基于假设检验的SAR图像分辨率判别准则与算法

SAR图像分辨率是衡量SAR系统成像质量的重要指标.常用的3dB主瓣宽度测量方法来源于光学成像中的瑞利准则,受人类视觉感知能力限制而且没有考虑噪声影响.本文基于假设检验理论,根据点目标散射模型建立相位历史域、复数据域以及图像域的两点目标二维分辨模型,得到统计意义上分辨率的计算表达式,并由此得出分辨率与图像质量或噪声水平之间的定量关系.该表达式与现有的分辨率定性理解相一致,克服了传统分辨率定量评价准则的缺陷.     

A tethered interferometric synthetic aperture radar (SAR) for atopographic mission

Analyzes a spaceborne interferometric synthetic aperture radar (SAR) for high-resolution, topographic applications. Two physical antennas are vertically spaced and are carried along parallel paths by two different platforms connected by a tether. Tethered space systems have been proposed by several authors for different applications and a joint US-Italian program exists to deploy in 1992 a small satellite (Tethered Satellite System) from a Space Shuttle. A system performance analysis is carried out, considering a theoretical study and a numerical simulation. By evaluating the tethered interferometric SAR impulse response, the height of various point scatterers is computed and an error budget is obtained. Results show that this system is capable of achieving a root-mean-square (RMS) error in height measurement adequate for several applications     

Detection and Analysis of Anisotropic Scattering in SAR Data

Scattering from man-made objects in SAR imagery exhibits aspect and frequency dependencies which are not well modeled by standard SAR imaging techniques. If ignored, these deviations will reduce recognition performance due to the model mismatch, but when appropriately accounted for, these deviations from the ideal point scattering model can be exploited as attributes to better distinguish scatterers and their respective targets. With this premise in mind, we have developed an efficient modeling framework that incorporates scatterer anisotropy. One of the products of our analysis is the assignment of an anisotropy label to each scatterer conveying the degree of anisotropy. Anisotropic behavior is commonly predicted for geometric scatterers (scatterers with a simple geometric structure), but it may also arise from volumetric scatterers (random arrangements of interfering point scatterers). Analysis of anisotropy arising from these two modalities shows a clear source-dependent relationship between the anisotropy classification and parameters of the scatterer. In particular, the degree of anisotropy is closely related to the size of the scatterer, and increasing the aperture size reduces the incidence of volumetric anisotropy but preserves the detection rate for geometric anisotropy. This result helps to address the question in the SAR community regarding the utility of wide-aperture SAR data for ATR since wide-aperture data reveals geometric anisotropy while resolving volumetric anisotropy into individual isotropic scatterers.     

任意构型双基地SAR二维频谱精度定量评估研究

双基地SAR频域成像算法,是目前SAR成像领域的研究热点。由于双基地SAR的距离历史具有双根号形式,利用驻定相位原理无法得到解析的二维频谱表达式,这给双基地SAR频域成像算法的研究带来了一定困难。本文通过引入点目标冲激响应,利用主瓣宽度、峰值旁瓣比和积分旁瓣比三个指标对扩展LBF、级数反演、二维驻定相位原理得到的点目标冲激响应进行了量化评估。同时,以星机双基地SAR成像为例,仿真不同频谱模型的二维聚焦效果,进一步验证了不同频谱模型的精度。      

Nonuniform azimuth image shift observed in the Radarsat images of ships in motion

This paper describes, for the first time to the authors' knowledge, the phenomenon of nonuniform azimuth image shift of a rigid body observed in the Radarsat synthetic aperture radar (SAR) images of cruising ships. The effect is caused by the different slant-range velocities of coherent scatterers across the hull associated with the ship motions. The slant-range velocity is estimated from the SAR image of an identified ship, and it is compared with that computed from a numerical model using the ship's specification and meteorological data. The result indicates that the dominant contribution to the nonuniform image shift is the pitching motion of the ship. Further comparison is made with the wave orbital velocity, and the results are shown to be in good agreement. Comparisons are also made between the SAR-derived slant-range velocities of two unknown ships and wave orbital velocities, and reasonable agreement is obtained. One of the ships' images exhibits not only nonuniform shift but also image skew. The latter skewing effect may be caused by rolling of the ship.     

