【作者】 赵小龙；

【导师】 黄际英；

【作者基本信息】 西安电子科技大学 ， 无线电物理， 2008， 博士

【摘要】 大气波导是由于大气表面层折射指数随高度迅速下降而形成的一种区别于标准大气的异常大气结构,它能够使电波射线向下弯曲的曲率大于地球表面的曲率,从而将电磁能量陷获在波导结构内形成大气波导传播。特别是在贴近海面的几十米高度空间内,大气表面层内海水与大气的双重交互作用使得大气结构变化更加剧烈,蒸发波导就是频繁出现在海洋环境中的一种大气波导形式。大气波导传播现象的出现不仅可以使电磁波偏离原来的传播方向,而且能够使电磁波以较小的衰减沿波导传播到视距以外很远的地方,严重影响了雷达、通信、侦察等基于电磁传播的无线电武器系统。如何充分利用大气波导传播这一现象来提高电子系统的工作效能,同时如何有效克服大气波导传播给电子系统性能带来的负面影响,使大气波导环境中电磁波的传播特性及其在雷达、通信等问题中的应用研究具有重要的理论意义和实用价值。本文在大气波导的环境特性参数描述及预测预报技术、大气波导中电磁波的传输特性及其在雷达、通信中的应用等方面进行了研究,主要工作内容及取得的成果如下:(1)研究了大气波导的形成、参数特征以及大气波导对电磁波传播的影响;分析了几种大气波导环境的预测预报技术,并对我国大气波导环境的试验测量以及环境特性进行了研究;利用大气边界层理论研究了蒸发波导环境中不同大气层结的修正折射率剖面以及理论分析研究中常用的几种修正折射率剖面参数模型,并对利用雷达海杂波反演大气波导中修正折射率的理论方法进行了分析。(2)几何光学理论是高频近似条件下描述电磁波传播问题的有效方法,本文利用射线描迹技术对标准大气、蒸发波导环境中的电波射线传播路径进行了分析,针对大气波导环境中雷达目标探测中的实际问题,利用射线描迹技术对视在距离、视在仰角、视在高度与目标高度间的关系进行了研究,并将大气波导环境中视在距离、视在仰角、视在高度的误差与标准大气中的误差进行了比较。射线描迹技术快速、直观、形象地说明了大气波导环境中电波射线弯曲以及超视距传播特征,并有助于大气波导环境中雷达目标探测的误差修正。(3)系统分析了完全可导光滑边界情形的标准抛物型方程及傅立叶分步算法(FSS),并对不规则地形环境中抛物型方程的平化坐标变换及有限可导边界条件的混和傅立叶分步算法(MFT)进行了深入研究,有效解决了大气波导环境中电波传播的重点、难点问题,即在大气折射指数水平分布不均匀、粗糙海面、不规则地形等复杂环境中的电波传播理论和解决方法。根据天线理论分析了辐射场问题中方向性函数与天线孔径分布函数的关系,并对适于大气波导环境中电波传播的低旁瓣、窄波束的波束成形方法进行了初步研究。针对雷达探测中关注的重点问题,利用抛物型方程的傅立叶分步算法以及电波传播理论的传输损耗分析,研究了天线高度、天线发射仰角、波导高度等因素对蒸发波导环境中雷达探测性能的影响。通过利用海浪谱实现了一维粗糙海面的Monte Carlo仿真;采用小波分析方法有效消除了不规则地形的刃形结构,极大地扩展了利用抛物型方程进行精确求解的雷达探测范围;分析了粗糙海面对波导环境中电波传输损耗的影响、不规则地形中的电波传播等复杂地形环境中电波传播的重点、难点问题,从而使抛物型方程方法成为解决非均匀大气环境、不规则地形环境中电磁波传播问题的有效算法。(4)针对海上大气波导环境中雷达目标的识别,提出了蒸发波导中基于零记忆非线性变换(ZMNL)的海杂波实现方法。该方法确定了海杂波K分布参数与蒸发波导高度间的关系,得到了不同蒸发波导高度、不同探测距离处的相关K分布杂波序列。应用零记忆非线性变换(ZMNL)方法产生杂波序列的仿真实验证明了该方法用于蒸发波导中海杂波建模的有效性。(5)利用我国东南沿海海域夏季微波超视距通信的外场测试数据,统计分析了测试信号的中值电平、衰落幅度、衰落深度、概率分布及信道可靠度,结果表明信号衰落幅度的日变化与我国海上蒸发波导高度的日变化具有显著的相关性。通过信道传输损耗理论值、测试值以及通信系统设备能力的比较,结合我国东南沿海蒸发波导环境,提出微波信号在海上的超视距传播途径主要为海上蒸发波导传播。(6)针对大气波导环境中电磁波的多径传播特性,根据容量有限的Markov生灭过程得到了大气波导中到达接收站的射线径数;利用差分时延试验数据的近似瑞利分布提出了大气波导中的三参数模型,并通过最小方差准则确定了模型参数;利用MUSIC算法对多径到达角进行最佳估计,数值仿真结果表明MUSIC算法极大地提高了到达角位置处的信噪比。根据以上参数提出了多特征路径信道模型,通过分析多特征路径信道模型多个时延周期散射分量与多径子特征路径散射分量之间的关系,得到了基于多径信号比(SMR)的子特征路径信号简化算法。最后,利用大气波导中的多特征路径信道模型通过Matlab的Simulink模块化程序设计对ASK、PSK、FSK三种调制方式的通信系统进行了仿真,分析了大气波导环境中通信系统的误码性能,对海上微波超视距通信系统的总体设计有重要参考价值。更多还原

【Abstract】 Atmospheric duct is a nearly permanent propagation mechanism due to the abrupt changes in the vertical temperature and humidity profiles just above the surface layer and a anomalous structure obviously different from the standard atmosphere, which can cause EM wave rays lying in the evaporation duct refracted towards the surface of the earth and make EM power trapped or guided in a thin tropospheric layer above the earth. Especially at tens of meter height close to the sea surface, interaction between the atmosphere and the sea water in surface layer makes the atmosphere structure more dramatic changes, and evaporation duct is one kind of the atmospheric duct frequently appearing in the marine environment. The resulting effect is an over-the-horizon propagation with less attenuation than in free-space conditions and radar detecting ranges can be greatly extended with duct propagation. The atmospheric duct phenomenon existence greatly strengthens or degrades the capability of the Radar, Communication, and Electronic Support Measures (ESM) system which are radio weapon system depending on electromagnetic radiation. How to overcome the negative impact on the electronic system resulting from the atmospheric duct propagation and how to make good use of the atmospheric duct