A sea surface height estimator using synthetic aperture radar complex imagery

The reflectivity density approximately describing electromagnetic scattering from a "two-scale" rough surface has a phase linearly dependent on surface height. The synthetic aperture radar (SAR) image is a complicated partially understood transformation of this reflectivity density, but in demonstrable situations the complex image's phase also contains sea height information: reported here is an initial study of an algorithm that exploits this. Envelope and phase demodulation, regression, and filtering algorithms are verified and applied to simulated and actual satellite radar SEASAT-SAR data. The simulation of a simplified stationary scene established tentative sufficient conditions on large-scale SAR and sampling parameters for accurate estimation of the large-scale structure's height and imply feasible system design. The algorithm accurately estimated a long wavelength low-amplitude large-scale sea height structure present in a SEASAT-SAR data record, consistent with available sea truth.     

Extension of synthetic aperture radar (SAR) technique to undersea applications

This paper presents results on the extension of synthetic-aperture radar (SAR) technique to underwater mapping applications. Due to the modest propagation speed of acoustic waves in water, the areal-mapping rate is found to be limited. A number of approaches are discussed in the paper which have the potential of significantly increasing the mapping rate. The map resolutions, both in azimuth and range, are derived for a synthetic array system employing multiple elements. The digital processing requirements, such as the complex multiplication rate and memory size are determined. It was found that the digital signal-processing requirements for many viable mapping systems are within the capability of existing processing hardware.     

一种增强型YOLOv3的合成孔径雷达(SAR)舰船检测方法

近年来,海战场成为现代战争的主要作战区域之一,舰船目标逐渐成为海上重点监测对象,能否快速准确地识别海战场舰船目标的战术意图,给指挥员的决策提供必要的支持,这关系到一场海上战役的成败.随着合成孔径雷达(synthetic aperture radar,SAR)成像技术的不断发展,大量SAR图像可用于舰船目标检测与识别.利...     

Signature extraction from mesoscale surface patterns observed onsynthetic aperture radar imagery

Two techniques for extracting internal wave signature from airborne synthetic aperture radar (SAR) imagery are presented. The first technique involves the extraction of a single-scan signature and appropriate for the validation of theoretical models which utilize sea truth from a limited area as input. The second technique involves creating a signature by averaging the results from a number of single scans within the same scene and is appropriate for trend analyses. The utility of both techniques is illustrated using internal wave signatures present in a multifrequency, airborne SAR data set     

Azimuthal ambiguities in synthetic aperture sonar and syntheticaperture radar imagery

It is shown that azimuthal ambiguities are not eliminated by the nulls of a sonar (or radar) beam pattern and have a definite influence on image quality. In synthetic aperture systems that are strongly limited in spatial sampling, particularly in ocean borne synthetic aperture sonar (SAS) and spaceborne synthetic aperture radar (SAR), azimuthal ambiguities will corrupt the images unless special measures are taken. These azimuthal aliases may be reduced by emphasizing the centermost portion of the available synthetic aperture length, and deemphasizing the endmost portions. This minimizes the effects from synthetic array elements that most strongly contribute to aliases     

Comments on "A sea surface height estimator using synthetic aperture radar complex imagery"

It is shown that sea-surface height may not be estimated directly from the phase of complex synthetic aperture radar (SAR) imagery.     

基于同步风场的机载SAR海浪参数反演方法

针对机载合成孔径雷达(SAR)对海探测特点,采用多入射角法从SAR数据本身得到与海浪参数反演区域时空匹配的同步海面风速和风向,并结合线性变换关系,计算得到海浪初猜谱对应的仿真SAR图像谱,将仿真SAR图像谱和观测SAR图像谱输入代价函数中进行迭代运算,通过非线性方程的解算得到最适海浪谱;采用交叉谱法去除海浪传播180°方向模糊,最终得到海浪参数。论文提出的基于同步风场的机载SAR海浪参数反演方法,充分利用了机载SAR海洋环境探测的优势,解决了传统SAR海浪参数反演中初猜谱构造依赖外部风场的问题,机载同步飞行试验的海浪参数反演结果与浮标观测值的有效波高、波向的均方根误差分别为0.23 m和13.23°,验证了该方法的有效性,可为机载SAR海浪参数反演业务化提供支持。     

