Internal solitary waves in the China seas observed using satellite remote-sensing techniques: a review and perspectives

Internal solitary waves (ISWs) occur ubiquitously in China’s waters: the South China Sea (SCS), the East China Sea (ECS), the Yellow Sea (YS), and the Bohai Sea (BS). ISWs have long attracted much research interest because of their important role in ocean acoustics, offshore engineering, ocean mixing, primary productivity, and submarine navigation. ISWs have sea surface signatures that can be detected by satellite synthetic aperture radar (SAR) and optical sensors. Satellite remote-sensing images provide excellent two-dimensional views of the ISW field. Our understanding of ISWs in the China Seas has been greatly improved using satellite remote-sensing techniques. The primary objectives of this paper are to review the development of remote-sensing techniques in the study of ISWs and to summarize ISW characteristics in the China seas, mainly demonstrated by remote-sensing techniques. In addition, several issues with remote-sensing techniques and interesting research topics are discussed.     

On the link between SAR‐derived sea ice melt and development of the summer upper ocean mixed layer in the North Open Water Polynya

We examine the ability of SAR‐derived landfast sea ice thermodynamic state surrounding the North Water Polynya to predict the timing of a shallow, stably stratified summer ocean mixed layer in the region. Radarsat‐1 ScanSAR is used to derive melt and pond onset dates over landfast sea ice surrounding the polynya, describing its thermodynamic evolution. Salinity and temperature profiles, connected in time and space, are complemented by potential density profiles and mixed layer depths, which describe the evolution of the upper ocean mixed layer between 4 April and 21 July 1998. Surface salinity driven potential density stratification and summer mixed layer depths are temporally coincident. Sea ice–ocean connection is first described at the local scale, then regionally. At the local scale, a reduction in surface salinity occurs between the melt and pond onsets over landfast sea ice. At the polynya scale, interpolated pond onset date is highly effective at predicting the timing of a thin, stably stratified summer ocean mixed layer; this has consequences for the potential prediction of timing of maximum ecosystem productivity.     

内波极性转变过程序列SAR图像仿真研究

基于Lynett和Liu的水平二维内孤立波传播模型,对内波的极性转变过程进行了序列SAR成像仿真研究。通过分析下降型内孤立波转变为上升型的序列SAR仿真图像,验证了内波极性转变时在SAR图像上出现信号减弱和特征消失的现象。对比于实地星载SAR图像,仿真结果也验证了内波在传播过程中受地形变化影响而出现的传播方向平行于等深线、分裂等现象。     

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。     

基于变系数Gardner方程的海洋内波研究

本文主要是基于同时含有二阶和三阶非线性项的变系数Gardner方程对海洋内孤立波的传播特性开展研究。在吕宋海峡海域,展示了下降型海洋内波的传播特性及其在SAR图像上的信号特征,并着重分析讨论了耗散项和微扰项对海洋内波所引起的表层流速变化的影响。     

On finite-time and infinite-time cost improvement of economic model predictive control for nonlinear systems

A novel two-layer economic model predictive control (EMPC) structure that addresses provable finite-time and infinite-time closed-loop economic performance of nonlinear systems in closed-loop with EMPC is presented. In the upper layer, a Lyapunov-based EMPC (LEMPC) scheme is formulated with performance constraints by taking advantage of an auxiliary Lyapunov-based model predictive control (LMPC) problem solution formulated with a quadratic cost function. The lower layer LEMPC uses a shorter prediction horizon and smaller sampling period than the upper layer LEMPC and involves explicit performance-based constraints computed by the upper layer LEMPC. Thus, the two-layer architecture allows for dividing dynamic optimization and control tasks into two layers for a computationally manageable control scheme at the feedback control (lower) layer. A chemical process example is used to demonstrate the performance and stability properties of the two-layer LEMPC structure.     

