极化合成孔径雷达测量海浪的研究

描述了极化合成孔径雷达海浪成像过程。提出了一种极化雷达反演海浪方向谱的方法,该方法利用合成孔径雷达图像谱与海浪方向谱之间简单的线性关系和最优方法反演得到海浪方向谱,并将结果与现场资料进行了比较。经过对同一海区机载雷达P、L、C 3个波段的图像进行反演,其方向谱相当一致,而且有效波高和波向与实测数据吻合相当好。     

干涉合成孔径雷达海浪遥感研究

海浪是重要的海洋环境参数, 它对港口建设、海上航行、油气田开发等具有重要的意义。合成孔径雷达(Syn thet ic Apertu re Radar, SAR ) 是目前大范围测量海浪的主要工具, 用它来估计海浪主波系统的波长和传播方向比较好, 但要定量的提供海浪的振幅以及SAR 图像强度与实际海面的关系是相当困难的, 而且由SAR 图像获取海浪方向谱也比较复杂。沿航迹干涉合成孔径雷达(Along-Track Interferometric Synthet ic Aperture Radar,ATI-SAR ) 是在平台飞行方向上安置两根天线的双天线合成孔径雷达。相对于合成孔径雷达, 真实孔径雷达调制传递函数对沿航迹干涉合成孔径雷达相位图像的影响很小, 因此沿航迹干涉合成孔径雷达更适合测量海浪。详细介绍了沿航迹干涉合成孔径雷达海浪遥感的成像机制, 为进一步反演海浪方向谱奠定了基础, 最后针对目前研究存在的问题以及今后有待研究的方向进行了探讨。     

SAR海浪方向谱的Monte Carlo仿真

利用Longuet-Higgins线性海浪模型和JONSWAP海浪谱来表示海浪方向谱。在Bragg波散射模型的假设下,推导了海浪谱与SAR图像谱之间的映射变换,它适用于线性海浪的成像范围。采用Monte Carlo方法,可产生具有随机性的海浪方向谱。对于时不变的海面来讲,该方法具有计算效率适中的特点。仿真结果表明,海浪谱与SAR图像谱之间存在着谱的畸变、谱的分裂和方位向谱的移位等特点。     

双峰海浪谱的SAR图像交叉谱仿真研究

由6参数双峰频谱和cos-2s型分布函数得到了双峰海浪方向谱的波数谱形式,并利用海浪谱到合成孔径雷达(Synthetic Aperture Radar,SAR)图像交叉谱的非线性转换关系,对双峰海浪谱的SAR图像交叉谱进行了仿真,分析了不同海浪参数和SAR平台参数对SAR图像交叉谱的影响,并与SAR图像谱进行了比较。     

利用星载SAR图像检测海洋锋的方法

海洋锋区剪切流、辐聚流与海洋表面波之间存在相互作用,将这种相互作用处理为对海浪谱的扰动,来分析其对海浪谱密度和谱梯度的影响;根据两尺度模型,分析了海面小尺度波(厘米级)和大尺度波(海浪)与雷达后向散射系数的关系,从而说明了海洋锋的SAR成像原理。在海面SAR图像中,海洋锋的尺度比海浪的尺度大2~3个数量级,可以通过二维空间谱分析,将海浪信息的主要部分滤除,再利用数字图像处理技术提取海洋锋的特征信息,由此形成了一套用海面SAR图像提取海洋锋特征参数的信息处理方法。     

基于文氏方向谱的海浪仿真技术研究

研究不同成长状态下的海浪真实性优化问题,海浪方向谱不仅与风速风向有关,还与风浪的成长状态息息相关.为获取海面波高数据,不仅要求正确地反映风速风向的影响,还应能够真实地反映海浪成长状态的变化.为解决上述问题,提出了一种采用能正确反映海浪成长状态的文氏方向谱来作为靶谱的三维海浪仿真方法,利用海洋谱能量散布随角频率以及不同风浪成长状态变化的具体特性,根据频率等分以及能量等分的采样原则,分别对多种成长状态下的海浪进行了数值仿真.仿真结果表明,新方法能够正确地反映海浪成长状态的变化以及风速风向因子的影响,仿真的海面波高数据与实际海况相符.     

基于外部DEM的InSAR图像配准方法研究

雷达图像的配准是进行雷达干涉测量(SAR Interferometry, InSAR)处理的关键,为了保证干涉相位图或形变相位图反映真实地面特性,需要雷达图像之间亚像元级精度的配准。首先综述了已有的基于外部DEM的InSAR图像配准方法的思路及其不足之处,并提出了一种全新的思路:以图像之间的相干性作为目标函数,利用搜索的方法实现了雷达成像方位向和距离向的最优时间常数的估计,从而实现雷达图像之间亚像元级配准;还进一步推导了数字高程模型(Digital Elevation Model, DEM)的误差对算法精度影响的一个更加严密的表示。结论表明,在利用精确轨道数据的情况下,美国航天飞机测地计划SRTM获得的地形数据的精度可以满足精确雷达图像配准的要求。结果表明,利用基于外部DEM算法配准雷达图像在山区和大的时间基线情况下要优于常规相干多项式配准方法,理论上可以达到百分之一个像素的配准精度。     

