The wavenumber shift in SAR interferometry

SAR surveys from separate passes show relative shifts of the ground wavenumber spectra that depend on the local slope and the off-nadir angle. The authors discuss the exploitation of this spectral shift for different applications: 1) generation of “low noise” interferograms benefiting phase unwrapping, 2) generation of quick-look interferograms, 3) decorrelation reduction by means of tunable SAR systems (TINSAR), 4) range resolution enhancement, and 5) the combination of SAR data gathered by different platforms (airborne and satellite) for a “long-time coherence” study     

基于广义似然比检验-动态规划的检测前跟踪算法

检测前跟踪(TBD)技术通过时间换取能量,可以有效提高微弱目标的检测和航迹处理性能.提出了一种距离多普勒域基于广义似然比检验-动态规划(GLRT-DP)的TBD算法.给出了目标回波信号模型,在此基础上,推导了基于二元复合假设检验的GLRT的TBD算法表达式,在把由表达式所得数据矩阵转换到离散距离多普勒域后,利用DP方法进行搜索寻优,实现所提算法的同时降低了运算量.利用虚警概率和检测概率对所提算法的性能进行了分析.通过仿真实验对所提算法进行了验证,实验结果表明:该算法对雷达微弱目标的检测性能优于传统TBD方法,可以有效实现低信噪比背景下微弱目标的检测和航迹处理.     

The consequences of sampling variability on the estimation of wavenumber and propagation direction from spaceborne SAR image spectra

Even if it were true that SAR (synthetic aperture radar) ocean surface imagery provides a perfect noiseless representation of ocean surface wave height, each spectrum computed from such imagery would be but a single realization of the ensemble-mean ocean spectrum. There would be sampling variability associated with the parameters of dominant wave number and propagation direction extracted from such a spectrum. The present study addresses two questions: (i) what statistical distribution is applicable to the spectra of SAR ocean images? and (ii) what are the consequences of such statistics on the precision with which wave number and propagation direction can be extracted? An examination is made of spectra computed from ocean imagery acquired during the SIR-B (Shuttle Imaging Radar-B) mission     

A systematic comparison of QuikSCAT and SAR ocean surface wind speeds

We performed a systematic comparison of wind speed measurements from the SeaWinds QuikSCAT scatterometer and wind speeds computed from RADARSAT-1 synthetic aperture radar (SAR) normalized radar cross section measurements. These comparisons were made over in the Gulf of Alaska and extended over a two-year period, 2000 and 2001. The SAR wind speed estimates require a wind direction to initialize the retrieval. Here, we first used wind directions from the Navy Operational Global Atmospheric Prediction System (NOGAPS) model. For these retrievals, the standard deviation between the resulting SAR and QuikSCAT wind speed measurements was 1.78 m/s. When we used the QuikSCAT-measured wind directions to initialize the inversion, comparisons improve to a standard deviation of 1.36 m/s. We used these SAR-scatterometer comparisons to generate a new C-band horizontal polarization model function. With this new model function, the wind speed inversion improves to a standard deviation of 1.24 m/s with no mean bias. These results strongly suggest that SAR and QuikSCAT measurements can be combined to make better high-resolution wind measurements than either instrument could alone in coastal areas.     

A comparison of several algorithms for SAR raw data compression

Proposes new algorithms for synthetic aperture radar (SAR) raw data compression and compares the resulting image quality with the quality achieved by commonly used methods. The compression is carried out in time and frequency domain, with statistic, crisp, and fuzzy methods. The algorithms in the time domain lead to high resolution and a good signal-to-noise ratio, but they do not optimize the performance of the compression according to the frequency envelope of the signal power in both range and azimuth directions. The hardware requirements for the compression methods in the frequency domain are significant, but a higher performance is obtained. Even with a data rate of 3 bits/sample, a satisfactory phase accuracy is achieved which is an essential parameter for polarimetric and interferometric applications. Preliminary analysis concerning the suitability of the proposed algorithms for different SAR applications shows that the compression ratio should be adaptively selected according to the specific application     

Rapid estimation of the range-Doppler scattering function

Under wide sense stationary uncorrelated scattering (WSSUS) conditions, the signal spreading due to a random channel may be described by the scattering function (SF). In an active acoustic system, the received signal is modeled as the superposition of delayed and Doppler spread replicas of the transmitted waveform. The SF completely describes the second-order statistics of a WSSUS channel and can be considered a density function that characterizes the average spread in delay and Doppler experienced by an input signal as it passes through the channel. The SF and its measurement will be reviewed. An estimator is proposed based on a two-dimensional (2-D) autoregressive (AR) model for the scattering function. In order to implement this estimator, we derive the conditional minimum variance unbiased estimator of the time-varying frequency response of a linear channel. Unlike conventional Fourier methods, the AR approach does not suffer from the usual convolutional smoothing due to the signal ambiguity function. Simulation results are given.     

