斜程大气湍流中激光回波的闪烁指数

根据光波斜程传输理论以及随高度变化的ITU-R大气折射率结构常数模型,考虑内尺度及从发射机到目标和从目标到接收机双斜程路径影响的条件下,应用修正Hill湍流折射率起伏功率谱,对反射器目标激光回波闪烁指数进行了研究。推导了回波闪烁指数随菲涅耳率的变化关系,数值分析了不同内尺度、传输距离、激光波长以及目标高度对回波闪烁指数的影响。结果表明:内尺度和目标高度对回波闪烁指数的影响要远大于波长和传输距离的影响。     

土壤表面与部分埋藏多目标复合散射的时域有限差分方法

采用Dobson半经验模型和电介质复介电常数表示土壤介电常数的实部和虚部,用指数型分布粗糙面和Monte Carlo方法模拟土壤表面,运用时域有限差分方法研究土壤表面与部分埋藏多个矩形截面混凝土柱复合模型的电磁散射.结果表明：复合散射系数随散射角振荡变化;土壤表面高度起伏均方根、土壤含水率、目标介电常数、入射角对复合散射系数影响较大;土壤表面相关长度、目标截面宽度、高度、间距、倾角对复合散射系数影响较小;目标埋藏深度对复合散射系数几乎没有影响.与其他数值计算方法比较,采用时域有限差分方法既可获得较高的准确性,又可减少计算时间和内存占用量.可以用来计算地、海粗糙面与附近任意多目标的复合散射.     

大气损耗对量子干涉雷达的影响机理

以马赫-曾德尔干涉仪作为基本模型对量子干涉雷达的探测原理进行分析,讨论了目标探测过程中光场量子态的具体演化情况,并采用宇称算符作为相位检测算符分析了量子干涉雷达的回波信号,将其与基于振幅检测的经典雷达回波信号进行比较,证明量子干涉雷达具有超越衍射极限的超分辨率特性.此外,针对大气损耗的进一步研究显示:量子干涉雷达分辨率受大气损耗影响较小,且可通过增大脉冲光子数N克服其影响;而量子干涉雷达的灵敏度则受到较大影响,尤其当两路光的损耗情况不同时,灵敏度随N的增加呈现先升高后降低的趋势;当两路光损耗情况相同时,系统灵敏度随N的增加而升高且正比于1/N~(1/2).综上,可根据探测光的大气损耗情况适当调节参考光的衰减来克服大气损耗带来的不良影响.     

Mie散射激光雷达研究大气边界层特性

基于Mie散射大气激光雷达,应用Klett算法对回波信号进行了反演,得到大气消光系数和后向散射系数。然后,通过对大气后向散射系数曲线进行拟合得到大气边界层混合层高度以及卷夹层厚度等特征参数。对比雷达实测数据与探空气球所测数据,证明二者存在较好相关性。基于实测数据讨论了四川盆地大气边界层特性对气候的影响。     

丛林地貌高功率微波传输散射特性

 采用时域有限差分方法仿真分析了高功率微波在丛林地貌传输的双站散射特性,得到了不同入射角度下的双站散射规律,对比分析了利用数值和解析两种方法得到的丛林散射系数与掠射角关系。理论分析和数值计算结果表明：散射系数曲线存在布儒斯特角,且地貌电参数越大,布儒斯特角越小;掠射角小于布儒斯特角时,散射系数随掠射角的增大而减小;掠射角大于布儒斯特角时,散射系数随掠射角的增大而增大。水平面情况下的不同丛林类型双站散射系数变化趋势一致;对于介电常数相对较大的丛林地貌类型,通过镜面反射方向时域有限差分方法得到的半功率波瓣宽度较宽;同一丛林地貌类型不同均方根高度条件下,均方根高度增大,相干分量变小,非相干分量变大;不同入射角、相同地貌和电参数情况下,散射系数的峰值向对应的镜面反射方向移动。     

雪层覆盖土壤表面与半埋柱体宽带复合散射FDTD方法

为满足半埋目标测量和检测的需要,应用指数型分布粗糙面模型和Monte Carlo方法模拟雪层、土壤表面,运用时域有限差分方法研究雪层覆盖的土壤表面与半埋矩形截面柱复合模型的宽带电磁散射,得出宽带复合散射系数的频率响应曲线,计算宽带复合散射系数随雪层表面、土壤表面高度起伏均方根、相关长度,雪层类型、雪层厚度,土壤层含水量,矩形截面柱几何参数、倾角、埋藏深度,电磁波入射角等参数的变化. 结果表明,雪层表面高度起伏均方根、相关长度对宽带复合散射系数影响较小,其它参数对宽带复合散射系数均有显著影响且较为复杂.     

