Fractal analysis of the signal scattered from the sea surface

This paper deals with the problem of the electromagnetic scattering from a sea fractal surface. The paper demonstrates that the sea-scattered signal retains some fractal characteristic of the sea surface. In detail, we analyze the signal scattered from a sea surface modeled by the one-dimensional (1-D) sea-surface fractal profile proposed by Berizzi, Mese, Pinelli and Andreoli (see Inst. Electr. Eng. Proc. Radar Sonar, Navigat., vol.146, p.55-66, 1999). The results show the graphs of the in-phase and quadrature components of the received signal are fractal curves with box-counting fractal dimension equal to the one of the sea profile. These results are validated by presenting and discussing some numerical examples     

GPS信号海面散射特性和初步实验结果

2006年9月至2007年3月,在福建厦门附近进行了为期半年的实验,观测和研究GPS海面反射信号特性.一天内至少有百分之九十六的时间可以同时接收到4颗卫星海面反射信号,最多时达到8颗.观测资料用于反演有效浪高、海面高度和潮汐等海洋参量.重点讨论海面散射特性,研究海面反射波与直达波的功率比值,以及它们随电波的入射角、海面等效浪高(SWH)等的依赖关系,为星载接收机设计提供科学数据.     

基于时频能量谱特征的跳频电台个体识别

针对于跳频电台的细微特征分类识别问题,提出基于跳频信号时频能量谱的细微特征提取算法。首先,利用跳频信号在时频域的稀疏特性,通过稀疏重构方法得到跳频信号时频能量谱;然后,在不同尺度条件下对时频能量谱进行分割,分别提取时频能量谱瑞利熵、多重分形维数和差分盒维数三种特征;最后,通过支持向量机分类器对提取特征集进行训练、分类和识别,实现跳频电台个体识别。利用四部电台的跳频信号,验证对比了本文算法与另外两种算法的识别性能。实验结果表明,本文方法所提取的细微特征集具有较强的分辨能力,避免了由单一特征的相似性而引起的误判问题,能够在少量训练样本条件下,保持较高的识别正确率。      

频率步进激励信号的时域特性研究

将频率步进扫频信号用于电声测量系统工作过程中的激励信号,测量精度相对较高,但测量速度慢。并且,在频率不同的两段正弦激励信号衔接播放时,时域信号存在冲击量,如不加以改善,既延长了激励信号的过渡过程时间,又降低了测量精度。使用希尔伯特变换计算激励信号的时域包络曲线,可以分析冲击量的强度,且通过在激励信号频率步进衔接段采用时域加窗及平滑方法,可以降低这一冲击量的强度。理论分析和实验试听表明,该方法可以提高正弦激励信号频率步进时信号播放时的平滑度,降低冲击量,缩短过渡过程时间,且为改善测量精度提供一定的支持。     

Scattering from a 2D sea fractal surface: fractal analysis of the scattered signal

For first part of the work see ibid., vol.50, no.4, p.426-34 (2002). We analytically demonstrate that the electromagnetic (EM) sea scattered signal retains certain fractal properties of the sea surface. Starting from the expression of the scattering coefficient derived in the first part of the work, we show theoretically that the fractal dimension of the in phase and quadrature components of the EM signal scattered from a sea fractal surface is linearly related to the fractal dimension of the sea surface. This result holds under the hypothesis of Kirchhoff approximation. Bearing in mind that the sea fractal dimension gives a measure of the degree of surface irregularity, we can exploit this result to make a remote estimation of the surface roughness through the fractal analysis of the scattered signal. This is a very interesting result and it could be successfully used in remote sensing applications for classification, target detection, sea surface traffic monitoring, sea current analysis, oil slick and ship wake recognition, pollution analysis etc.     

