The singularity expansion method and its application to targetidentification

The singularity expansion method (SEM) for quantifying the transient electromagnetic (EM) scattering from targets illuminated by pulsed EM radiation is reviewed. SEM representations for both induced currents and scattered fields are presented. Natural-resonance-based target identification schemes, based upon the SEM, are described. Various techniques for the extraction of natural-resonance modes from measured transient response waveforms are reviewed. Particular attention is given to the aspect-independent (extinction) E-pulse and (single-mode) S-pulse discriminant waveforms which, when convolved with the late-time pulse response of a matched target, produce null or mono-mode responses, respectively, through natural-mode annihilation. Extensive experiment results for practical target models are included to validate the E-pulse target discrimination technique. Finally, anticipated future extensions and areas requiring additional research are identified     

Extraction of the natural frequencies of a radar target from a measured response using E-pulse techniques

A new scheme is introduced for extracting the natural resonance frequencies of a radar target from a measured response. The method is based on theE-pulse technique and is shown to be relatively insensitive to random noise and to estimates of modal content. Verification of the technique is accomplished by comparing the natural frequencies extracted from the measured responses of a thin cylinder and a circular loop with those obtained from theory. The applicability of the technique to low-Qtargets is also demonstrated, using the measured response of a scale model aircraft.     

Detection and discrimination of radar targets

A new method for detection and discrimination of radar targets is described. The basis for this method is that the gross structure of a radar target can be identified from scattered fields of the target at harmonic radar frequencies located just in the low resonance region. This is in sharp contrast to many target signature schemes that operate at much higher frequencies and observe many of the details of the target in lieu of its gross features. Multiple frequency radar scattering data and the complex natural resonant frequencies of radar targets are integrated into a predictor-correlator processor. The method is illustrated using as target models both classical shapes and thin-wire configurations of simple geometry. Integral equation programs are utilized to calculate multiple frequency backscatter data for the wire geometries and to deduce the complex natural resonant frequencies of the wire structures. Discrete multiple frequency radar scattering data spanning a particular spectral range are shown to be desirable for optimum capability but discrimination and detection can be achieved using just two near-conventional radars, even if the radars are located at different sites and hence view the target from different aspects.     

Experimental discrimination of wire stick targets usingmultiple-frequency amplitude returns

An experimental radar target discrimination algorithm using multiple-frequency scattering amplitude without phase data is investigated. The technique is based on the concept of natural resonance frequencies, which are aspect-independent so that prior information of the aspect angle is not necessary. The radar cross sections of two wire stick models are measured at different aspect angles for distinguishing these two similar targets in the resonance frequency range. By the one-number method, the risk values of different test targets are calculated and compared using the minimum-risk strategy. The results show that the discrimination algorithm works well under experimental conditions if the natural resonance frequencies of different targets are unlike enough. The effect of noise is also investigated     

Ultra-wideband radar signals generated technology with two-channel

Synthesis of ultra-wideband (UWB) linear frequency modulation radar signals is a very important technology for microwave imaging, target identification and detection of low radar-cross-section (RCS) targets. In this paper a new method of UWB radar signals generation with two-channel is presented. The realization structure is given, and the principle of signal synthesis is analyzed. At the same time, an automatic adjustment measure of signal phase is proposed for phase discontinuity of waveform in this method. The simulation experiment and analysis results show that radar signals with large instantaneous bandwidth can be generated by means of this method.     

Radar target discrimination using the extinction-pulse technique

An aspect independent radar target discrimination scheme based on the natural frequencies of the target is considered. An extinction-pulse waveform upon excitation of a particular conducting target results in the elimination of specified natural modal content of the scattered field. Excitation of a dissimilar target produces a noticeably different late-time response. Construction of appropriate extinction-pulse waveforms is discussed, as well as the effects of random noise on their application to thin cylinder targets. Also presented is experimental verification of this discrimination concept using simplified aircraft models.     