非直视紫外光通信单次散射传输模型研究

依据Lambert定律建立了非直视紫外光通信系统单次散射传输模型。模型表明:大气传输中的两次斜向传输引入的大气透射比衰减,大气散射衰减和距离平方反比衰减是导致接收光功率随距离增加迅速衰减的主要因素。为便于实际计算,对建立的散射传输模型做了合理的计算简化,经实验验证简化后的传输模型计算结果与实验结果吻合较好。利用简化后的传输模型可以方便有效地计算出非直视紫外光通信接收机在不同距离下的接收功率和路径传输损耗,进而可以迅速、有效地计算和评估已知参数的非直视紫外光通信系统的极限工作距离。     

基于文氏改进谱的二维粗糙海面模型及其电磁散射研究

基于文氏改进功率谱和Donelan方向分布的实验结论,结合经典的Monte Carlo法提出了一种适用于不同水深、不同风浪成长阶段的二维粗糙海面模型.在经典的双尺度法计算海面后向散射的基础上,结合小斜率近似法的仿真结果,对电磁散射计算方法进行改进.通过与典型海杂波均值散射系数GIT模型的拟合结果进行对比,验证了文氏改进谱在中国近海海面模拟的适应性与改进后电磁散射计算方法的有效性.     

窄带大功率高温超导滤波器的研制

本文中,我们介绍一个中心频率大约为2.015GHz,相对带宽小于0.35%,通带内回波损耗大于20dB,带外零点高度大约为-45dB,通带内插损小于0.2dB的窄带四阶双模片型高温超导滤波器.我们对该滤波器在不同输入功率下的传输特性曲线进行了测试,测试结果表明,在输入微波信号功率值为11.7W的情况下,该器件的传输特性曲线没有发生显著退化.     

大气风场和温度对无线电声波探测系统探测高度影响的数值研究

从声波扰动介质中的电波波动方程出发, 使用时域有限差分(FDTD)方法, 结合声波传播的FDTD 模型, 构建了描述声波和电波相互作用的数值模型, 并运用该模型分析风场和温度对无线电声波探测系统的探测高度的影响. 数值模拟结果表明: 温度与风场剖面的存在改变声波和电波散射回波的传播轨迹; 温度梯度剖面主要影响声波的传播速度, 风场剖面导致作为电波散射体的声波波阵面的偏移, 降低电波散射回波的强度并改变回波路径, 使得接收数据减少, 限制无线电声波探测系统的探测高度; 在强风背景下, 若降低声波散射体高度, 电波散射回波“聚束点”的偏移会有较大的改善, 但同时意味着探测高度的降低. 为了改善风场背景下无线电声波探测系统的探测高度, 可以使用双基地雷达或者增大接收天线面积等方法来实现.     

Investigation of EM scattering from electrically large sea surface with breaking wave at low grazing angles

According to the high-resolution radar data, backscattering scattering from sea surfaces may give rise to super events of strong scattering and large Doppler offsets for horizontal (HH) polarization at low grazing angles (LGA), which are usually considered to be caused by breaking waves. In this paper, the mesoscale breaking wave model is introduced to account for the contribution of breaking waves, which can be responsible for sea spikes and high Doppler shift. The backscattering radar cross-section (RCS) from a single breaking wave is computed and the scattering from sea surfaces is analyzed by a composite surface scattering model, in which the phase factor of every facet is modified by the capillary waves. By using the slope criterion, the spatial distribution of the breaking waves is obtained. At the same time, the RCS of sea surfaces with breaking waves is calculated and the results agree with measured data well at LGA for HH polarization. Considering the Bragg phase velocity, orbital motion of facets and wind drift, the Doppler spectrum of sea surface with breaking wave is simulated, and when compared with the result without breaking wave, there is a good improvement for HH polarization, and for the vertical polarization, the revision is little, which are agreeable with the results of recent researches.     

起伏海面环境下水声信道特性及估计方法*

为了研究随机起伏海面的散射对水声信道特性的影响,以及不同信道估计算法在该信道条件下的性能问题,该文从利用Monte Carlo方法模拟的基于PM海浪谱的二维随机起伏海面出发,提取了与风向不同夹角的一维随机起伏海面,并利用Kirchhoff近似法计算了其散射强度。将得到的声散射特性和海面环境信息融入到信道模型中,建立了基于PM海浪谱的随机信道模型。通过仿真分析了不同风速、传播距离条件下,声波的传播损失和信道冲激响应变化,并利用l_0-最小均方误差法、匹配追踪算法和正交匹配追踪算法进行了信道估计。给出了不同环境条件下,三种算法估计性能的综合比较结果,验证了算法针对该信道模型的有效性。     

浅海起伏海面下气泡层对声传播的影响

分析了起伏海面下风浪引起的气泡层对海面反射损失和对声传播的影响.一方面,气泡层会改变原来水中的声速剖面;另一方面,气泡层会对声波产生散射和吸收作用.考虑以上两方面的因素,分析了不同风速下气泡层对海面反射损失和声传播损失的影响,仿真发现,在风速大于10 m/s时,对于2 k Hz以上频率时气泡层对小掠射角下海面反射损失的影响不可忽视.在给定的水声环境中,当声源深度和接收深度都为7 m时,风速为16 m/s的风浪下生成的气泡层,在10 km处对3 k Hz的声传播损失的影响达到8.1 d B.当声源深度和接收深度都为18 m时,风速为16 m/s的风浪下生成的气泡层,在10 km处对3 k Hz的声传播损失的影响达到4 d B.     