基于海浪谱的Gerstner波浪模拟

本文从海洋学现有的观测和研究成果出发,结合海浪的Gerstner模型,提出一种风力作用下的基于海浪谱的三维Gerstner海浪模型。首先,通过分析Gerstner模型各个参数的随机性,利用海浪谱和方向谱相关公式,获得固定风速下模型各随机参数值,建立固定风速下的海浪模型;然后,在此基础上进一步构造了基于海浪谱的风力模型,模拟了风力作用下的海浪;最后,采用基于视点的网格拓扑结构,实现了海浪的实时绘制。实验结果表明,该模型不仅能更真实地模拟海浪,还具有较好的交互性。     

水下航行器测量海浪方向分布研究

研究水下优化测量海浪信息问题,由于海浪谱准确实时测量比较难,针对传统的海浪方向谱测量方法均需要大量的仪器设备和长时间的数据测量处理的不足,为提高测量效率,提出一种通过水下航行器测量海浪方向分布的方法.首先利用水下航行器携带的流速测量设备得到所经过海域的三维流速序列,通过计算三维流速的互谱和自谱,得到描述海浪方向分布的傅立叶系数,利用系数推算出各个频率波浪的主波向角,用海浪能量密度加权处理后得到整个海域内海浪的主波向角进行仿真.仿真结果表明了不同海浪参数下估计结果,验证了方法的有效性.改进方法不仅可以获取某一特定地点的海浪方向谱,还可以随水下航行器的位置变化准确实时获取相应海区的海浪方向谱,为设计提供了依据.     

一种X波段雷达反演算法的研究

研究了X波段雷达数据反演算法,为了验证反演算法的准确性,利用Longuet-Higgins海浪模型,通过选用适当的海浪谱和方向函数,仿真了三维随机海浪数据,利用反演模型得到的数据和仿真的海浪数据进行拟合,结果表明反演算法能够有效地实现海浪相关信息的提取;为了进一步检验算法的实用性,将反演算法得到的参数和实际的波浪浮标数据进行了对比,结果表明反演算法有一定的可行性。     

Measurement of ocean surface slopes and wave spectra using polarimetric SAR image data

Methods have been investigated which use fully polarimetric synthetic aperture radar (SAR) image data to measure ocean slopes and wave spectra. Independent techniques have been developed to measure wave slopes in the SAR azimuth and range directions. The azimuth slope technique, in particular, is a more direct measurement than conventional, intensity based, backscatter cross-section measurements.In the azimuth direction, wave-induced perturbations of the polarimetric orientation angle are used to sense the wave slopes. In the range direction, a new technique involving the alpha parameter from the Cloude-Pottier H-A-? (Entropy, Anisotropy, and (averaged) Alpha) polarimetric scattering decomposition theorem is used to measure slopes. Both measurement types are sensitive to ocean wave slopes and are directional. Taken together, they form a means of using polarimetric SAR (POLSAR) image data to make complete measurements of either ocean wave slopes, or directional wave spectra.These measurements must still contend with fundamental nonlinearities in the SAR image processing (i.e., azimuth direction “velocity bunching”) that are due to wave velocity and acceleration effects.NASA/JPL/AIRSAR L-, and P-band data from California coastal waters were used in the studies. Wave parameters measured using the new methods are compared with those developed using both conventional SAR intensity based methods, and with in situ NOAA National Data Center buoy measurement products.     

Cover photograph Image from GOES-E spacecraft

Azimuthally travelling ocean waves are seldom well imaged by microwave real aperture radar (RAR) operating with conventional HH or VV polarizations. Attenuation of image intensity modulation in the azimuthal direction implies that ocean wave spectra derived from such images also will not be accurate. Real aperture radar cross-section modulation by long ocean waves is normally attributed to two principal sources, tilt modulation and hydrodynamic modulation. In ocean radar images both of these modulation sources are significantly attenuated in the azimuthal direction. Therefore, complete two-dimensional k-space wave spectra derived from microwave data often are quite different than physical ocean spectra. This paper uses fully-polarimetric radar measurements of ocean backscatter to identify a new source of backscatter modulation that is strongest in the azimuthal direction. This modulation source has potential for augmenting tilt and hydrodynamic modulation sources in the azimuthal direction where their weakness causes poor wave visibility. The predicted improvement in the measurement of ocean wave spectra using an optimized polarization is investigated by means of a RAR ocean imaging model. Fully-polarimetric, and conventional, radar spectrometers are proposed which are specifically optimized to sense wave-tilts in the azimuthal direction.     