基于机载双基地SAR的两种成像算法的比较

首先建立了机载双基地SAR的信号模型,然后简要描述了RD算法以及波数域算法的基本成像原理和算法流程,最后基于双基地SAR的信号模型运用RD算法及波数域算法对点目标及面目标进行成像,并分析比较了在正侧式飞行模式下用两种成像算法的优缺点。     

A comparison of time versus frequency domain antenna patterns

This paper compares the performance of antenna arrays that use sinusoidal and ultra-short pulse waveforms. There are two parts to the paper: first, a comparison of narrowband versus ultra-wideband linear arrays using an analytic approach and second, a series of computer simulations used to extend the analytic results and to show the characteristics of nonstandard array configurations. Analysis shows that antennas using ultra-short waveforms have highly desirable pattern properties that do not appreciably degrade under conditions of high array sparsity, failed elements, or antenna element position errors. Patterns are given that show these properties directly. These results are of particular importance to the emerging technology of base-band or time-domain communications and remote sensing. It addresses the practical problem of designing extremely large aperture space-based arrays without populating those arrays densely with antenna elements and also reducing the traditional fabrication tolerances.     

Quantitative comparison of the performance of SAR segmentationalgorithms

Methods to evaluate the performance of segmentation algorithms for synthetic aperture radar (SAR) images are developed, based on known properties of coherent speckle and a scene model in which areas of constant backscatter coefficient are separated by abrupt edges. Local and global measures of segmentation homogeneity are derived and applied to the outputs of two segmentation algorithms developed for SAR data, one based on iterative edge detection and segment growing, the other based on global maximum a posteriori (MAP) estimation using simulated annealing. The quantitative statistically based measures appear consistent with visual impressions of the relative quality of the segmentations produced by the two algorithms. On simulated data meeting algorithm assumptions, both algorithms performed well but MAP methods appeared visually and measurably better. On real data, MAP estimation was markedly the better method and retained performance comparable to that on simulated data, while the performance of the other algorithm deteriorated sharply. Improvements in the performance measures will require a more realistic scene model and techniques to recognize oversegmentation.     

SAR图像幅度域增强的正则化方法

正则化方法通过增加先验信息约束,达到增强SAR图像特征的目的。在分析现有先验信息约束条件的基础上,提出SAR图像幅度域上的正则化增强方法,从理论上证明了SAR图像复数域上正则化方法和幅度域上正则化方法的等价性,进一步解释了正则化方法的相位保持特性,提高了计算效率。通过参数变换,结合先验信息,给出正则化参数的选取方法,该方法操作简单,能有效地抑制噪声,保护目标。最后利用仿真图像和MSTAR图像验证了该方法的有效性。     

基于Contourlet域K-L变换的SAR图像相干斑抑制

针对SAR图像相干斑噪声去除问题,提出了一种基于多尺度分解的Contourlet域K-L变换的SAR图像去噪的新方法。方法首先对源图像进行Contourlet分解,在不同频段的子带图像中,利用K-L变换进行能量保持即提出信号的主要特征,用重构图像来进行去噪,最后通过Contourlet逆变换得到去噪之后的图像。在SAR图像上的实验结果表明,方法不仅较好地保持了图像的纹理和细节特征及边缘特征,且信噪比也较高。     

Modeling and reduction of SAR interferometric phase noise in the wavelet domain

This paper addresses the problem of interferometric phase noise reduction in synthetic aperture radar interferometry. A new phase noise model in the complex domain is introduced and validated by using both simulated and real interferograms. This noise model is also derived in the complex wavelet domain, where a novel noise reduction algorithm, which is not based on a windowing process and without the necessity of phase unwrapping, is addressed. The use of the wavelet transform allows to maintain the spatial resolution in the filtered phase image and prevents to filter low coherence areas. By using both, simulated as well as real interferometric phase images, the performance of this algorithm, in terms of noise reduction, spatial resolution maintenance, and computational efficiency, is reported and compared with other conventional filtering approaches.     

Evaluation of the SAR induced in a multilayer biological structure and comparison with SAR in homogeneous tissues

Wireless systems usage has evolved, for instance, with the recent increase in the use of a hands-free kit, the mobile phone is used more and more in a body-worn position. Therefore, to check the compliance to the international limits, new methods have to be developed. In this study, we analyze the relevance of using the equivalent head liquid for the biological structure of organs that are different from that of the head. This paper compares the Specific Absorption Rate (SAR) values assessed using simulations in a flat phantom filled with the liquid used to test the compliance of mobile phone close to the head to those values obtained using a multilayer model representing the tissues of the trunk. The multilayer structures are derived from the anatomical analysis of the visible human model and corresponding to reasonable positions of a handset in a body-worn configuration. The employed sources are half-wavelength dipoles placed at different distances from those structures and operating at frequencies between 300 MHz and 6 GHz.     

