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。     

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。     

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.     

基于SWIM、SCAT联合观测小卫星的总体设计要点

海洋对全球气候、气象、环境和经济发挥着重要作用,因此,有必要对其进行监测、研究及预测发生在海洋内部和海洋表面的运动过程。海面风场信息、海面波浪谱信息能够体现并反演海洋气象、海洋动力学特性、海洋资源、海洋污染情况、海洋经济性和海岸环境。波谱仪(SWIM,Surface Wind Investigation and Monitoring)和微波散射计(SCAT,Scatterometer )可以用于获取海面波浪谱信息和海面风场信息。首先对这两套载荷系统进行了介绍,并简要对基于这类主动微波遥感载荷配置的卫星设计进行了任务分析,最后给出了基于波谱仪、微波散射计联合使用在小卫星平台应用背景下卫星系统级设计的几个主要方面进行了研究和分析。     

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。     

基于谱的三维随机海浪数值模拟研究

为有效地模拟三维波浪,从已有的统计观测结果出发,应用P-M谱和SWOP方向函数的基础上,建立了风浪的三维随机波面数学模型,该模型采用多个随机余弦波叠加来模拟随机波浪曲面。最后利用Matlab实现了该算法,结果表明这种方法能有效地模拟三维随机海浪。     

SAR图像船只尾迹检测研究综述

利用星载合成孔径雷达(SAR)图像进行海面船只尾迹检测的研究在海洋领域具有重要意义。SAR海洋图像中尾迹线性特征的检测和精确定位是个难点,研究了SAR图像中船只尾迹的检测方法,根据近年来SAR图像海面船只尾迹检测的发展,总结了尾迹类型及其成像机理,分析了影响尾迹检测的因素和目前国内外的尾迹检测算法,通过对这些方法的对比分析得出它们的优缺点,并对今后的研究方向进行了展望。     

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。     

基于非线性海面微波段多普勒回波的建模算法

基于线性波浪理论和尖波模型,分别仿真得到时变线性和非线性海面,并比较了不同海况下两种海面模型的统计特征。采用小斜率近似法求解微波段的电磁散射,并分析了不同条件下的回波多普勒谱。通过采用海浪反演算法验证仿真数据,证明了该建模算法的正确性。结果表明,非线性海浪表现出斯托克斯波浪的典型特性,海面的非线性效应在掠入射时对多普勒谱的影响更显著,并且进行了非线性修正的海面模型的电磁散射特性与实际海况的结果更加吻合。     

波浪数值预报可视化系统的设计与实现

针对全球及中国近海海域,采用第三代海浪数值模式WAVEWATCHIII建立波浪数值预报模型,由此计算得到研究区域的波要素。基于预报模型计算结果建立波浪数值预报可视化系统,采用MSchart控件实现了全球及中国近海海面风场输入口功能和单点波要素静态图显示功能,同时依据提供的全球及中国近海海面风场预报7天全球及中国近海海浪场,采用GRADS软件实现全球及中国近海预报波浪场动态可视化的功能。     

SAR detection and characterization of sea surface slicks

In this paper we demonstrate SAR system capability for detecting and characterizing marine surface slicks. During an aircraft measurement campaign over the Gulf of Genoa (Italy), a multi-frequency SAR system, operating in P-, L- and C-bands, explored a sea area heavily covered by slicks. At the same time in situ measurements were performed with an interferential microwave probe, installed on board a small boat, capable of measuring high resolution sea spectra up to frequencies of capillary waves. By plotting SAR pixel intensity versus sea wave Bragg frequency we obtained wide portions of the sea spectrum region affected by the surface film damping. Spectra derived from SAR imagery and from gauge data present comparable slopes and furthermore the ratio between clean to slicked water spectrum obtained with the two techniques were surprisingly similar. This demonstrates the multi-frequency SAR systems ability to detect and characterize sea surface films assuming the Bragg mechanism in the radar backscatter. The outlined analysis suggests a simple methodology to monitor coastal water quality by using airborne SAR.     

On the interpretation of SAR imagery from the Sea Empress oil spill

A method for monitoring oil spills using SAR imagery is suggested, based on the simulation of the wave spectrum using modelled surface winds. A first order separation of the purely wind-driven backscatter distribution and its modification due to surfactant was made by parametrizing the effect of surfactant on the wave growth rate and on the reflective properties of the sea surface. The technique was applied to a SAR image showing the Sea Empress oil spill, in south-west Wales, UK.     

Estimated surface-wave contributions to radar Doppler velocity measurements of the ocean surface

For simulated ocean conditions, we estimate the magnitude of the Doppler velocity contributions produced by unresolved surface waves that typical spaceborne synthetic aperture radars (SAR) would measure. The mechanism for generating Doppler velocities is the correlation between wave phase and radar cross section. The contributions analyzed include those of linear gravity waves, second-order wave-wave interactions, Bragg-wave scatterers and breaking waves. For gravity waves, we consider both wave tilt and hydrodynamic modulation transfer functions (MTFs). We find that for nominal sea conditions, the Doppler velocity is significant, on the order of 1 m/s, and exhibits large variation as a function of incidence angle and look with respect to the sea direction. The most important contributors are gravity waves and the Bragg scatterers, followed by sea spikes. Effects produced by second-order wave solutions are argued to be inconsequential.     