短波广播信号的特征及其监测技术的研究

简要介绍了短波广播信号的优势及短波广播频段的使用现状,说明了短波广播信号的普遍特征,重点提出了短波广播信号的监测方法以及联合测向方法.通过多辆移动监测车配合固定站进行联合测向定位,很好地解决了短波无线电测向的盲区问题,再通过将测量结果与短波广播台站数据库进行比对,进一步验证了该方法的可行性.     

正弦波纹太赫兹慢波结构高频特性研究

针对真空电子太赫兹辐射源的发展需要,本文分析和设计了一种微型正弦波纹慢波结构。利用三维电磁仿真软件研究并分析了其高频特性,并对主要结构参数进行了优化,给出了不同参数下的色散特性和耦合阻抗。仿真结果表明:增加波纹深度可降低电磁波相速,同时增大耦合阻抗;缩短周期长度可提高工作频率;增大横截面宽度可降低相速;降低电子注高度可提高耦合阻抗等。     

基于网络的射频信号预处理器设计

文章通过对射频信号接收的研究和设计,实现射频信号的预处理,可以显著地改善复杂电磁环境下接收机或频谱仪的测量效果,同时也可以提高测试系统的自动化水平。     

REBUILDING THE SEA WAVE HEIGHT SPECTRUM BY USING MICROWAVE SCATTERING CHARACTERISTICS

The microwave back-scattering characteristics are numerically analysed by the elec-tromagnetic scattering theory,and the sea wave height spectrum that reflects the wind field isrebuilt by comparing the calculated data with experimental data which were measured by airbornescatter.According to the wave height spectrum,the relationship between the back-scattering co-efficients and wind speeds can be obtained.Once the back-scattering coefficients of the sea surfaceare measured,then the wind speeds over the sea surface can approximately be determined.It isuseful for the microwave remote sensing in ocean.     

Estimation of the transfer ratio frequency characteristic of HF radio channel by using the LFM signal

A technique for determining the transfer ratio frequency relationship of a short-wave ionospheric radio channel with high resolution has been considered using the continuous linear-frequency-modulated signal. This technique is based on the filtering of modes of ionospheric propagation at the receiver output following the compression method processing in the frequency domain. The estimates of measurement errors were also presented.     

Millimeter wave radio telescopes: Gain and pointing characteristics

The basic gain and pointing characteristics of millimeter-wave radio telescopes are described in this paper. Formal definitions of relevant telescope parameters are given. Performance limits set by conventional technology are discussed and compared with measurements of existing telescopes. Methods are also suggested for minimizing pointing errors.     

电离层对高频电波吸收衰减的影响研究

提出了一种计算电离层吸收损耗的方法.利用国际参考电离层模型(IRI)和离散电离层背景下的射线追踪模型,对穿过电离层区域的高频雷达电波的吸收损耗进行了定量分析,得到了在不同电波频率和不同发射仰角情况下的吸收损耗的模拟结果,并给出了初步结论.     

一种射频体声波滤波器的版图设计与仿真

射频体声波滤波器品质因子高,尺寸小,其性能已超过声表面波滤波器,将其替代传统的射频滤波器极具性能和价格优势。本文建立了适用于体声波滤波器性能分析的巴特沃斯—范戴克（MBVD）模型,采用梯形级联方式设计了一种射频体声波滤波器的版图。在此基础上以中心频率为1.99GHz,带宽56MHz的体声波滤波器为例,对不同连接级数梯形滤波器的插入损耗、阻带抑制进行了仿真与分析讨论,在4阶滤波器中其带外衰减达到了-29.708dB。采用微机电机械系统工艺制备的2阶和3阶滤波器传输特性的测试曲线与仿真结果基本吻合,表明射频体声波滤波器具有广泛的应用前景。该模拟结果可作为射频体声波滤波器设计的一个重要参考。     