phenomenon to improve the system performance of Radar, Communication and so on, the researches on the propagation characteristic of electromagnetic wave and its application in atmospheric duct have great theoretical significance and are of great value to the military.This dissertation gives a thorough research in depth on the subject including the parameter description and the forecast or prediction of the atmospheric duct and, and the propagation characteristic of electromagnetic wave and its application on Radar and Communication system in atmospheric duct. The specific investigations are followed below as:First, the formal condition and the parameter characteristics of the atmospheric duct and the impact on the EM-wave propagation are studied. Some kinds of the forecast and prediction technologies of the atmospheric duct are discussed, and the experimental measurement and the environment characteristics of the atmospheric duct in China sea are analyzed by some experiment data. By the atmospheric boundary layer theory, the dissertation discusses the modified refractivity profile of the different atmosphere layer in evaporation duct and the some parameter models of the modified refractivity, and the modified refractivity inversion by radar sea clutter are analyzed in atmospheric duct environment.Secondly, Geometrical Optics theory is an effective way to describe the electromagnetic-wave propagation problem in a condition of high-frequency approximation. By the ray-tracing technique, the dissertation analyzes the propagation path of the radio-wave rays at standard atmosphere and in evaporation duct for vividly describing the ray curvature and the over-the-horizon propagation. Based on the actual problems of the radar target detection in atmospheric duct environment, the ray-tracing technique is used to analyze the sight range, the sight elevated angle and the sight height. The error modification of the sight range, the sight elevated angle and the sight height respectively at the standard atmosphere and in the evaporation duct is compared, which is important to the radar target detection in atmospheric duct.Thirdly, the parabolic equation and the Fourier split-step method (FSS) with perfectly conducted smooth boundary are analyzed, and the flattening transform of the parabolic equation with irregular terrain and the mixed Fourier split-step method (MFT) with finitely conducted boundary are studied further. The radio-wave propagation theory and the solution with non-uniform horizontal refractivity in atmospheric duct, rough sea surface and irregular terrain are obtained. Based on the antenna theory, the relation between the direction function and the aperture distribution for the radiation of the EM field is discussed, and the beam-forming method for low sidelobe and narrow main beam suitable for the atmospheric duct propagation is analyzed. With the parabolic equation method and the basic theory of the radio-wave propagation, the transmission loss was calculated at the neutral atmospheric stratification in the evaporation duct near the sea surface and at standard atmosphere. The space characteristic of the electromagnetic field was presented in evaporation duct. The impaction of the duct height, antenna height and grazing angle on radar detection ability was analyzed under the neutral atmospheric stratification condition in the evaporation duct and at standard