进行目标探测,探测距离远,范围广,可应用于气象、军事预警、海面检测等领域.但是对于海上目标,尤其是舰船等慢速目标,由于海杂波的强烈干扰,致使信杂比很低,要求较长时间的相干积累时间.而电离层的时变性和多径效应的存在严重影响相干积累的效果.本文主要论述国内外学者就此问题提出的各种去电离层相位污染方法,并对其做了对比和分析.     

Determination of the ionospheric electron content utilizing satellite signals

Various methods for the determination of the ionospheric electron content are possible when satellite-borne signals can be used. The paper compares the results from Doppler shift, Faraday rotation, Doppler data combined with Faraday rotation data, and topside and bottomside soundings. The often neglected prerequisite for the application of Doppler and Faraday methods is radio propagation along the same ionospheric path for the different frequencies involved. Criteria for correct application are discussed. The comparison of inflection time moments is chosen for data selection. The usage of the Doppler slopes at the time of simultaneous inflection provides reliable data of the electron content. The diurnal variation is given. The Faraday rotation rate measurement has been made a continuous one by special equipment of G. Vogt. The simultaneous use of two Doppler slopes at different frequencies and of the Faraday rotation rate at one of these frequencies results in the measurement of the effective component of the geomagnetic field. Thus, a method free from any assumptions is on hand for the electron content. From the radio-frequency sounding of the topside-sounder satellites and from simultaneous bottomside soundings a complete ionospheric profile is obtained, the integration of which results in data for the total content. A comparison of the results of the different methods is permitted within the limitations brought about by the various theoretical and experimental assumptions made.     

Ionospheric effects on satellite land mobile systems

Satellite-ground radio systems are now so sensitive that ionospheric changes can disrupt their performance. This paper discusses some satellite-to-ground propagation problems in the UHF and L bands caused by the Earth's ionosphere. Such problems include signal time delay, signal dispersion, Faraday rotation, and scintillation.     

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.     

电磁波在任意磁偏角等离子体中的传播

本文将电流密度卷积时域有限差分（Current Density Convolution Finite Difference Time Domain,JEC-FDTD）方法扩展到求解任意磁偏角电磁波在磁化等离子体中的传播和共振吸收问题.首先,验证数值算法正确性,分析了法拉第旋转角效应,以及任意磁偏角电磁波在等离子体中的传播特性.然后,求解电磁波在磁化等离子体中的等离子体朗缪尔共振、电子回旋共振、高频混杂共振吸收特性.结合在电离层加热中的应用,重点分析了等离子体高频混杂共振吸收特性,得到了高频混杂共振激发的频率匹配条件.数值结果表明,高频混杂共振吸收是电离层加热的有效方式,对于解释电离层加热机制具有重要意义.     

Ionospheric effects for L-band 2-D interferometric radiometry

Ionospheric effects are a potential error source for the estimation of surface quantities such as sea surface salinity, using L-band radiometry. This study is carried out in the context of the SMOS future space mission, which uses an interferometric radiometer. We first describe the way the Faraday rotation angle due to electron content along the observing path varies across the two-dimensional field of view. Over open ocean surfaces, we show that it is possible to retrieve the total electron content (TEC) at nadir from radiometric data considered over the bulk of the field of view, with an accuracy better than 0.5 TEC units, compatible with requirements for surface salinity observations. Using a full-polarimetric design improves the accuracy on the estimated TEC value. The random uncertainty on retrieved salinity is decreased by about 15% with respect to results obtained when using only data for the first Stokes parameter, which is immune to Faraday rotation. Similarly, TEC values over land surfaces may be retrieved with the accuracy required in the context of soil moisture measurements. Finally, direct TEC estimation provides information which should allow to correct for ionospheric attenuation as well.     

The effects of polarization rotation and phase delay with frequency on ionospherically propagated signals

When a CW skywave signal is received on a linearly polarized antenna, polarization (Faraday) rotation produces a variation of received signal strength with radio frequency. The resulting dependence of received signal amplitude on radio frequency may impose a bandwidth limitation on pulsed signals where waveform preservation is important. A measure of this limitation, termed polarization bandwidth, is defined to correspond to the bandwidth in which the plane of polarization rotates90deg. Computer ray-tracing calculations were performed using a single Chapman-layer ionospheric model to determine the 1-hop polarization bandwidth as a function of geomagnetic azimuth and radio frequency. The polarization bandwidth was found to decrease with increasing radio frequency and with increasingly close alignment of the propagation path with the longitudinal component of the earth's magnetic field. Assuming a critical frequency of 9 MHz and a path length of 2000 km, the polarization bandwidth increased from a minimum of 140 kHz at 10.5 MHz and from a minimum of 70 kHz at 17.5 MHz, as the propagation direction varied from geomagnetic north to east. A model for the 1-hop ionospheric signal channel is proposed whose parameters are the rate of change of polarization rotation with frequency and the phase versus frequency characteristic of the path. These two parameters are shown to be readily determined from FM-CW or equivalent oblique-path sounding records. Using this model, predictions are made of the effects of polarization rotation with frequency, and also of ionospheric dispersion or phase distortion, on the envelope shape of short-pulse signals (of from 1.5 to50 mus duration). A pronounced waveshape distortion due to the effects of polarization rotation on the pulse envelope was observed when the signal bandwidth appreciably exceeded the "polarization bandwidth" for the path.     

The gradient in the ionospheric electron content over Texas as calculated from Faraday fading of radio waves from satellites

A method is shown for calculating the ionospheric electron content as a function of longitude and latitude. The results of the analysis of the calculations made of Faraday fading from 251 transits of satellites Explorer 22 and Explorer 27 provide a gradient in the ionospheric electron content over Texas.