基于物理光学方法二面角反射器一维距离像特征信号计算

二面角是众多雷达目标中的典型散射结构,是典型的二次散射体,其特征识别对于复杂目标的识别具有重要意义。首先在频域内应用ＰＯ光学计算方法得到二面角的散射信号,通过ＩＦＴ变换得到时域散射信号。采用幅度调制信号与目标的时域散射信号进行卷积,即得到目标的距离像。一维距离像是二维合成孔径成像的基础。     

状态空间法在超宽带雷达动目标速度及距离像估计中的应用

该文通过状态空间(State-Space, SS)法高分辨估计超宽带(Ultra-Wide Band, UWB)雷达运动目标径向速度及其距离像。首先,结合具有伸缩的UWB雷达回波模型获得散射体频域冲激响应,以构建SS所需的Hankel矩阵。其次,利用SS估计UWB雷达高速运动目标径向速度及距离像。进一步,分别推导了上述估计参数的克拉美-罗下界(Cramr-Rao Lower Bound, CRLB)。SS法不但能获得目标高分辨距离像,而且较好地规避了多普勒随信号带宽内频率变化,即多普勒色散对径向速度估计的影响。该方法亦具有良好的噪声抑制能力。此外,其只需目标频域冲激响应,并未涉及具体发射波形,因此未受发射信号形式的限制,这不同于利用小波变换估计相关参数的方法。仿真实验验证了该方法的可行及有效性。     

Cross-calibration experiment of JPL AIRSAR and truck-mountedpolarimetric scatterometer

When point calibration targets are used to calibrate a SAR image, the calibration accuracy is governed by two major factors. The first factor stems from the stringent requirement on the radar cross section (RCS) of the point calibration target. To reduce the effect of radar return from the background, the RCS of a point calibration target must be much larger than that of the background. Calibration targets with large RCS require large physical dimensions for passive targets or high amplifier gain for active targets, which in practice leads to uncertainty in the nominal RCS of the targets. The second factor is related to the fact that point calibration targets are used to develop a calibration algorithm which is applied to distributed targets. To this end, accurate knowledge of the impulse response (ambiguity function) of the SAR system is required. To evaluate the accuracy of such a calibration process, a cross-calibration experiment was conducted at a test site near Pellston, MI, using the JPL aircraft SAR and the University of Michigan truck-mounted polarimetric scatterometer. Five different types of distributed surfaces, all in the same area, were chosen: three of these were bare surfaces with varying roughnesses, and the other two were covered with vegetation. Trihedral corner reflectors were used for calibrating the aircraft SAR, and the UM scatterometer was calibrated using a metallic sphere. The scatterometer data were collected at L and C bands immediately after the aircraft flew over the test site. This paper presents results of the cross calibration between the polarimetric SAR and ground-based polarimetric scatterometer measurements at L and C bands. Comparison of the data measured by the two radar systems shows that SAR calibration with trihedrals may lead to unreliable results. A distributed-target calibration technique is introduced and applied to the data with good results     

Resolution of a controversy surrounding the focusing mechanisms ofsynthetic aperture radar images of ocean waves

This paper addresses key problems regarding the focusing of synthetic aperture radar (SAR) images of ocean surface waves, explaining why applying a processor defocus will generally yield an enhanced image, why the same defocus applies to both image modulations brought about by the radar cross section and by the velocity bunching process, and why the effects apply to both single-look and multilook systems independently of look relocation. Two interpretations are given for the case when surface scatterers are stationary, but modulated in reflectivity (radar cross section) by a propagating wavefield. The first interpretation is what will be called a “degrade-and-shift” model. In it, a processor focusing adjustment degrades a point image. However, the overall image can be enhanced because an appropriate defocus results in a shifting of points in such a way that the image can most closely resemble the image of the time-invariant (or “frozen”) reflectivity. The second interpretation is a “defocus-and-refocus” model in which the image of a time-varying reflectivity is defocused and may be refocused to enhance the image. In justifying this “defocus-and-refocus” model, it is shown that the radar return from stationary scatterers of time-varying reflectivities is identical to that from physically moving scatterers of constant reflectivity. Thus, the two interpretations are not contradictory; they are, fundamentally, equivalent. The models support the use of a processor defocus corresponding to one half the wave phase velocity. Both qualitative and quantitative illustrations of the effects are given. Finally, it is shown that the same defocusing effect applies to image modulations brought about by the velocity bunching process     

低频UWB SAR的冲击响应函数

提取系统的冲击响应函数(impulse response function,IRF)是测量雷达系统性能指标的前提,IRF来自理想点目标的雷达图像。对于低频超宽带合成孔径雷达(ultra wide band synthetic aperture radar,UWB SAR),参考点目标的电磁散射特性是频率和方位角的函数,不满足理想点目标的散射特性要求。本文从IRF的定义出发,结合低频UWBSAR系统处理模型,利用二维数字聚焦方法消除杂波的影响,同时利用低频电磁散射数值建模的方法计算参考点目标的电磁散射特性,最后给出了计算系统IRF的方法。     