Spectral characteristics of a microwave radar signal backscattered by the sea surface at small incidence angles

We present a theoretical analysis of the Doppler-spectrum properties of a microwave radar signal scattered by the sea surface at small incidence angles. The dependences of Doppler-spectrum width and shift on the wind velocity and wave development stage and their azimuthal dependence are analyzed. The case of mixed sea (wind wave plus swell) is also considered. The JONSWAP spectrum model is used to describe sea waves. The study shows that Doppler-spectrum parameters are sensitive to variation of sea-surface state; for example, for the case of developed sea waves, an increase in wind velocity by 1 m/sec leads to increases in the Doppler-spectrum width and shift by 15 Hz and 3 Hz, respectively. It is shown that for the case of a moving radar the Doppler spectrum remains sensitive to variation of sea- surface state with a sufficiently narrow radar directivity pattern. Estimates show that in the case of a single sea- wave system on the surface, using Doppler-spectrum parameters we can, in principle, determine wave type (developing wind wave, developed wind wave, or swell), dominant wavelength, wave propagation direction, and wave height; wind velocity, direction, and acceleration distance can be determined for wind waves.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 517–526, May, 1996.This paper was supported by the Russian Foundation for Fundamental Research (project 93-02-15892).     

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.     

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.     

Performance comparison of detection methods for weak target based on two-dimensional fractal sea surface

A detection method of the weak radar target is studied by applying fuzzy theory and multifractal correlation theory based on a two-dimensional fractal sea surface model. Firstly, a two-dimensional fractal sea surface model and its backscattering coefficient are introduced, the backscattering coefficient is a universal model affected by seawater permittivity, electromagnetic wave incidence angle, incident frequency, wind speed and wind direction factors. A novel two-dimensional wideband radar echo model, which is considered as a time-domain convolution of the stepped frequency signal radiated by airborne radar and the backscattering coefficient, is derived. Secondly, multifractal correlation theory is elaborated and a computation method of a membership degree of multifractal correlation spectrum is proposed, fuzzy theory and the AdaBoost algorithm are applied to the target detection. Finally, several target detection methods are compared with CA-CFAR and works of the predecessors. The results of the comparative study show its rationality of the two-dimensional wideband radar echo model and the superiority of wideband radars in detection performance, it is also seen that the multifractal correlation spectrum outperforms the multifractal spectrum in the probability of detection.     

海面冰层对声波的反射和散射特性

北极海面冰层复杂多变,其对声波的反射和散射严重影响冰下水声信道的传输特性,建立海面冰层的声波反射和散射模型对冰下水声通信研究具有重要意义.假设海面冰层为多层固体弹性介质且冰-水界面粗糙,满足微扰边界条件,导出声波从海水介质入射到海面冰层时相干反射系数满足的线性方程组.对相干反射系数随声波频率、掠射角、冰层厚度的变化进行数值分析.进一步引入根据散射声场功率谱密度计算散射系数的方法,改变掠射角,对冰层厚度、散射掠角对散射系数的影响进行研究.     

Doppler spectrum of polarimetric scattering field from two-dimensional time-varying nonlinear sea surfaces

Polarimetric scattering model of second-order small-slope approxi-mation combined with “choppy wave" model (CWM) for describing nonlinear hydrodynamic interactions between ocean waves is utilized in this paper to investigate the influence of sea surface nonlinearities on backscattering coefficient as well as Doppler spectrum signatures including Doppler shift and spectral bandwidth. Simulation results show that at moderate to large incidence angles the Doppler shift and spectral bandwidth of the CWM nonlinear sea surfaces are significantly larger than those of linear sea surfaces, in particular at low grazing angles. In addition, Doppler signatures show distinct polarization dependence, and most importantly the cross-polarized Doppler signatures significantly differ from the co-polarized ones. It is also indicated that co-polarized Doppler shift increases obviously with wind speed increasing, whereas the cross-polarized Doppler shift looks less sensitive to wind speed variations. The difference of Doppler signatures between co- and cross-polarization is potentially valuable for ocean remote sensing applications, especially for observing very high winds.