Radar scattering measurements of cones and computation of transient response

The far zone scattered signal amplitude and phase for two small cone and cone-like targets (sharp and spherically blunted) were measured using a ten-harmonic20degbistatic radar system. As many as five of the first ten harmonics of 1.085 GHz were transmitted simultaneously. The received vector sum of the harmonics was measured (amplitude and phase) by a network analyzer and stored in a computer. Data were collected as the target moved a half wavelength at the fundamental, and individual data for each harmonic were extracted by Fourier analysis. In either system, empty pedestal (background) data were vectorially subtracted and sphere data were used for system calibration. Data were collected for cones at aspect angles of 0, 10, 20, and 30 degrees forkafrom 0.1 to nearly 4. The behavior of the data is compared to the Rayleigh theory for the lower frequencies and the geometrical theory of diffraction (GTD) for the higher frequencies. These theories do not overlap, and the data allow us to "fill the gap." The data were Fourier transformed to the time domain, and the time response of the targets is discussed.     

Tree-structured vector quantization of CT chest scans: image quality and diagnostic accuracy

The authors apply a lossy compression algorithm to medical images, and quantify the quality of the images by the diagnostic performance of radiologists, as well as by traditional signal-to-noise ratios and subjective ratings. The authors' study is unlike previous studies of the effects of lossy compression in that they consider nonbinary detection tasks, simulate actual diagnostic practice instead of using paired tests or confidence rankings, use statistical methods that are more appropriate for nonbinary clinical data than are the popular receiver operating characteristic curves, and use low-complexity predictive tree-structured vector quantization for compression rather than DCT-based transform codes combined with entropy coding. The authors' diagnostic tasks are the identification of nodules (tumors) in the lungs and lymphadenopathy in the mediastinum from computerized tomography (CT) chest scans. Radiologists read both uncompressed and lossy compressed versions of images. For the image modality, compression algorithm, and diagnostic tasks the authors consider, the original 12 bit per pixel (bpp) CT image can be compressed to between 1 bpp and 2 bpp with no significant changes in diagnostic accuracy. The techniques presented here for evaluating image quality do not depend on the specific compression algorithm and are useful new methods for evaluating the benefits of any lossy image processing technique.     

Oscillating intensity display of soft tissue lesions in MRI

A new postprocessing method for improving visualization of soft tissue lesions in MR images is described. Abnormal tissues are detected by a computerized tissue characterization algorithm which is based on measurements of intensity in a spatially matched pair of T1- and T2-weighted images. Simultaneous display of information from this pair of static images is achieved by using a temporal parameter (amplitude or frequency of intensity oscillation) to encode abnormal pixels. Specifically, a movie is created in which pixel intensities of abnormal tissues are made to oscillate so that the amplitude (or frequency) of oscillation is proportional to an abnormality index which depends on the difference between intensities of normal and abnormal tissues in the original image pair. The visual effect is that of a churning motion within the lesion, while surrounding normal tissues are displayed as stable structures. This technique increases the conspicuity of the lesion by exploiting the eye's great sensitivity to motion.     

一种基于波程差补偿的InISAR图像配准方法

逆合成孔径雷达(ISAR)图像配准是干涉逆合成孔径雷达(InISAR)成像领域一个关键的课题,可以实现同一散射点在不同ISAR图像中的对齐,以便于后续的ISAR图像干涉处理。该文分析了ISAR图像失配准的原因,即散射点到不同天线之间的波程差,并据此提出一种基于波程差补偿的方法来实现不同天线ISAR图像之间的精确配准。首先通过调频傅里叶变换估计目标相对于雷达的转速;进一步根据波程差与目标转动角速度的关系构建补偿相位消除散射点到不同天线间的波程差,并通过2维傅里叶变换获得配准之后的ISAR图像。最后利用干涉处理获得目标真实的3维结构。该文方法可以在回波域通过波程差补偿实现ISAR图像配准,配准之后的各散射点在图像中的位置相同;而经过相关法配准之后的ISAR图像中的各散射点之间有一个像素单元的错位,即该方法的配准效果更精确。此外,基于相关法的图像配准方法耗时达到万秒级,而基于该方法的ISAR图像配准时间仅为秒级,即该方法计算效率更高。最终的InISAR 3维成像结果中,该方法的散射点坐标重构误差为0.3034,而基于相关法的成像结果的误差(45.8529)远大于此。因此,基于所提出方法的InISAR 3维成像结果精度更高。      

基于傅里叶和小波变换的电力系统谐波分析

针对目前电力系统谐波分析中存在的无法同时对稳态谐波分量和非稳态谐波分量进行准确检测的问题,提出一种利用两阶段法结合傅里叶变换和小波变换的算法来实现电力系统谐波分析的方法。首先将原始谐波信号通过小波多分辨率分析分解为低频和高频两部分,然后利用傅立叶变换分析得到低频成分中稳态谐波的幅值、相位等参数,并利用小波包进行分析得到高频部分非稳态谐波的时域信息。     

SAR amplitude probability density function estimation based on a generalized Gaussian model.