On the synthetic aperture radar imaging of ocean surface waves

A process of synthetic aperture radar imaging of ocean surface waves is considered on the basis of the two-scale model of microwave scattering by a disturbed sea surface. Analytical expressions are obtained to relate characteristics of a large-scale wave image, averaged over an ensemble of realizations of the small-scale ripple, with the wave, radar, and viewing scheme as parameters. It is shown that the wave image would be defocused as an image of a target moving in the along-track direction with a speed equal to a half of the wave phase speed projection on the line of flight. The defocusing magnitude was measured experimentally for the ocean swells images, obtained with an airborneS-band radar, and the results are found to be in satisfactory agreement with the model prediction.     

Effects of random phase changes on the formation of synthetic aperture radar imagery

The effects of Gaussian random and linear phase change on the response of the matched azimuth processor of a synthetic aperture imaging radar is analyzed numerically.     

复合雷达后向散射模型与合成孔径雷达、散射计和高度计海面雷达后向散射观测的比较分析

海洋微波散射模型相比于以经验统计建立的地球物理模式函数具有不受特定微波频率限制的优势。组合布拉格散射模型和几何光学模型形成了复合雷达后向散射模型。利用南海北部气象浮标2014年海面风速风向实测值作为散射模型输入,分别比较了复合雷达后向散射模型与RADARSAT-2卫星C波段SAR、HY-2A卫星Ku波段微波散射计的海面后向散射系数,偏差分别为(?0.22±1.88) dB (SAR)、(0.33±2.71) dB (散射计VV极化）和(?1.35±2.88) dB (散射计HH极化);以美国浮标数据中心(NDBC)浮标2011年10月1日至2014年9月30日共3年的海面风速、风向实测值作为散射模型输入,分别比较了复合雷达后向散射模型与Jason-2、HY-2A卫星Ku波段高度计海面后向散射系数,偏差分别为(1.01±1.15) dB和(1.12±1.29) dB。中等入射角和垂直入射下的卫星传感器后向散射系数观测值与复合雷达后向散射模型模拟值比较,具有不同的偏差,但具有相同的海面风速检验精度,均方根误差小于1.71 m/s。结果表明,复合雷达后向散射模型可模拟计算星载SAR、散射计和高度计观测条件下的海面雷达后向散射系数,且与CMOD5、NSCAT-2、高度计业务化海面风速反演的地球物理模式函数的计算结果具有一致性;复合雷达后向散射模型可用于微波遥感器的定标与检验、海面雷达后向散射的模拟。     

全极化合成孔径雷达近岸风场反演研究

Coastal winds are strongly influenced by topology and discontinuity between land and sea surfaces. Wind assessment from remote sensing in such a complex area remains a challenge. Space-borne scatterometer does not provide any information about the coastal wind field, as the coarse spatial resolution hampers the radar backscattering. Synthetic aperture radar （SAR） with a high spatial resolution and all-weather observation abilities has become one of the most important tools for ocean wind retrieval, especially in the coastal area. Conventional methods of wind field retrieval from SAR, however, require wind direction as initial information, such as the wind direction from numerical weather prediction models （NWP）, which may not match the time of SAR image acquiring. Fortunately, the polarimetric observations of SAR enable independent wind retrieval from SAR images alone. In order to accurately measure coastal wind fields, this paper proposes a new method of using co-polarization backscattering coefficients from polarimetric SAR observations up to polarimetric correlation backscattering coefficients, which are acquired from the conjugate product of co-polarization backscatter and cross-polarization backscatter. Co-polarization backscattering coefficients and polarimetric correlation backscattering coefficients are obtained form Radarsat-2 single-look complex （SLC） data.The maximum likelihood estimation is used to gain the initial results followed by the coarse spatial filtering and fine spatial filtering. Wind direction accuracy of the final inversion results is 10.67 with a wind speed accuracy of 0.32 m/s. Unlike previous methods, the methods described in this article utilize the SAR data itself to obtain the wind vectors and do not need external wind directional information. High spatial resolution and high accuracy are the most important features of the method described herein since the use of full polarimetric observations contains more information about the space measured.This article is a useful addition to the work of independent SAR wind retrieval. The experimental results herein show that it is feasible to employ the co-polarimetric backscattering coefficients and the polarimetric correlation backscattering coefficients for coastal wind field retrieval.     