Persistent iceberg groundings in the western Weddell Sea, Antarctica

We use the time-average of a series of satellite synthetic aperture radar (SAR) images from 2005 to 2007 to identify grounded icebergs in the western Weddell Sea. Satellite laser altimetry along repeated ground tracks confirms that regions of high mean backscatter are associated with stationary, tabular icebergs. The altimeter data also provide iceberg freeboard, from which we infer maximum ice draft assuming the lightly-grounded limit and a two-layer density model consistent with a source at Filchner-Ronne Ice Shelf. The maximum iceberg draft agrees very well with the ocean depth at a point where the bathymetry is well constrained by ship soundings. However, for an extensive region near 71°S, south of Larsen C Ice Shelf, the maximum grounding depth for several icebergs reveals the ocean to be locally more than 200 m shallower than in bathymetry products. This previously uncharted bank is expected to have a profound effect on the northward flow of the western Weddell Gyre and the cross-slope transport of offshore warm deep water towards the eastern Antarctic Peninsula ice shelves. Passive microwave data also show that sea-ice concentration is reduced in the vicinity of this group of grounded icebergs, indicating an iceberg influence on ocean/atmosphere heat and freshwater exchanges.     

基于矩阵分解的海洋SAR图像舰船检测

针对海洋原始图像与低秩和稀疏矩阵分解模型数据结构不一致的问题,本文提出一种新的基于矩阵分解的海洋SAR图像舰船检测方法。首先该方法需对结构化相似的海洋SAR图像进行重组;然后根据重组矩阵特性适应性设计一个分解精度更高、分解速度更快的新矩阵分解模型,并利用增广拉格朗日乘子法求解模型,在不依赖任何杂波模型和检测统计量的前提下,实现代表舰船目标的稀疏成分的提取;最后利用形态学处理进行优化,实现海洋SAR图像舰船目标的检测。基于高分三号SAR卫星数据的实验结果表明,相比已有的基于鲁棒主成分分析的舰船检测方法,本文方法在处理复杂海况时,能更快速度地以较好的形状从海杂波中准确提取舰船目标,具有更好的鲁棒性。     

A comparative assessment on the use of SAR and high-resolution optical images in ocean dynamics studies

A comparison of Synthetic Aperture Radar (SAR) images acquired by the ERS-2 and RADARSAT-1 satellites with Landsat Thematic Mapper (TM) images acquired close in time is presented. Similarities and differences on the kind of ocean information they provide are discussed. The images were acquired over the Brazil–Malvinas (Falkland) Confluence under different solar elevation and azimuth angles. Ocean features observed by both types of sensors are discussed. These observations show that (1) at high solar angles, sun glint prevails over upwelled water-leaving radiance resulting in optical images that tend to provide dynamical information similar to that obtained from SAR imagery and (2) at low solar zenith and azimuth angles, upwelled radiation tends to prevail over sun glint, providing typical ocean colour and water quality information that can still complement and aid in the interpretation of SAR observations.     

Alignment of mountain lee waves viewed using NOAA AVHRR imagery,MST radar,and SAR

Recent radar observations of mountain waves in the troposphere and lower stratosphere above Aberystwyth (52.4°N, 4.0°W) indicate that, on average, the wave alignment is related more closely to the wind direction within the boundary layer than to the alignment of mountain ridges. This is investigated using independent data NOAA AVHRR imagery of both mountain-wave clouds and convective cloud streets, combined with surface synoptic wind measurements. The mountain-wave cloud bands are found to be aligned not at exactly 90° to the surface wind but rotated a further 18° clockwise. Similarly, in an important backup test, the cloud streets are found not to be parallel to the surface wind but rotated 12° clockwise, which agrees with over 30 years of observations, most recently of wind rows on the ocean by synthetic aperture radar (SAR). Because the wind rotates, on average, clockwise with increasing height in the northernhemisphere boundary layer, the mountain-wave clouds will be at 90° to the wind direction in the middle of the boundary layer. Therefore, the satellite images independently confirm earlier mesosphere-stratosphere-troposphere (MST) radar observations. Mountain lee waves may corrupt SAR measurements of surface wind above the ocean, so knowledge of their alignment is useful; two examples of lee waves modulating the sea roughness west of Aberystwyth are discussed.     

Fusion of multisensor multitemporal satellite data for land cover mapping

Efficient integration of remote sensing information with different temporal, spectral and spatial resolutions is important for accurate land cover mapping. A new temporal fusion classification (TFC) model is presented for land cover classification, based on statistical fusion of multitemporal satellite images. In the proposed model, the temporal dependence of multitemporal images is taken into account by estimating transition probabilities from the change pattern of a vegetation dynamics indicator (VDI). Extension of this model is applicable to Synthetic Aperture Radar (SAR) images and integration of multisensor multitemporal satellite images, concerning both temporal attributes and reliability of multiple data sources. The feasibility of the new method is verified using multitemporal Landsat Thematic Mapper (TM) and ERS SAR satellite images, and experimental results show improved performance over conventional methods.     