Monte-Carlo simulation studies of the nonlinear imaging of a two dimensional surface wave field by a synthetic aperture radar

Abstract

The imaging of ocean surface waves by synthetic aperture radar (SAR) is investigated using two-dimensional Monte-Carlo simulations. The properties of the SAR imaging mechanism for windseas and swell in the Bragg scattering regime are discussed as a function of a few governing non-dimensional parameters formed from a combination of SAR and ocean wave parameters. The parameter ranges may be classified into three regimes corresponding to linear and weakly nonlinear, medium nonlinear and strongly nonlinear imaging. The nonlinearities are induced by motion effects (velocity bunching, velocity spread and acceleration smearing), while the real aperture radar (RAR) tilt and hydrodynamic modulation processes are regarded as linear. In the strongly nonlinear imaging regime, the velocity bunching mechanism causes a rotation of the spectral peak towards the range direction and a stretching of the peak wavelength. In addition, the azimuthal resolution is degraded through the Doppler spreading arising from the different facet velocities within a SAR resolution cell. The imaging properties in this regime are largely governed by two non-dimensional parameters, the velocity bunching and velocity smearing parameter. The nonlinear imaging distortions are strongest for broad spectra (windseas) and are significantly weaker for narrow-band swell. In the linear and weakly nonlinear imaging regime, the superposition of the hydrodynamic and tilt cross-section modulation and the velocity bunching transfer function normally produces a rotation of the spectral peak towards the azimuthal direction. The interference characteristics of these different modulation mechanisms depends on the wave propagation direction and can lead to a significant distortion of the image. This is often seen in large differences in the image modulation depths of waves propagating parallel and anti-parallel to the flight direction.     

Interferometric SAR imagery of a monochromatic ocean wave in the presence of the real aperture radar modulation

Abstract

The model which allows one to simulate the Synthetic Aperture Radar (SAR) and the along track Interferometric SAR (INSAR) imagery of a monochromatic ocean wave is presented. The model accounts for the effects of the real aperture radar (RAR) modulation of the radar backscatter cross-section. The comparison of the SAR and the INSAR imagery of the wave system under consideration is carried out and the performance of the two imaging systems is studied under various operational conditions. The results of the study indicate that the interferometric SAR is less sensitive to the RAR modulation as compared lo the regular SAR. This fact further augments the assertion that INSAR has considerable advantages over SAR in its potential of providing quantitative information about the ocean wave lengths, directions and amplitudes     

Spectral signal to clutter and thermal noise properties of ocean wave imaging synthetic aperture radars

Abstract

The high wavenumber detection cut-off is determined above which the spectrum of ocean waves imaged by a synthetic aperture radar (SAR) is lost in the background noise spectrum consisting of the clutter noise associated with the Rayleigh statistics of the backscattering surface and the thermal noise originating in the SAR system itself. For given power, the maximum detection cut-off wavenumber is attained if the SAR resolution is chosen such that the clutter and noise spectra are equal at the cut-off wavenumber. Assuming a constant modulation transfer function relating the image modulation and wave slope spectra, the cut-off wavenumber is in this case proportional to (ρ_aρ_g)^?1/2, where ρ_a and ρ_g represent the full bandwidth (single look) azimuthal and ground range resolutions, respectively. The same proportionality holds (but with a cut-off wavenumber increased by a factor √2) for a clutter limited cut-off, the normal operating condition of an SAR. To first order, incoherent multilook averaging has no influence on the signal-to-background detection cut-off wavenumber, provided the reduced Nyquist cut-off wavenumber resulting from the reduced multilook spatial resolution remains greater than the signal-to-background cut-off wavenumber. Estimates of the detection cut-off wave-numbers are given for the Seasat SAR and the SAR proposed for the European Remote Sensing Satellite ERS-1.     

基于交叉极化比的SAR图像矿物油膜与生物油膜的区分方法

海洋表面矿物油膜、生物油膜等在SAR图像上都呈现为暗色特征,使得单极化SAR图像对矿物油膜和生物油膜的区分存在困难。分析了矿物油膜和生物油膜后向散射系数的极化比,提出一种基于交叉极化比的多极化SAR图像矿物油膜和生物油膜的区分方法,并用SIR\|C多极化数据验证了该方法的有效性。     

Improving SAR data processing with polarimetric reference functions in the range Doppler algorithm

Synthetic aperture radar (SAR) is a form of radar that can be used to create images of objects and landscapes. The main important application of the polarimetric SAR can be found in surface and target decomposition process of its image processing. In this article, we propose a method of polarimetric SAR data processing using two new polarimetric reference functions of canonical targets with the intention to apply in coherent decompositions. Our experiment uses polarimetric backscatter characteristics of the dihedral and trihedral reflectors as the targets under a ground-based SAR geometry to create the polarimetric reference functions for azimuth compression in the SAR data processing. We process the data using Pauli decomposition to investigate the effect of our functions on the RGB (red, green, and blue) properties of the processed images. The results show that Pauli decomposition using our functions produces images with different distribution and intensity of RGB colours in the image pixels with some signs of improvement over the traditional range Doppler algorithm. This demonstrates that our polarimetric reference function can be used in the decomposition steps of the traditional SAR data processing and can potentially be used to reveal some useful quantitative physical information of target points of interest and improve image and surface classification.