The Seasat-A synthetic aperture radar system

The objective of the Seasat-A synthetic aperture radar (SAR) system was to detect ocean waves from orbital altitudes. An SAR system operating at 1275 MHz (23-cm wavelength) was designed and built to meet this objective. The choice of wavelength was the result of imagery obtained by aircraft during 1972 to 1974 and ease of implementation. The Seasat. A SAR system was turned on in orbit on July 4, 1978, and gathered imagery until the spacecraft bus failed in orbit in October 1978.     

A novel synthetic aperture radar scattering model for sea surface with breaking waves

In this study a novel synthetic aperture radar(SAR) scattering model for sea surface with breaking waves is proposed. Compared with existing models, the proposed model considers an empirical relationship between wind speed and wave breaking scattering to present the contribution of wave breaking. Moreover, the scattering weight factor p, and wave breaking rate q, are performed to present the contribution of the quasi-specular scattering term, Bragg scattering term, and wave breaking scattering t...     

基于合成孔径雷达的长江口海表流场反演

Range Doppler velocities derived from the Envisat advanced synthetic aperture radar（ASAR） wide swath images are analyzed and assessed against the numerically simulated surface current fields derived from the finite volume coastal ocean model（FVCOM） for the Changjiang Estuary. Comparisons with the FVCOM simulations show that the European Space Agency（ESA） Envisat ASAR based Doppler shift anomaly retrievals have the capability to capture quantitative information of the surface currents in the Changjiang Estuary. The uncertainty analysis of the ASAR range Doppler velocity estimates are discussed with regard to the azimuthal and range bias corrections, radar incidence angles, inaccuracy in the wind field corrections and the presence of rain cells.The corrected range Doppler velocities for the Changjiang Estuary area are highly valuable as they exhibit quantitative expressions related to the multiscale upper layer dynamics and surface current variability around the East China Sea, including the Changjiang Estuary.     

Phase and doppler errors in a spaceborne synthetic aperture radar imaging the ocean surface

Data processing in a spaceborne synthetic aperture radar (SAR) imaging the ocean surface is affected by earth rotation, orbit eccentricity, and wave motion. Without compensation these sources will cause the images to shift in range and in-track positions and also cause defocusing. Ionospheric granularities may degrade image quality. Calculations of the magnitudes of these effects are presented.     

Nonlinear ocean internal waves observed by multifrequency synthetic aperture radar

A numerical model which consists of the Korteweg-de Vries (KdV) equation, the action balance equation and the radar backscat- tering model is developed to simulate the frequency dependence of synthetic aperture radar (SAR) remote sensing of nonlinear o- cean internal waves. Muhifrequency data collected by NASA SIR - C SAR and NASA JPL AIRSAR are used as comparison. Case studies show that the results of simulation agree well with the results of SAR data.     

一种快速非局部平均合成孔径雷达海洋图像斑点噪声抑制方法

合成孔径雷达在海洋环境监测和海洋研究中扮演着越来越重要的角色。受其成像机制的影响,合成孔径雷达图像总是受到斑点噪声的污染。斑点噪声的存在会增大目标识别、跟踪和分类的难度,也会降低雷达信号的信噪比。合成孔径雷达海洋图像具有一些特殊的性质：海洋现象在雷达图像中主要呈现为条带状或斑块状的结构。这些条带状或斑块状的结构呈现出高度的自相似性或信息冗余。非局部平均方法能够衡量图像中不同图像块之间纹理结构的相似性,并利用图像的自相似性对图像进行去噪。但非局部平均去燥方法存在计算量巨大、计算耗时长的缺点,这几乎限制了其实际应用。本文采用一种自适应方法将雷达图像中的像素点区分为纹理区像素点和平坦区像素点。对纹理区像素点,采用较大的相似窗和搜索窗,对平坦区像素点,采用较小的相似窗和搜索窗,从而提高计算速度。进一步,本文基于计算统一设备并行架构（CUDA）技术,利用计算机图形处理器（GPU）对前述算法进行并行加速。与经典非局部平均算法相比,加速后算法的计算效率提高了200倍。     