Nonsinusoidal radar signal design for stealth targets

The detection of stealth point targets challenges the design of conventional radars using sinusoidal carriers since the objective of stealth technology is to reduce the radar cross section (RCS) of targets to a level where the radar receiver cannot detect the target. While there are a number of techniques employing different technologies to reduce the RCS of targets, shaping and coating the target with absorbing material are the most useful ones. The analysis and design of nonsinusoidal radar signals is based on modeling stealth point targets by a two-layer structure consisting of a metal surface covered with a coat of absorbing material. The design is presented for two classes of signals: uncoded signals and pulse compression signals using Barker codes. The relationship between target response, absorbing material time delay, time variation of transmitted pulses and coding features are determined and analyzed. While sliding correlators are used for detection and selection of various target responses, their output autocorrelation functions are determined analytically in terms of transmitted signal autocorrelation functions. Thumbtack range-velocity resolution functions are obtained for transmitted signal characters with a single pulse and characters with coded waveforms, for different pulse duration. It is shown that the range resolution can be improved by the proper choice of the transmitted signal duration relative to absorbing material time delay. Thumbtack range-velocity resolution functions similar to those of conventional point targets can also be realized     

Radar waveform synthesis method--A new radar detection scheme

A new scheme for radar detection and discrimination, the radar waveform synthesis method, is investigated. This scheme consists of synthesizing the waveform of an incident radar signal which excites the target in such a way that the return radar signal from the target contains only a single natural resonance mode of the target. When the synthesized incident radar signal for a known preselected target is applied to a wrong target, the return radar signal will be significantly different from that of a natural mode of resonance, thus the wrong target can be sensitively discriminated. A simple geometry of a thin wire illuminated by a radar signal at normal incidence has been studied. The induced current on the target and the backscattered field from the target are obtained in terms of natural resonance modes. The required waveforms for the incident radar signal for exciting return radar signals which contain various single natural mode of resonance are obtained. When an incident radar signal synthesized to excite a natural resonance mode of a thin wire is applied to a slightly shorter or longer thin wire, the return radar signal from the wrong target is shown to be significantly different from that of a pure natural mode of resonance.     

Image processing of synthetic aperture radar range ambiguoussignals

If wide-range swath scenes are illuminated by a synthetic aperture radar with high-pulse repetition frequencies, ambiguous scattered signals from an undesired range may appear in its receiver at the same time from consecutive transmitted pulses. This simplified analysis shows that range ambiguous signal returns result in a defocused image because they are convolved with the phase history of the desired range signal. A satellite borne SAR example is presented     

Determination of the natural frequencies of a thin wire elliptical loop

The natural frequencies of a thin wire scatterer formed into an elliptical loop are determined theoretically by solving the homogeneous transform domain electric field integral equation (EFIE). Comparison with frequencies extracted from measurements of the time domain scattered field responses of elliptical and circular loops at various aspects validates the assumption of a natural mode expansion of the late-time response of conducting radar targets.     

Selection of Contributing Natural Poles for the Characterization of Perfectly Conducting Targets in Resonance Region

In the resonance region, the radar scattering response of any target can be modeled by natural poles, with the formalism of the singularity expansion method. The mapping of poles gives useful information for the discrimination of radar targets. In this paper, we show that a reduced number of natural poles is sufficient to characterize such objects. Furthermore, we propose a procedure for selecting the poles that actually contribute to the scattering response. Results are presented for various perfectly conducting (PC) canonical targets and for a PC complex shape target.     

An ultrafast wide-band millimeter-wave (MMW) polarimetric radar for remote sensing applications

With the advent of high-frequency radio frequency (RF) circuits and components technology, millimeter-wave (MMW) radars are being proposed for a large number of military and civilian applications. Accurate and high-resolution characterization of the polarimetric radar backscatter responses of both clutter and man-made targets at MMW frequencies is essential for the development of radar systems and optimal detection and tracking algorithms. Toward this end, a new design is developed for ultrafast, wide-band, polarimetric, instrumentation radars that operate at 35 and 95 GHz. With this new design, the complete scattering matrix of a target (magnitude and phase) can be measured over a bandwidth of 500 MHz in less than 2 /spl mu/s. In this paper, the design concepts and procedures for the construction and calibration of these radars are described. In addition, the signal processing algorithm and data-acquisition procedure used with the new radars are presented. To demonstrate the accuracy and applicability of the new radars, backscatter measurements of certain points and distributed targets are compared with their analytical radar cross section (RCS) and previously measured /spl sigma//spl deg/ values, respectively, and good agreements are shown. These systems, which can be mounted on a precision gimbal assembly that facilitates their application as high-resolution imaging radar systems, are used to determine the MMW two-way propagation loss of a corn field for different plant moisture conditions.     

Phase signature radars

The identification and classification of radar targets is facilitated by a newly developed technique based on measurements of the differential phase shift of the target scattering obtained at harmonic phase-locked frequencies. These phase differences are independent of target range and relative motion. It is mathematically proved that for a smooth conducting body in the Rayleigh region the copolarized terms of the scattering matrix have zero phase shift. Based on this, the absolute target phase can be measured. The differential phase shift between two harmonic frequencies has been measured for various types of targets. The experimental results indicate that the differential phase shift is a useful parameter for classifying or identifying targets, and one that can be readily measured in practical radars.     