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。     

A New Ship Target Detection Algorithm based on SVMin High Resolution SAR Images

The characteristics of ocean background and target in the high resolution synthetic aperture radar (SAR) images are analyzed.Aiming at the requirements of ship detection in high-resolution synthetic aperture radar (SAR) image,the detection accuracy,intelligence level,real-time and processing efficiency,we put forward a high resolution SAR images ship detection algorithm based on support vector machine.The algorithm designs a pre-training support vector machine (SVM) classifier and complete the screening of the ship target block area,then the algorithm of optimal entropy thresholds proposed by Kapur,Sahoo,Wong(KSW) will be used on the target area selected for fine detection of ship targets.In this paper,several commercial satellite data,such as TerraSAR-X,are used to verify the experiment.Comparing with the classical CFAR detection algorithm,Experimental results show that the algorithm can improve the false alarm caused by the speckle noise and ocean clutter background inhomogeneity.At the same time,the detection speed is also increased by 20% to 35%.     

海洋监视监测SAR卫星任务初步分析

以海浪、船舶、溢油、内波和海冰等海洋现象为主要的观测目标,海洋监视监测卫星利用多极化SAR的高分辨率模式进行精细成像,识别目标细节特征,利用宽观测带扫描模式进行快速覆盖观测,获取全球海洋遥感信息。针对各观测目标要素,借鉴ERS和RADARSAT-1的观测结果,结合理论分析与仿真,初步分析了海洋监视监测SAR卫星轨道参数和有效载荷工作模式等卫星任务。     

应用Radon变换方法检测窄V形船舶航迹

ＳＡＲ航天遥感图像中的舰船航迹影象包含有舰船的进行速度、行进行方向、船体长度等重要信息,而充分开发利用与运动舰船航迹相关的信息资源,能够加强对上舰船航行的监测能力。通过对舰船航迹在卫星遥感ＳＡＲ图象中不同模式及Ｒａｄｏｎ变换原理和特性的分析,提出了应用Ｒａｄｏｎ变换对ＳＡＲ图象中的窄Ｖ形舰船航迹进行了检测的方法,通过实例测试,效果比较理想。     

不确定海流环境下水下机器人最优时间路径规划

为解决海流预测不精确条件下,现有基于确定性海流路径规划算法鲁棒性差和规划的路径有可能为不可行路径的问题,本文提出一种基于区间优化的水下机器人(AUV)最优时间路径规划算法.该算法采用双层架构,外层用蚁群系统算法(ACS)寻找由起点至终点的候选路径;内层以区间海流为环境模型,计算候选路径航行时间上下限,并分别通过区间序关系和基于可靠性的区间可能度模型将航行时间区间转换为确定性评价函数,并将评价函数值作为候选路径适应度值返回到外层算法.仿真结果表明,相对于确定海流场路径规划方案,提出的方案增强了路径规划器的鲁棒性并解决了结果路径不可行问题.     

空间约束层次加权Gamma混合模型的SAR图像分割

目的 合成孔径雷达（SAR）图像中像素强度统计分布呈现出复杂的特性,而传统混合模型难以建模非对称、重尾或多峰等特性的分布。为了准确建模SAR图像统计分布并得到高精度分割结果,本文提出一种利用空间约束层次加权Gamma混合模型（HWGaMM）的SAR图像分割算法。方法 采用Gamma分布的加权和定义混合组份;考虑到同质区域内像素强度的差异性和异质区域间像素强度的相似性,采用混合组份加权和定义HWGaMM结构。采用马尔可夫随机场（MRF）建模像素空间位置关系,利用中心像素及其邻域像素的后验概率定义混合权重以将像素邻域关系引入HWGaMM,构建空间约束HWGaMM,以降低SAR图像内固有斑点噪声的影响。提出算法结合M-H（Metropolis-Hastings）和期望最大化算法（EM）求解模型参数,以实现快速SAR图像分割。该求解方法避免了M-H算法效率低的缺陷,同时克服了EM算法难以求解Gamma分布中形状参数的问题。结果 采用3种传统混合模型分割算法作为对比算法进行分割实验。拟合直方图结果表明本文算法具有准确建模复杂统计分布的能力。在分割精度上,本文算法比基于高斯混合模型（GMM）、Gamma分布和Gamma混合模型（GaMM）分割算法分别提高33%,29%和9%。在分割时间上,本文算法虽然比GMM算法多64 s,但与基于Gamma分布和GaMM算法相比较分别快600 s和420 s。因此,本文算法比传统M-H算法的分割效率有很大的提高。结论 提出一种空间约束HWGaMM的SAR图像分割算法,实验结果表明提出的HWGaMM算法具有准确建模复杂统计分布的能力,且具有较高的精度和效率。     

