On the cross spectrum between individual-look synthetic apertureradar images of ocean waves

Cross spectra of individual-look synthetic aperture radar (SAR) images of the ocean surface are used to retrieve ocean wave spectra. A quasilinear transform is derived that relates ocean wave spectra to SAR image cross spectra. Furthermore, Monte Carlo simulations are also carried out for those cases where quasilinear imaging does not apply. It is shown that, as the time separation between the individual-look SAR images increases (within a limit determined by the Doppler bandwidth of the original single-look complex SAR image), the spectral energy density of the imaginary part of the SAR image cross spectra increases, while the spectral energy density of the real part decreases. The integration time has a small effect on the SAR image cross spectra as long as the integration time is large compared to the scene coherence time. In order to retrieve ocean wave spectra from SAR data by using cross-spectral analysis techniques, the authors suggest calculating two SAR image cross spectra: one with a short time separation and one with a large one between the individual-look SAR images. The real part of the SAR image cross spectra calculated from individual-look SAR images with the short time separation is used for retrieving ocean wave spectra, which have a 180° ambiguity in wave propagation direction. The imaginary part of the SAR image cross spectra calculated from individual-look SAR images with the long time separation is used for removing this 180° ambiguity     

Special Phenomena of the Shadow Region in the High Resolution Synthetic Aperture Radar Image due to Synthetic Aperture

With the development of several High Resolution (HR) Synthetic Aperture Radar (SAR) systems, many special phenomena appear in the SAR image, especially for the SAR image with millimeter wave. We firmly believed that every detail in the SAR image should have its own special mechanisms and these details may provide some key clues for us to build up the frame work on understanding the SAR image. The synthetic aperture is one of the important particularities about SAR, and the radar is moving during the data is collected, which leads many special phenomena in the SAR image; one of these is the shadow with blurred boundary. In this work, the effect on the shadow region in the SAR image by synthetic aperture is expanded on. The blurred boundary of the shadow is analyzed using imaging formation theory, and the Quadratic Phase Errors (QPE) brought by the synthetic aperture progress is deduced for the first time, which builds up the relationship between the parameters of the shadow caster and the behavior of the shadow in the SAR image. It is found that the QPE is approximately a linear function of the height of the shadow caster. Furthermore, an approach for shadow enhancement based on height variant phase compensation is proposed and it could provide a better effect on shadow enhancement than the traditional technique called Fixed Focus Shadow Enhancement (FFSE), which is proved by theoretical analysis and experiments. Based on the analysis, some typical application of the shadow in SAR image is designed and some mini-SAR image with Ku-band is analyzed about the shadow region. It is expected that the work in this paper could be some helpful for the SAR image understanding and the microwave imaging with high resolution.     

Resolution of the ocean wave propagation direction in SAR imagery

There is an inherent 180° ambiguity in the derived wave propagation direction when using conventional spectral analysis techniques on standard synthetic aperture radar (SAR) image products. Three different techniques are successfully used to resolve this ambiguity in propagation direction using a single pass of airborne SAR data. The fact that the SAR is characterized by a large time-bandwidth product is used to advantage. A sequence of individual looks extracted from the Doppler spectrum represents images of the scene collected at a sequence of discretely delayed intervals of time. The techniques utilized include cross-correlation-based motion analysis of a pair of looks, phase weighting based upon a pair of looks and the ocean wave dispersion relation, and a three-dimensional spectral analysis. The phase weighting technique is also demonstrated for a Seasat SAR scene     

Spectral estimation techniques for multilook SAR images of oceanwaves

When deriving a directional ocean wave spectrum from a synthetic-aperture radar (SAR) image of the ocean surface, spectral analysis of the image data is a key step. In this paper, a quantitative comparison of spectral analysis techniques for analysis of multilook SAR image data of ocean waves is presented. Performance is rated using a spectral SNR, defined as the ratio of the desired spectral signature to the speckle noise as it appears in the SAR image spectrum. Spectral techniques considered cover all of those now in use (including SAR processor focus adjustment) as well as new experimental methods. It is shown that the spectral-phase-shift technique, introduced in this paper, is the best approach: it maximizes the spectral SNR, it allows resolution of the 180° directional ambiguity, and it applies simultaneously to all wave components present in the scene     

A wavelet-based algorithm to estimate ocean wave group parameters from radar images