In the context of remotely sensed data analysis, an important problem is the development of accurate models for the statistics of the pixel intensities. Focusing on synthetic aperture radar (SAR) data, this modeling process turns out to be a crucial task, for instance, for classification or for denoising purposes. In this paper, an innovative parametric estimation methodology for SAR amplitude data is proposed that adopts a generalized Gaussian (GG) model for the complex SAR backscattered signal. A closed-form expression for the corresponding amplitude probability density function (PDF) is derived and a specific parameter estimation algorithm is developed in order to deal with the proposed model. Specifically, the recently proposed "method-of-log-cumulants" (MoLC) is applied, which stems from the adoption of the Mellin transform (instead of the usual Fourier transform) in the computation of characteristic functions and from the corresponding generalization of the concepts of moment and cumulant. For the developed GG-based amplitude model, the resulting MoLC estimates turn out to be numerically feasible and are also analytically proved to be consistent. The proposed parametric approach was validated by using several real ERS-1, XSAR, E-SAR, and NASA/JPL airborne SAR images, and the experimental results prove that the method models the amplitude PDF better than several previously proposed parametric models for backscattering phenomena.     

基于相干叠加与模均等矢量分解的光学图像加密算法

为了解决光学图像加密技术将密文相位信息主要集中在纯相位掩码中,使其存在轮廓显现问题,该文提出相干叠加与模均等矢量分解的光学图像加密算法。首先,对输入图像进行归一化处理;基于映射,利用输入图像的像素特性,生成其初值,通过迭代映射,输出混沌相位掩码;借助相位掩码,对图像完成调制,并结合Fourier变换,对调制图像进行处理,输出其Fourier频谱;再对该频谱分别进行等模分解,获得两个掩码;再基于不同分数阶的Fourier机制,对两个掩码进行变换;最后,引入相位-幅度截断编码技术,设计单向编码机制,通过激光束的相干叠加,输出编码密文的幅度与相位信息,将相位部分视为编码密文,把幅度信息作为解密密钥。通过等模分解技术,将输入明文演变为4个不同的相位与幅度信息,有效解决了轮廓显现问题。实验结果显示：与当前基于干涉原理的图像加密机制相比,所提算法的安全性更高,有效消除了轮廓显现问题。     

Near-field investigation of uniform periodic monopole arrays

Measurements of amplitude and phase of the fields close to a number of driven periodic arrays of monopoles have been made to determine frequency dependence of the propagation constant for such structures. The results have been analyzed, where possible, in terms of a single traveling wave along the structure, and dispersion curves are given which include attenuation as well as phase shift. These curves clearly illustrate the possibility of obtaining leading phase shift along the periodic structure that will produce backfire radiation. Experimental radiation patterns corroborate the near-field results. A simple theory is given for the backfire phasing in terms of the space-harmonic content of a Fourier series expansion for the fields of periodic structures. This theory predicts beam scanning for the periodic structures which is readily observed in the experimental models. The measured data are approximately applicable to tapered versions of the periodic structures which have log-periodic geometry. Examples of periodic structures that have frequency independent log-periodic counterparts as well as some that do not are discussed.     

投影仪标定中的相位误差补偿

为了减小由投影仪-摄像机γ非线性引起的红蓝棋盘格标定板相位误差,分析了非线性相位误差的产生原因和表现特征,建立了非线性相位误差和相位主值之间的数学模型,由该模型得到相位误差补偿矩阵并修正展开相位;同时采用MATLAB仿真并结合相关试验对该算法进行了验证。结果表明,误差补偿后棋盘格平面展开相位的均方根误差从0.0836rad下降到0.0218rad;由γ非线性引起的棋盘格角点在标靶图像中像素坐标的最大误差值约从补偿前的0.3pixel降低到0.1pixel。该方法有效减小了由投影仪-摄像机γ非线性引起相位误差,提高了标靶图像的角点坐标精度。     

The nonlinear wave equation for higher harmonics in free-electron lasers

The nonlinear wave equation and self-consistent pendulum equation are generalized to describe free-electron laser operation in higher harmonics; this can significantly extend their tunable range to shorter wavelengths. The dynamics of the laser field's amplitude and phase are explored for a wide range of parameters using families of normalized gain curves applicable to both the fundamental and harmonics. The electron phase-space displays the fundamental physics driving the wave, and we use this picture to distinguish between the effects of high gain and Coulomb forces.     