Ocean remotely sensed image analysis using two-dimensional continuous wavelet transforms

Ocean remote sensing is a useful way to obtain ocean wave information. Due to possible inhomogeneities from remotely sensed images, the current work proposes issues concerning ocean wave image analysis using the two-dimensional continuous wavelet transforms (2-D CWTs) to calculate local wave image spectra from inhomogeneous images. To optimize the algorithm of the 2-D CWT for wave image analysis, this work explores ideal parameter values for the wavelet function. The current study also analyses the limits of spatial image resolution and wave image size. After implementing the 2-D CWT on satellite and X-band radar images, this study presents local image spectra and ocean wave information from all the ocean images. These local image spectra reveal the phenomenon of wave refraction and wave nonlinearity nearshore. Compared to real wave spectra, the wavelet spectra present accurate results to describe local wave features in the spatial frequency domain.     

Planar block adjustment and orthorectification of Chinese spaceborne SAR YG-5 imagery based on RPC

Yaogan-5(YG-5), launched in December 2008, is a Chinese high-resolution spaceborne synthetic aperture radar (SAR) satellite, with a ground resolution of 3 m. However, the direct geometric positioning accuracy of YG-5 slant range images is low and so is the mosaic accuracy of the orthoimages. To improve the geometric accuracy of YG-5 orthoimages, this article proposes a strategy to calculate the rational polynomial coefficients for each SAR image and then uses a planar block adjustment method to solve for the orientation research parameters of the SAR images to achieve the orthorectification while a auxiliary digital elevation model is necessary for height constraint. Compared with the traditional orthorectification method using a single image, this strategy can ensure both uniformity in positioning accuracy of orthorectified images and high mosaic accuracy of adjacent orthoimages based on a small number of ground control points (GCPs). Tests using Chinese YG-5 satellite data over Xi’an and Xianning, China show that, using four GCPs positioned in the four corners of the test area, we can achieve independent check point plane accuracies better than ±4 m after the planar block adjustment. Finally, this article demonstrated that seamless mosaic geometry levels can be attained after the block orthorectification.     

Semi-automated feature-tracking of East Antarctic sea ice from Envisat ASAR imagery

While feature tracking of sea ice using cross-correlation methods on pairs of satellite Synthetic Aperture Radar (SAR) images has been extensively carried out in the Arctic, this is not the case in the Antarctic. This is due to the dynamic nature of Antarctic pack ice, its microwave signature, the tendency for SAR swath paths to be poorly aligned with the often narrow sea ice zone around the continent and inadequate satellite sampling. A semi-automated system, known as IPADS (IMCORR [IMageCORRelation] Processing, Analysis and Display System), has been developed to map fast ice and pack ice in Antarctica using multiple pairs of SAR images. The software processing pipeline uses overlapping image pairs which are geocoded and roughly registered using only data contained in the image headers. Next, fast ice maps are rapidly generated using zero motion features located within ocean regions. This also provides precise image registration. Finally, the same image pairs are re-examined for pack ice motion in a slower off-line batch process. The pack and fast ice are identified using a cluster-based search method which compares both location and motion information. Each image pair generates a NetCDF file which adds to a growing database of Antarctic sea ice motion and ice roughness. Five image-pair examples are presented to illustrate the methods used as well as their strengths and limitations. Substantial pack ice motion can often be detected in the marginal ice zone on SAR images only a few days apart.