Reciprocity and the concept of the brewster wavenumber

Reflection of planewaves at the planar interface of two biisotropic media has been examined in order to obtain the Brewster wavenumber. This examination shows that the Brewster wavenumber is, at least, not necessarily affected by the reciprocity or the non-reciprocity of the media on either side of the planarinterface.     

Speckle removal from SAR images in the undecimated wavelet domain

Speckle reduction is approached as a minimum mean-square error (MMSE) filtering performed in the undecimated wavelet domain by means of an adaptive rescaling of the detail coefficients, whose amplitude is divided by the variance ratio of the noisy coefficient to the noise-free one. All the above quantities are analytically calculated from the speckled image, the variance and autocorrelation of the fading variable, and the wavelet filters only, without resorting to any model to describe the underlying backscatter. On the test image Lena corrupted by synthetic speckle, the proposed method outperforms Kuan's local linear MMSE filtering by almost 3-dB signal-to-noise ratio. When true synthetic aperture radar (SAR) images are concerned, empirical criteria based on distributions of multiscale local coefficient of variation, calculated in the undecimated wavelet domain, are introduced to mitigate the rescaling of coefficients in highly heterogeneous areas where the speckle does not obey a fully developed model, to avoid blurring strong textures and point targets. Experiments carried out on widespread test SAR images and on a speckled mosaic image, comprising synthetic shapes, textures, and details from optical images, demonstrate that the visual quality of the results is excellent in terms of both background smoothing and preservation of edge sharpness, textures, and point targets. The absence of decimation in the wavelet decomposition avoids typical impairments often introduced by critically subsampled wavelet-based denoising.     

Quantitative comparison of classification capability: fullypolarimetric versus dual and single-polarization SAR

Addresses the land-use classification capabilities of fully polarimetric synthetic aperture radar (SAR) versus dual-polarization and single-polarization SAR for P-, L-, and C-band frequencies. A variety of polarization combinations are investigated for application to crop and tree age classification. Based on the complex Wishart distribution for the covariance matrix, maximum likelihood classifiers for all polarization combinations are used to assess quantitative classification accuracy. Thus, this allows optimally selecting the frequency and the combination of polarizations for various applications     

Time domain comparison of pulse-width modulation schemes

This paper shows that classical space vector modulation (SVM) is functionally identical to double-sided uniform-sampled pulse width modulation (UPWM). Consequently, direct conclusions about harmonic distortion, losses, DC bus utilization, and ease of implementation are made that clarify some existing misconceptions about SVM. Since UPWM is conceptually simple and involves few steps, it is possible that computation may be reduced in practice. UPWM algorithms can avoid the sector and switch sequence tracking tasks in an SVM algorithm. The linear modulation range extension "inherent" to SVM is associated with triplen harmonic injection, and does not provide advantages over conventional third-harmonic injection techniques. The equivalence of SVM and UPWM means that SVM has spectral distortion in baseband as is known to occur in UPWM. This has implications for naturally-sampled sine-triangle PWM (NPWM)-which is known not to generate baseband distortion.     

A semi-synchronous SAR ADC

A semi-synchronous clocking scheme is proposed for successive approximation register (SAR) analog-to-digital converters (ADCs). The conversion time is dynamically allocated to the comparator decision and to the DAC settling in every bit cycle. This significantly improves the conversion speed.     

A comparison between a time domain and continuous wave small animal optical imaging system

We present a phantom study to evaluate the performance of the eXplore Optix (Advanced Research Technologies-GE Healthcare), the first commercially available time-domain tomography system for small animal fluorescence imaging, and compare its capabilities with the widely used IVIS 200 (Xenogen Corporation-Caliper) continuous wave planar imaging system. The eXplore Optix, based on point-wise illumination and collection scheme, is found to be a log order more sensitive with significantly higher detection depth and spatial resolution as compared with the wide-area illumination IVIS 200 under the conditions tested. A time-resolved detection system allows the eXplore Optix to measure the arrival time distribution of fluorescence photons. This enables fluorescence lifetime measurement, absorption mapping, and estimation of fluorescent inclusion depth, which in turn is used by a reconstruction algorithm to calculate the volumetric distribution of the fluorophore concentration. An increased acquisition time and lack of ability to image multiple animals simultaneously are the main drawbacks of the eXplore Optix as compared with the IVIS 200.