In recent years, new remote sensing techniques have been developed to measure two-dimensional (2-D) sea surface elevation fields. The availability of these data has led to the necessity to extend the classical analysis methods for one-dimensional (1-D) buoy time series to two dimensions. This paper is concerned with the derivation of group parameters from 2-D sea surface elevation fields using a wavelet-based technique. Wave grouping is known to be an important factor in ship and offshore safety, as it plays a role in dangerous resonance phenomenons and the generation of extreme waves. Synthetic aperture radar (SAR) data are used for the analysis. The wavelet technique is introduced using synthetic ocean surfaces and simulated SAR data. It is shown that the group structure of the ocean wave field can be recovered from the SAR image if the nonlinear imaging effects are moderate. The method is applied to a global dataset of European Remote Sensing satellite (ERS-2) wave mode data. Different group parameters including the area covered by the largest group and the number of groups in a given area are calculated for over 33 000 SAR images. Global maps of the parameters are presented. For comparison, classical 1-D grouping parameters are calculated from colocated wave model data showing good overall agreement with the wavelet-derived parameters. ERS-2 image mode data are used to study wave fields in coastal areas. Waves approaching the island of Sylt in the North Sea are investigated, showing the potential of the wavelet technique to analyze the spatial wave dynamics associated with the bottom topography. Observations concerning changes of wavelength and group parameters are compared to linear wave theory.     

Unconstrained inversion of waveheight spectra from SAR images

A procedure for inverting the nonlinear relationship between the waveheight spectrum and the SAR image spectrum is presented, and this procedure is evaluated using simulated data as well as actual ERS SAR data collected near Duck, NC. Results of this nonlinear inversion are compared with those obtained from a quasi-linear estimation procedure using simulated data, in order to illustrate the effects of nonlinearities in the imaging process. These effects include the well-known azimuth falloff effect as well as the generation of harmonics which appear in the background region of the spectrum. The nonlinear inversion technique is able to reproduce the input image spectrum to high accuracy, although the wave spectrum obtained by this procedure is not necessarily the same as the input wave spectrum. In general, the estimated wave spectrum is quite similar to the portion of the input wave spectrum within the SAR passband region, but none of the energy outside the passband is recovered. The background signals due to nonlinear effects can cause large errors in the quasi-linear estimation procedure because these signals appear in regions of the spectrum where the SAR modulation transfer function (mtf) is small. Results using actual SAR data also indicate that energy within the passband is recovered fairly accurately, although energy outside the passband is clearly lost     

Multilook images of ocean waves by synthetic aperture radars

A property of multilook processing of synthetic aperture radar (SAR) data is that a time lapse exists between subapertures, so that they contain information about a scattering surface at different times. Reported here is a theoretical study on the images of dynamic ocean waves processed by this technique. It is shown that due to the time lapse the subimages of a moving ocean wave differ in position depending on the look number and the wave phase velocity. Such images cannot be enhanced by the incoherent addition so much as those of stationary surfaces. The difference in image position can be corrected by defocusing the azimuth reference signal by the same amount as for the correction of defocusing induced by the wave motion. Discussions are presented on the correction of image positions and on the effect of defocusing. The property of the time lapse could be applied to estimating not only the phase velocity of ocean waves but also temporal changes in general scattering surfaces.     

Synthetic aperture radar imaging of moving ocean waves

A theory for the radar imaging of ocean waves is presented under the assumptions that a swell propagates through an ensemble of Bragg scatterers and that the integration time of the synthetic aperture radar (SAR) is small compared to the angular velocity of the swell. Results are prsented which show image development and distortions caused by the radial velocities and accelerations of the swell. Neglecting small wave bunching and tilts due to the longer underlying waves, and considering only one-dimensional geometries, the mechanism of wave motions are considered and their efforts on the production of the usual intensity Pattern representing the wave image are studied. The analysis shows that in certain situations a processed image can appear which has twice the spatial period of the actual long wave on the ocean, which can confuse the interpretation of ocean wave analysis.     

Imaging ocean waves by synthetic aperture radars with long integration times

Previously developed theories for the imaging mechanism of ocean waves by synthetic aperture radars (SAR's) apply only for short integration timesT(typically,T < 1s). To what extent long integration times modify these theories is investigated. First, an analytical expression describing the effect of systematic wave motions on radar imagery valid for an arbitrary integration time is derived. Numerical evaluation of the expression shows only small deviations from the previous expression valid for short integration times. Thus the restriction of the previous theory, (hat{omega}T/2) ll 1, wherehat{omega}is the radian frequency of the long waves, can be relaxed to(hat{omega}T/2) lsim 1. Second, the influence of the coherence time of the scene, randomness of wave field, on the imaging mechanism is investigated. It is argued that the scene coherence time is usually larger than the SAR coherent integration timeT, implying that the azimuthal "image smear" for the case of ocean wave imaging is usually due to systematic wave motion rather than the scene coherence time.     