基于射频指纹技术的卫星信号个体识别方法

现有射频指纹识别研究主要集中于射频指纹产生及提取的通信机理,忽略了实际应用中信号采集数据的清洗筛分、识别算法模型的效率等工程性问题。为此,分析了卫星通信号的载波信息提取的原理与方法,并针对现有射频指纹方法的不足,围绕卫星信号识别领域,利用海量采集的射频指纹数据,深入研究基于自组织神经网络的射频指纹识别算法,提出了对应的算法模型,并与现有常见的无监督算法进行了比较。结果表明,所提方法可以取得更优的算法聚类效果和时间开销,可作为设计实现卫星频谱管理系统的基础。     

加热电离层区域对高频波特性的影响研究

利用二维磁化电离层欧姆加热数值模型和离散电离层背景下的射线追踪模型,对穿过高频加热电离层区域的超视距雷达(OTHR)诊断电波的波特性(主要是doppler频移和相位偏移)进行定量的分析,得到了在加热开机2 min内的诊断电波doppler频移和相位偏移的模拟结果.并结合国外相近参数的试验结果进行对比分析,给出了初步结论.     

基于离散时间状态空间模型的平衡式C-W电路频率特性研究

对于分辨率优于0.25 nm的200 k V级透射电子显微镜,要求配备稳定度优于2×10-6/min的高压电源,这是高稳定度输出的重要保证。C-W电路是高压发生的核心部分,其传递函数是高压电源闭环系统中不可或缺的环节之一。由于CW电路采用级联的整流二极管-电容网络拓扑,受二极管器件本身非线性效应的影响,常规传递函数的求解方法已不符合其非线性的特点。通过研究平衡式C-W电路的原理,结合其物理特征将系统非线性处理为离散时间下的线性系统,建立了离散时间状态空间模型,进行z变换后推导了三阶平衡式C-W脉冲传递函数通式。应用Matlab仿真一款平衡式C-W电路的幅频特性曲线和相频特性曲线,进一步分析了其频率特性,为平衡式C-W电路系统特性的研究提供了一种理论方法。     

A step in time Changes in standard frequency and time signal broadcasts

An improved Coordinated Universal Time (UTC) system has been adopted by the International Radio Consultative Committee (CCIR).* It was implemented internationally by the standard frequency and time broadcast stations on January 1, 1972. The new UTC system eliminates the frequency offset of 300 parts in 1010 between the old UTC and Atomic Time, thus making the broadcast time interval, the UTC second, constant and defined by the resonant frequency of cesium atoms. The new time scale will be kept in synchronism with the rotation of the earth within ±0.7 second by step-time adjustments of exactly 1 second, when needed. A time code will be added to the disseminated time signals that will permit Universal Time to be obtained from the broadcasts to the nearest 0.1 second for users requiring such precision.     

加强频率台站管理做好大型活动无线电安全保障

在工业和信息化部、北京市政府的领导下,在军队和相关省市的大力支持下,北京市无线电管理局先后圆满完成了2008年北京奥运会、残奥会和首都国庆60周年活动无线电安全保障任务。以下主要介绍北京市无线电管理工作所面临的形势以及保障大型活动无线电安全的几点做法和体会。     

A signal theory approach to the space and frequency variant filtering performed by the human visual system

A mathematical representation is given of the processing performed by the peripheral nervous cells of the human visual system. This representation consists essentially of a set of shift-variant convolutions, implying non-uniform resolutions in both the space and frequency domains. A remarkable property of such “convolutions” is the form-invariance of their outputs with respect to (size) linear scalings of the inputs. It is shown that this latter property is a necessary support of the hypothesis according to which the visual system translates linear scalings of the (retinal) inputs into shifts of the (cortical) outputs.     

光载射频信号在多模塑料光纤中的传输特性研究

实验研究了光载射频信号在多模塑料光纤(POF)中的传输特性,将24GHz的正弦波信号与1.5Gbt/s的数字信号进行混频后再通过光强度调制器产生双边带调制光信号,将双边带调制信号通过多模POF发送至接收机,在接收机转化为24GHz的射频信号。实验结果表明,这种双边带调制的光载波射频信号可以在多模POF中传输50m后而功率代价可以忽略不计。