双站SAR距离徙动算法

由于空间关系复杂，双站合成孔径雷达（SAR）数据的成像处理是一个难点。但在合成孔径时间内可以近似认为他们的轨道是直线且相互平行。利用驻定相位原理，推导双站SAR在航迹平行、飞行速度相同时回波信号的二维频域表达式；然后采取一定近似，提出双站的距离徙动算法（RMA），并用仿真结果验证算法的有效性。     

A Diffraction Transfer Function Approach to the Calculation of the Transient Field of Acoustic Radiators

A computationally-efficient approach to the calculation of the transient field of an acoustic radiator was developed. With this approach, a planar or curved source, radiating either continuous or pulsed waves, is divided into a finite number of shifted and/or rotated versions of an incremental source such that the Fraunhofer approximation holds at each field point. The acoustic field from the incremental source is given by a 2-D spatial Fourier transform. The diffraction transfer function of the entire source can be expressed as a sum of Fraunhofer diffraction pattern of the incremental sources with the appropriate coordinate transformations for the particular geometry of the radiator. For a given spectrum of radiator velocity, the transient field can be computed directly in the frequency domain using the diffraction transfer function. To determine the accuracy of the proposed approach, the impulse response was derived using the inverse Fourier transform. The results obtained agree well with published data obtained using the impulse response approach. The computational efficiency of the proposed method compares favorably to those of the point source method and the impulse response approach.     

Simulation of ocean waves imaging by an along-track interferometricsynthetic aperture radar

A two-dimensional (2D) model for describing the imaging of ocean waves by an along-track interferometric synthetic aperture radar (AT-INSAR) is derived. It includes the modulation of the normalized radar cross section by the long waves, velocity bunching, and azimuthal image smear due to orbital acceleration associated with long waves and due to the orbital velocity spread within the AT-INSAR resolution cell (parameterized by the scene coherence time). By applying the Monte-Carlo method, AT-INSAR amplitude and phase image spectra are calculated for different sea states and radar configurations. The Monte-Carlo simulations show that velocity bunching affects the AT-INSAR imaging mechanism of ocean waves, and that a unimodal ocean wave spectrum may be mapped into a bimodal AT-INSAR phase image spectrum due to an interference between the velocity term and the velocity bunching term in the AT-INSAR imaging model. It is shown that the AT-INSAR imaging mechanism of ocean waves depends on the ratio of the scene coherence time and the time separation between the observations by the two antennas. If this ratio is larger than one, the AT-INSAR phase image spectra are distorted. Furthermore, the simulations show that the AT-INSAR phase image spectrum is quite insensitive to the ocean wave-radar modulation transfer function. Comparing AT-INSAR with conventional SAR imaging of ocean waves, the authors find that the azimuthal cut-off in AT-INSAR phase image spectra is shifted toward higher wavenumbers than in conventional SAR image spectra. This implies that AT-INSAR can resolve shorter azimuthal wavenumbers than conventional SAR. Thus the authors conclude that AT-INSAR phase images are better suited for measuring ocean waves spectra than conventional SAR images     

分布式卫星SAR沿航迹干涉编队构型优化

沿航迹干涉(ATI)SAR可用来测量地面运动目标的径向速度,分布式卫星系统能改善ATI测速精度并增加系统可检测速度范围。合理地设计小卫星间的空间编队构型是保证分布式卫星系统性能的关键。该文给出一种综合评价分布式卫星SAR系统测速精度的模型,该模型可用来对满足绕飞轨道的分布式卫星群的构型进行优化设计。采用随机搜索算法进行了仿真试验,试验结果验证了本文方法的有效性。     

ScanSAR processing using standard high precision SAR algorithms

Processing ScanSAR or burst-mode SAR data by standard high precision algorithms (e.g., range/Doppler, wavenumber domain, or chirp scaling) is shown to be an interesting alternative to the normally used SPECAN (or deramp) algorithm. Long burst trains with zeroes inserted into the interburst intervals can be processed coherently. This kind of processing preserves the phase information of the data-an important aspect for ScanSAR interferometry. Due to the interference of the burst images the impulse response shows a periodic modulation that can be eliminated by a subsequent low-pass filtering of the detected image. This strategy allows an easy and safe adaptation of existing SAR processors to ScanSAR data if throughput is not an issue. The images are automatically consistent with regular SAR mode images both with respect to geometry and radiometry. The amount and diversity of the software for a multimode SAR processor are reduced. The impulse response and transfer functions of a burst-mode end-to-end system are derived. Special attention is drawn to the achievable image quality, the radiometric accuracy, and the effective number of looks. The scalloping effect known from burst-mode systems can be controlled by the spectral weighting of the processor transfer function. It is shown that the fact that the burst cycle period is in general not an integer multiple of the sampling grid distance does not complicate the algorithm. An image example using X-SAR data for simulation of a burst system is presented     