Ocean wave extraction from RADARSAT synthetic aperture radarinter-look image cross-spectra

This study is concerned with the extraction of directional ocean wave spectra from synthetic aperture radar (SAR) image spectra. The statistical estimation problem underlying the wave-SAR inverse problem is examined in detail in order to properly quantify the wave information content of SAR. As a concrete focus, a data set is considered comprising six RADARSAT SAR images co-located with a directional wave buoy off the east coast of Canada. These SAR data are transformed into inter-look image cross-spectra based on two looks at the same ocean scene separated by 0.4 s. The general problem of wave extraction from SAR is cast in terms of a statistical estimation problem that includes the observed SAR spectra, the wave-SAR transform, and prior spectral wave information. The central role of the weighting functions (inverse of the error covariances) is demonstrated, as well as the consequence of approximate (based on the quasilinear wave-SAR transform) versus exact linearizations on the convergence properties of the algorithm. Error estimates are derived and discussed. This statistical framework is applied to the extraction of spectral wave information from observed RADARSAT SAR image cross-spectra. A modified wave-SAR transform is used to account for case-specific geophysical and imaging effects. Analysis of the residual error of simulated and observed SAR spectra motivates a canonical form for the SAR observation error covariance. Wave estimates are then extracted from the SAR spectra, including wavenumber dependent error estimates and explicit identification of spectral null spaces where the SAR contains no wave information. Band-limited SAR wave information is also combined with prior (buoy) spectral wave estimates through parameterization of the wave spectral shape and use of regularization     

The effect of orbital motions on synthetic aperture radar imagery of ocean waves

The formation of wave-like patterns in synthetic aperture radar (SAR) images of the ocean surface caused by orbital motions is investigated. Furthermore, the degradation in azimuthal resolution due to these motions is calculated by applying a least square fit to the phase history. Formulas are given which describe the variation of intensity in azimuthal direction in the image plane as well as the degradation in azimuthal resolution as a function of ocean wave amplitude, wave frequency, direction of wave propagation, and radar wavelength, incidence angle, and integration time.     

Tutorial review of synthetic-aperture radar (SAR) with applications to imaging of the ocean surface

A synthetic aperture radar (SAR) can produce high-resolution two-dimensional images of mapped areas. The SAR comprises a pulsed transmitter, an antenna, and a phase-coherent receiver. The SAR is borne by a constant velocity vehicle such as an aircraft or satellite, with the antenna beam axis oriented obliquely to the velocity vector. The image plane is defined by the velocity vector and antenna beam axis. The image orthogonal coordinates are range and cross range (azimuth). The amplitude and phase of the received signals are collected for the duration of an integration time after which the signal is processed. High range resolution is achieved by the use of wide bandwidth transmitted pulses. High azimuth resolution is achieved by focusing, with a signal processing technique, an extremely long antenna that is synthesized from the coherent phase history. The pulse repetition frequency of the SAR is constrained within bounds established by the geometry and signal ambiguity limits. SAR operation requires relative motion between radar and target. Nominal velocity values are assumed for signal processing and measurable deviations are used for error compensation. Residual uncertainties and high-order derivatives of the velocity which are difficult to compensate may cause image smearing, defocusing, and increased image sidelobes. The SAR transforms the ocean surface into numerous small cells, each with dimensions of range and azimuth resolution. An image of a cell can be produced provided the radar cross section of the cell is sufficiently large and the cell phase history is deterministic. Ocean waves evidently move sufficiently uniformly to produce SAR images which correlate well with optical photographs and visual observations. The relationship between SAR images and oceanic physical features is not completely understood, and more analyses and investigations are desired.     