Filter design for the detection/estimation of the modulus of a vector: Application to polarization data

We consider a set of M images, whose pixel intensities at a common point can be treated as the components of a M-dimensional vector. We are interested in the estimation of the modulus of such a vector associated to a compact source. For instance, the detection/estimation of the polarized signal of compact sources immersed in a noisy background is relevant in some fields like astrophysics. We develop two different techniques, one based on the maximum likelihood estimator (MLE) applied to the modulus distribution, the modulus filter (ModF) and other based on prefiltering the components before fusion, the filtered fusion (FF), to deal with this problem. We present both methods in the general case of M images and apply them to the particular case of three images (linear plus circular polarization). Numerical simulations have been performed to test these filters considering polarized compact sources immersed in stationary noise. The FF performs better than the ModF in terms of errors in the estimated amplitude and position of the source, especially in the low signal-to-noise case. We also compare both methods with the direct application of a matched filter (MF) on the polarization data. This last technique is clearly outperformed by the new methods.     

基于FFT的近场——口径场变换方法

本文提出的近场－口径场变换方法基于探头与被测天线（AUT）的耦合方程,由Fourier变换求出AUT发射场的平面波谱的切向分量,然后由Fourier反变换求出被测天线口径场的切向分量,从而达到诊断天线口径场幅相分布的目的。口径场变换中的大部分计算采用了FFT算法,并且引入空间域的重构法,数值精度好,数据处理效率高,可以达到任意的诊断分辨率。通过数值模拟和诊断实验,说明了该方法的正确性和工程实用性。     

Anharmonic frequency analysis: the complement to Fourier analysis

Fourier analysis of a finite time series allocates amplitudes and phases to a given set of frequencies which are integer multiples of a basic frequency determined by the length of the time series. Anharmonic frequency analysis (Afa)collects the spectral components between those frequencies and the results are the average amplitude and phase for each band defined by two consecutive frequencies. A repetition of the transform involved for a later reference time determines the effective frequency within each band by the time change of the phase. This process is by far superior to Fourier analysis especially if unknown discrete frequencies are present in the time series and those frequencies are not harmonics of one common basic frequency. The principles of this method, some of its properties and a few results are presented.     

Precision of harmonic amplitude estimation on jitter corrupted data using sine fitting

The effect that jitter and phase noise have on the precision of amplitude estimation of the fundamental and harmonics of a periodic signal is studied. The analysis carried out is applicable to both Three-Parameter Sine Fitting as well as Coherent Discrete Fourier Transform procedures. An analytical expression for the standard deviation of estimated amplitude is presented. It shows that it is proportional to the jitter/phase noise standard deviation (for small amounts of jitter/phase noise) and inversely proportional to the square root of the number of data samples. The expression presented can be used to compute the estimation confidence intervals or determine the minimum number of samples that should be acquired in order to guarantee a given bound on the estimation precision.     

A nonparametric statistical method for image segmentation using information theory and curve evolution.

In this paper, we present a new information-theoretic approach to image segmentation. We cast the segmentation problem as the maximization of the mutual information between the region labels and the image pixel intensities, subject to a constraint on the total length of the region boundaries. We assume that the probability densities associated with the image pixel intensities within each region are completely unknown a priori, and we formulate the problem based on nonparametric density estimates. Due to the nonparametric structure, our method does not require the image regions to have a particular type of probability distribution and does not require the extraction and use of a particular statistic. We solve the information-theoretic optimization problem by deriving the associated gradient flows and applying curve evolution techniques. We use level-set methods to implement the resulting evolution. The experimental results based on both synthetic and real images demonstrate that the proposed technique can solve a variety of challenging image segmentation problems. Futhermore, our method, which does not require any training, performs as good as methods based on training.     

一种基于t图的多尺度边缘检测方法

在多尺度边缘检测方法中，滤波器的滤波尺度的选取是关键，本文通过检验一给定像素邻域的灰度分布来判断此像素是否位于平滑区，并产生一幅五维向量图－t图，然后根据t图自适应地选取小波变换的尺度。实验结果表明，应用这种方法进行多尺度小波边缘检测具有较强的抗噪能力。     

相位相关法及其在偏振图像配准中的应用研究

分析了遥感偏振探测的原理和与遥感强度探测相比所具有的优越性,指出了偏振图像的特点。采用偏振测量方法,可以获得地物的偏振信息,有助于增强对地物的探测能力,但是对图像像元不对准有很高的敏感性,为了得到可靠的偏振信息,配准的精度应该至少到达0．1个像素。介绍了一种基于傅立叶变换的配准方法：相位相关法,并把该方法应用于偏振图像配准中。相位相关法根据傅立叶变换的平移性质,能够解决具有平移关系的图像配准问题,实验证明该方法具有对噪声和图像灰度变化不敏感的优点。     

The multitap lock-in CCD with offset subtraction

An image sensor for the two-dimensional (2-D) synchronous detection of temporal properties of modulated optical wave fields is presented. Each pixel of this multitap lock-in CCD is capable of acquiring all optical data for the extraction of modulation amplitude, relative phase, background level, and higher order Fourier coefficients. By employing the pixels' offset subtraction stages, a dynamic range of >100 dB was obtained experimentally