Digital processing considerations for extraction of ocean wave image spectra from raw synthetic aperture radar data

The digital processing requirements of several algorithms for extracting the spectrum of a detected synthetic aperture radar (SAR) image from the raw SAR data are described and compared. The most efficient algorithms for image spectrum extraction from raw SAR data appear to be those containing an intermediate image formation step. It is shown that a recently developed compact formulation of the image spectrum in terms of the raw data is computationally inefficient when evaluated directly, in comparison with the classical method where matched-filter image formation is an intermediate result. It is also shown that a proposed indirect procedure for digitally implementing the same compact formulation is somewhat more efficient than the classical matched-filtering approach. However, this indirect procedure includes the image formation process as part of the total algorithm. Indeed, the computational savings afforded by the indirect implementation are identical to those obtained in SAR image formation processing when the matched-filtering algorithm is replaced by the well-known "dechirp-Fourier transform" technique. Furthermore, corrections to account for slant-to-ground range conversion, spherical earth, etc., are often best implemented in the image domain, making intermediate image formation a valuable processing feature.     

台风中降雨对中国高分3号合成孔径雷达观测信号的影响

Gaofen-3(GF-3), a Chinese civil synthetic aperture radar(SAR) at C-band, has operated since August 2016.Remarkably, several typhoons have been captured by GF-3 around the China Seas over its last two-year mission.In this study, six images acquired in Global Observation(GLO) and Wide ScanSAR(WSC) modes at verticalvertical(VV) polarization channel are discussed. This work focuses on investigating the observation of rainfall using GF-3 SAR. These images were collocated with winds from the European Centre for Medium-Range Weather Forecasts(ECMWF), significant wave height simulated from the WAVEWATCH-III(WW3) model, sea surface currents from climate forecast system version 2(CFSv2) of the National Centers for Environmental Prediction(NCEP) and rain rate data from the Tropical Rainfall Measuring Mission(TRMM) satellite. Sea surface roughness,was compared with the normalized radar cross section(NRCS) from SAR observations, and indicated a 0.8 correlation(COR). We analyzed the dependences of the difference between model-simulated NRCS and SARmeasured NRCS on the TRMM rain rate and WW3-simulated significant wave height. It was found that the effects of rain on SAR damps the radar signal at incidence angles ranging from 15° to 30°, while it enhances the radar signal at incidence angles ranging from 30° to 45° and incidence angles smaller than 10°. This behavior is consistent with previous studies and an algorithm for rain rate retrieval is anticipated for GF-3 SAR.     

Semisupervised heterogeneous ensemble for ship target discrimination in synthetic aperture radar images

Ship detection using synthetic aperture radar (SAR) plays an important role in marine applications. The existing methods are capable of quickly obtaining many candidate targets, but numerous non-ship objects may be wrongly detected in complex backgrounds. These non-ship false alarms can be excluded by training discriminators, and the desired accuracy is obtained with enough verified samples. However, the reliable verification of targets in large-scene SAR images still inevitably requires manual interpretation, which is difficult and time consuming. To address this issue, a semisupervised heterogeneous ensemble ship target discrimination method based on a tri-training scheme is proposed to take advantage of the plentiful candidate targets. Specifically, various features commonly used in SAR image target discrimination are extracted, and several acknowledged classification models and their classic variants are investigated. Multiple discriminators are constructed by dividing these features into different groups and pairing them with each model. Then, the performance of all the discriminators is tested, and better discriminators are selected for implementing the semisupervised training process. These strategies enhance the diversity and reliability of the discriminators, and their heterogeneous ensemble makes more correct judgments on candidate targets, which facilitates further positive training. Experimental results demonstrate that the proposed method outperforms traditional tri-training.