On the focusing issue of synthetic aperture radar imaging of oceanwaves

It is widely accepted that the imaging of ocean surface waves by synthetic aperture radar (SAR) can be adequately described by velocity bunching theory in conjunction with the two-scale wave model. However, it has been conjectured that this theory is incapable of explaining why, under certain conditions, the image contrast of airborne SAR imagery of ocean waves can be enhanced by defocusing the SAR processor. In the present study the velocity bunching theory is defended     

一种星载SAR图像的系统级几何校正技术

在星历参数和星载SAR空间几何模型的基础上，本文实现了星载SAR图像的系统级几何校正，给出了一种基于空间几何模型的星载SAR图像像素定位技术，说明了经纬网中任意点所在距离线被雷达波束中心照射的时刻的估算方法。最后利用仿真数据验证了这种几何校正方法的有效性。     

MIMO雷达近场成像校准与互耦合补偿方法

毫米波频段的合成孔径雷达（Synthetic Aperture Radar, SAR）因拥有良好的方位向分辨率而受到广泛关注,同时在发射端和接收端采用多根天线多输入多输出（Multiple?Input Multiple?Output, MIMO）的方式可以极大提高信道容量。然而MIMO?SAR图像重建计算较为复杂,且多个通道间幅相不一致和相互耦合容易导致图像出现伪影,严重影响成图质量。基于此,本文对引入非均匀快速傅里叶变换（Nonuniform Fast Fourier Transform, NUFFT）简化的距离迁移算法（Range Migration Algorithm, RMA）成像算法进行了研究;在分析扫描架运动抖动对相位影响的基础上,探究了通过照射金属反射面的MIMO阵列校准方法,实现了192个通道的同时校准;并搭建了毫米波SAR系统实验平台,对伪影消除、成像分辨率等开展了验证实验。实验结果实现了图像重构,成像分辨率达到了2 mm,完成200 mm×200 mm孔径扫描时间缩短至120 s。     

与图像对比度准则相结合的PGA自聚焦算法

根据图像对比度准则对标准的PGA算法。作了改进，分析和实测数据试验表明：改进后的PGA算法在计算量得以下降的同时，仍保持了PGA算法对高分辨率合成孔径雷达图像中未知相位误差的补偿能力，并且对场景的适用性优于标准PGA算法。     

Hasselmann方法从SAR图像反演海浪方向谱及其印证研究

介绍了从星载SAR海浪图像反演海浪方向谱的Hasselmann方法.在第一猜测谱的选取上采用了文氏谱的最新结果.设计了一个针对该方法的印证实验.反演所得的有效波高、有效波周期与浮标实测结果基本一致.     

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     

Sea surface imaging with an across-track interferometric syntheticaperture radar: the SINEWAVE experiment

An across track interferometric synthetic aperture radar (InSAR) is used to image ocean waves. Across track InSAR data were acquired during the SAR INnterferometry Experiment for validation of ocean Wave imaging models (SINEWAVE) in the North Sea using an airborne X-band radar with horizontal polarization. A wind sea system was imaged at different flight levels and with different flight directions with respect to the ocean wave propagation direction. Simultaneously, ocean wave spectra were measured by a directional wave rider buoy. Thus, the experiment data comprises synthetic aperture radar (SAR) intensity, coherence, and phase images together with in situ measurements. As shown in a recent theoretical study by Schulz-Stellenfleth and Lehner (2001), across track InSAR provides distorted (bunched) digital elevation models (DEMs) of the sea surface. Using SINEWAVE data the DEM bunching mechanism is verified with in situ ocean wave measurements available for the first time. It is shown that significant waveheight as well as one-dimensional (1D) wavenumber spectra derived from bunched DEMs and buoy data are in good agreement for small nonlinearities. Peak wave directions and peak wavelength detected in bunched DEMs and SAR intensity images are compared with the buoy spectrum. Peak rotations of up to 30° with respect to the buoy spectrum are found depending on flight direction and flight level. Two-dimensional (2D) spectra of bunched DEMs, corresponding coherency maps, and SAR intensity images are intercompared. The signal-to-noise ratio (SNR) of bunched DEM spectra is shown to be about 5 to 10 dB higher than the SNR of SAR intensity image spectra     

扁率摄动对地球同步轨道SAR成像聚焦的影响分析

针对地球同步轨道星载合成孔径雷达(Geosynchronous Synthetic Aperture Radar, GEOSAR)长合成孔径成像受地球扁率摄动影响的问题,推导了卫星不同轨道根数受摄动所导致的雷达回波多普勒调频率和2次相位公式,通过分析扁率摄动对成像的影响,得到结论:地球扁率摄动使雷达回波产生附加的2次相位调制,相位调制的主导成分是卫星轨道长半轴受摄分量,相位调制幅度与成像所采用的轨道弧段有关,2次相位调制总量经过分钟量级的长合成孔径累积几乎在卫星运动全周期超过45的容限。雷达成像聚焦不能简单忽略扁率摄动的影响,必须采取相应的补偿措施,否则会造成图像散焦。