谐振区雷达目标识别的模块化神经网络方法

本文研究了基于模块化神经网络的谐振区雷达目标识别方法,该方法先用BP网络进行波形预测,再用最大后验概率准则或修正最小平方误差准则进行分类。通过计算机模拟,证实了该方法有较好的识别性能,且对信号留数变化,即目标的姿态变化不敏感。另外,该方法还具有实现简单,结构扩展方便等优点。     

提取自然频率问题的对偶解法和连续化Prony法

本文提出了提取雷达目标自然频率的对偶空间解法,该解法包含了前人提出的Prony法、奇异值分解法和E脉冲法,从理论上认识了这3种算法的性能和相互间的内在联系。进而从对偶解法导出了一种新的算法连续化Prony法。相对于上述3种算法,该算法对信号中的噪声和模数都不敏感,是一种更为有效、实用的算法。文中作了数值比较和实验验证。     

基于正向建模的目标超宽带电磁脉冲雷达回波仿真

为了快速获取超宽带（ultra-wideband,UWB）电磁脉冲激励下雷达目标的时域电磁响应,提出了一种基于散射中心正向建模的目标时域回波仿真方法.从目标几何模型出发,利用空间射线分集技术对强散射源进行分离,通过模型参数正向确定方法构建出目标的属性散射中心模型,用以表征目标高频电磁散射特性,并在UWB电磁脉冲激励下进行仿真运算,获得目标时域回波信号.以典型目标SLICY为例,基于正向建模的散射中心模型,快速获取不同UWB电磁脉冲激励下的雷达回波信号,与高频仿真方法得到的一维距离像（high resolution radar profile,HRRP）进行对比分析,吻合良好.由此验证了本文提出的回波仿真方法的有效性,为不同辐射源激励下目标的快速电磁响应研究提供了一种新思路.     

一种基于线性预处理的米波雷达低仰角处理算法

由于多径效应的存在,使得米波雷达在低仰角时接收到的回波是两组相干信号,即直达信号和反射信号的矢量叠加.本文提出了一种米波雷达在低仰角情况下DOA(波达方向估计)的新方法,先对实测数据进行线性变换,然后对线性变换后的数据和实测数据的差分结果应用角度超分辨算法,实现米波雷达低仰角的波达方向估计.算法对信号所处环境不敏感,可以有效地克服多径效应.理论分析和计算机仿真都表明新算法的优越性.     

Radar imaging from ramp response signatures

A new approach to synthetic image generation of radar targets from electromagnetic scattering measurements is described and illustrated. Complex harmonic samples of the backscattered response at ten frequencies lying in the low resonance range of the target response spectrum are used as input data for this approach. A periodic ramp response waveform synthesized from these data is shown to be correlated to the cross-sectional area versus distance along the line-of-sight, or profile function, of the targets measured. A technique using profile function data at three orthogonal target look angles for the mathematical specification of a three-dimensional "approximate limiting surface" is described. Visual images simulating an isometric view of the surface are then constructed. Images produced from model measurements of several target shapes are presented to illustrate some characteristics of this imaging process.     

散射中心的时频像特征研究

散射中心是高频区电磁散射的重要特征,其属性特征,如散射幅度、位置,对方位的依赖性对于雷达成像及目标识别具有重要意义。与其它雷达图像相比,时频图像能更完整地反映出散射中心的属性特征,但目前关于不同散射中心的时频像特征研究还不完整。该文首先基于散射中心模型,从理论上分析了各个散射中心时频像的特征,然后通过全波法电磁计算得到了典型结构目标的散射数据,从数值上验证了对时频图像特征的理论分析,最后总结了不同散射中心的时频像特征,此结论有助于从时频像中直观地判断目标的散射中心类型和其对应的物理结构特点,可为基于时频像的雷达目标特征提取与识别提供一定的理论参考。     

用瞬态电磁场测量目标的冲激响应和宽带雷达截面

突破传统的时域散射区域技术的限制，采用基于瞬态电磁测试系统的实验性冲激雷达，在实际空间对导体球和导体长椭球进行了瞬态磁脉冲响应测量。用修正最速下降法由实测瞬态电磁脉冲响应较发地反演了导体球和导体长椭球的冲激响应。提出了时域测量目标宽带雷达截面的瞬态响应法，并用此法测得了导体球、导体长椭球的雷达截面和谐振频率。