An assessment of a FMICW ground-wave radar system for ocean wave studies

In February 1982 the first successful trial of a frequency modulated, interrupted, continuous wave (FMICW), monostatic ground-wave radar for sea backscatter studies was carried out. Since then data have been collected in a number of different meteorological conditions over a range of HF frequencies. The data are presented and compared with simulated Doppler spectra obtained by evaluating the integral equation, which relates the sea backscatter spectrum to the ocean wave directional spectrum (Barrick 1972). The dynamic range performance of the system is as good as or better than has been achieved by pulse radars currently in operation. Good agreement with the theoretical formulation is demonstrated     

Radar imaging of Kelvin arms of ship wakes

The wave pattern generated by a moving ship is formed by two dominant features: the turbulent wake and a 'V'-shaped pattern trailing the ship, consisting of the two Kelvin arms. In this paper we investigate the radar imaging mechanism of Kelvin arms, which are formed by the cusp waves. A composite surface model for the radar backscattering at the ocean surface is used. The radar signatures of Kelvin arms can be attributed to tilt and hydrodynamic modulation of Bragg waves by the cusp waves. The proposed model allows the computation of the normalized radar backscattering cross-section (NRCS) as a function of radar frequency, polarization, incidence angle, wind speed and direction, and wavelength, direction, and slope of the cusp waves. By using this imaging model, radar signatures of cusp waves are calculated for several spaceborne Synthetic Aperture Radars (SARs): (1) the SEASAT L-band HH-polarized SAR, (2) the ERS-1/-2 VV-polarized SAR, (3) the RADARSAT C-band HH-polarized SAR, and (4) the X-, C- and L-band multipolarization SARs of the Space Radar Laboratory flown on the space shuttle during the SIRC/X-SAR mission in 1994. The results of the simulations are compared with SEASAT and SIR-C/X-SAR imagery of ship wake patterns. It is shown that the dependence of the observed radar signatures of Kelvin arms on radar look direction is consistent with the proposed imaging theory and that the measured relative mean NRCS values induced by Kelvin arms can be fairly well reproduced by the proposed model. The simulations indicate that ship wake signatures should be more clearly visible on SEASAT L-band SAR than on ERS-1/-2 or RADARSAT C-band SAR images. The radar signatures of Kelvin arms are strongest at low wind speeds and are not very sensitive to wind direction.     

High-resolution satellite measurements of coastal wind field and sea state

Methods to derive wind speed and sea state by simple empirical models from synthetic aperture radar (SAR) data are presented and applied for use in high-resolution numerical modelling for coastal applications. The new radar satellite, TerraSAR-X (TS-X), images the surface of the sea with a high resolution up to 1 m. Therefore, not only wind information and integrated sea state parameters but also individual ocean waves with wavelengths down to 30 m are detectable. Two-dimensional information on the ocean surface retrieved using TS-X data is validated for different oceanographic applications: derivation of finely resolved wind fields (XMOD algorithm) and integrated sea state parameters (XWAVE algorithm). Both algorithms are capable of taking into account fine-scale effects in coastal areas. Wind and sea state information retrieved from SAR data are applied as the input for a wave numerical spectral model (wind forcing and boundary condition) running at a fine spatial horizontal resolution of 100 m. Results are compared to collocated buoy measurements. Studies are carried out for varying wind speeds and comparisons against wave height, simulated using original TS-X-derived wind data, showing the sensitivity of waves to local wind variation and thus the importance of local wind effects on wave behaviour in coastal areas. Examples for the German Bight (North Sea) are shown. The TS-X satellite scenes render well-developed ocean wave patterns of developed swell at the sea surface. Refraction of individual long swell waves at a water depth shallower than about 70 m, caused by the influence of underwater topography in coastal areas, is imaged on the radar scenes. A technique is developed for tracking wave rays depending on changes in swell wavelength and direction. We estimate the wave energy flux along wave tracks from deep water to the coastline based on SAR information: wave height and wavelength are derived from TS-X data.     

利用卫星SAR监测海上航行船舶

星载合成孔径雷达能够得到高分辨率的遥感图像。船舶目标以及船舶航迹在一些卫星ＳＡＲ海洋图像中清晰可见。文中就利用卫星ＳＡＲ对船舶目标和船舶航迹的监测问题进行了论述。首先简要介绍了船舶目标及其航迹的ＳＡＲ成像原理,然后对不同成像条件得到的航迹图像进行了分类,最后分析了船舶目标的检测、船舶航迹特征的检测以及船舶的相应参数的估计问题。通过对上述问题的讨论,认为利用卫星ＳＡＲ监测海上航行的船舶是可行的,而且是一项很有意义的工作。     

Review Article. The imaging of waves by satelliteborne synthetic aperture radar: the effects of sea-surface motion

The effects of sea-surface velocities in the imaging of waves by synthetic aperture radar (SAR) are considered using the ‘facet’ concept of the backscattering process. It is shown that if the sea wave spectrum is divided at the nominal limit of resolution of the SAR the effect of the long and short wavelength parts can be considered separately, the former being treated by numerical simulation and the latter by statistical methods. It is found that the motions due to: the short wavelengths produce an azimuthal smearing which can be represented by a Gaussian low-pass filter acting on the azimuthal component of wavenumber in the image. The cut-off wavelength is typically some hundreds of metres in moderate winds. Images obtained with the SEASAT SAR frequently show such an effect.