atmosphere by the pseudo-color map. The numerical simulation can present the space field distribution effectively rather than the ray-tracing technique. The research offers the reliable basis for predicting the blind area of the radar system. The one-dimension rough sea surface is simulated by Monte Carlo method on the basis of the sea spectrum. The knife-edge clearance of maritime digital terrain is effectively realized by the Wavelet analysis to broaden greatly the radar detection range for the modified parabolic equation. Based on the parabolic equation, the radio-wave propagation with rough sea surface in evaporation duct and with irregular terrain is analyzed. Fourthly, A method based on zero memory nonlinearity (ZMNL) is proposed to simulate sea clutter in evaporation duct environment for modeling radar signalling. The method identifies the relation between the K-distributed parameter of sea clutter and the evaporation duct height. The correlated K-distributed sea clutter is simulated at a certain duct height and detecting range. The application process of this method for clutter simulation is described. The simulation results prove the validity of the method for modeling sea clutter in evaporation duct.Fifthly, With the experimental data on the over-the-horizon microwave communication in China sea on summer day, the median level of signal, the fading amplitude, the fading depth and probability distribution are analyzed. The results show that the day variation of the fading amplitude has a high degree of significant correlation with the day variation of the evaporation duct height. Comparing with the communication equipment capability and the propagation loss on the basis of the evaporation duct characteristic in southeast sea of China, the propagation mode of the over-the-horizon microwave communication is mainly the anomalous propagation of the evaporation duct. Statistical analysis of the measured data of the over-the-horizon microwave communication is of great value to the system design and optimize the performance of microwave communication systems.Sixthly, based on the characteristics of EM multi-path propagation in marine atmospheric duct and the Markov birth-and-death process in finite space, path number in marine atmospheric duct is presented. The excess delay three-parameter model is proposed according to the experimental data with approximate Rayleigh distribution, and the model parameters are determined by the minimum variance criteria. The angles of arrival (AOA) is estimated by MUSIC algorithm and simulation results show MUSIC algorithm can significantly increase the SNR at AOA. Furthermore, a multi-eigenpath channel model is proposed to simulate the microwave transhorizon communication signal in atmospheric duct environment The channel model identifies the relation between multi-periodical delay components and subeigen-components. The simplified algorithm of subeigen-signal is proposed based on signal-to-multipath ratio (SMR). The multi-eigenpath channel model contributes to the system design and performance optimization of marine microwave communication system. Finally, the communication system modulated by ASK, PSK and FSK is simulated at the basis of the multi-eigenpath channel model of the atmospheric duct, and the bit error performance of the system is analyzed for designing the marine over-the-horizon microwave communication system.更多还原