若干层析SAR成像方法在解叠掩性能上的对比分析

层析技术因具有解译城区SAR影像上复杂叠掩场景的能力而备受关注。层析成像包含两个部分:估计散射体在高程向的分布和确定散射体在混叠像元内的真实数目。该文以中科院空天院峨眉数据的机载阵列干涉系统参数为基础,选取了若干代表性的方法,包括OMP, SLIM和MUSIC等层析谱估计方法以及BIC和GLRT等模型定阶方法,进行了模拟叠掩目标的层析反演实验,使用了克拉默-拉奥界和重建成功率来评估实验结果。实验表明:在机载阵列数很有限的条件下,(1)使用2阶统计量反演的高程估计量的方差比单个观测矢量反演结果的方差更小;(2)叠掩散射体间的幅度比、相位差和散射间距会影响层析算法解叠掩的成功率;(3)叠掩散射体间的相位差会使层析算法的高程估计发生偏差。      

低波段大波束角SAR 脉冲响应函数特性研究

对合成孔径雷达(SAR)而言,其成像性能的优劣或SAR 图像质量可通过成像系统的脉冲响应函数(IRF)来表征。脉冲响应函数可看作是一个理想点目标的成像结果,这一函数由中央主瓣和一些旁瓣组成。SAR 系统的图像质量可以通过描述IRF 的空间分辨率、峰值旁瓣比、积分旁瓣比等主要特征进行描述和评价。文中主要对工作于VHF/ UHF 频段的低频大波束角SAR 系统的脉冲响应函数(IRF鄄USAR)特性进行了研究,推导了超宽带大波束角条件下的脉冲响应函数。基于点目标的傅里叶支撑谱与脉冲响应函数之间的对应解析关系,分析了IRF鄄USAR 的响应特性,并基于仿真及实测数据对IRF-USAR 响应特性进行了验证。     

Validating the SAR Wavenumber Shift Principle With the ERS–Envisat PS Coherent Combination

Continuity of the European Remote Sensing Satellite Synthetic Aperture Radar (ERS SAR) archive by means of Envisat Advanced SAR (ASAR) data acquired from March 2002 has introduced the problem of the coherent combination of images coming from sensors with slightly different frequencies. The spectral shift principle states that in case of extended distributed targets, the frequency shift is equivalent to a change of looking angle. In this paper, the same principle is exploited to analyze the behavior of permanent scatterers (PSs) with an extension that is smaller than the ground resolution cell. The conditions under which the PSs identified by ERS can be continued by Envisat are then theoretically determined and experimentally validated. Moreover, this analysis shows that acquisitions characterized by different frequencies can be used to identify the slant-range position of scatterers with high subcell accuracy (tens of centimeters). From the processing side, a very precise images coregistration step is required to get the results described in this paper.     

Reconsideration of Azimuth Ambiguities in SAR

The traditional method of specifying and controlling azimuth ambiguities in SAR is through integrated energy balance measures. However, the most frequently observed azimuth ambiguities arise from ensembles of strong point reflectors in the principal sidelobes of the antenna, which in turn are aliased into the processed Doppler bandwidth by the radar PRF. This paper considers the dependence of these ambiguities on radar wavelength and PRF. It is shown that such ambiguous image elements are strengthened in proportion to ? 2 and PRF-1. The theoretical structure is based on orbital SAR geometry, including Earth rotation. The work is applied to a SIR-B L-band radar scene in which azimuth ambiguities are clearly observed. The level and spatial position of these ambiguities are measured in the digital image. The results are extrapolated to higher frequency radars such as ERS-1 and Radarsat. It is concluded that for these C-band radars the point azimuth ambiguity restraint is more relevant (and more binding) than the traditional energy balance method.     

A Three-Dimensional Formulation for Synthetic Aperture Radar Images of Ocean Waves in Orbital Motions

An analytic model of the ocean surface SAR images with a three-dimensional framework is developed following the formalism presented by Swift and Wilson (1979); a trochoidal swell propagates through a uniform field of Bragg-type distributed scatterers. Two- dimensional SAR images are calculated for the interpretation and prediction of actual SAR images of the ocean surface as a function of ocean wave amplitude, wave frequency, propagation direction, and radar frequency, off-nadir angle of the antenna, and spatial resolutions.