A model of satellite radar altimeter return from ice sheets

Abstract

Satellite-borne radar altimetry offers a unique opportunity for measuring the form and mass balance of the polar ice sheets. Changes in ice-sheet mass balance are intimately linked to climatic change and variations in the global mean sea level. However, previous altimeter measurements of ice-sheet topography have been made without the use of a well-validated model of the altimeter return. Here, we present a theoretical model of the return which, supported by both observational and experimental evidence, suggests that over vast areas of the higher altitude regions of the ice-sheets, significant radar penetration of the firn occurs at frequencies commonly used for space altimetry. This implies the need for a previously-neglected correction to height measurements which can be as much as 3-3 m, depending on the retracking method and location. Since the degree of radar penetration may exhibit variability over a range of time-scales, failure to account for the effect could lead to erroneous estimates of surface elevation change. The detection of variability in the degree of penetration is of considerable interest from the point of view of monitoring the processes of accumulation and ablation of snow over the ice-sheet surface, as the return is particularly sensitive to conditions within a few centimetres of the surface. The model has wider applications as it may be used in modified form to simulate altimeter return from all smooth surfaces which exhibit a combination of surface and volume scattering, including deserts and the surfaces of the terrestrial planets and their satellites.     

Scattering cross sections for composite surfaces with large mean square slopes—full wave analysis

Abstract

In this work the full wave approach to rough surface scattering is applied to composite models of rough surfaces with large mean square slopes. It is shown that both specular point scattering as well as Bragg scattering are accounted for in the analysis and the results are compared with earlier solutions based on a combination of physical optics and perturbation theories. Using the full wave approach it is not essential to decompose the rough surface into individual surfaces with different roughness scales unless it is desired to separate the specular point contribution from the Bragg contribution to the scattering cross sections. Shadowing is accounted for in the analysis.     

插值曲率线的直纹面可展设计

目的 曲率线在微分几何中起着非常重要的作用,它在曲面分析中是一个很有用的工具。可展曲面是曲面造型中最简单也最常用的一类曲面,目前大部分工作都是研究在给定曲面上寻找或者计算曲率线,而其反问题研究工作较少,为此,提出一种插值曲率线的可展曲面构造方法,并进一步将它应用到曲面造型中。方法 利用Frenet标架表示直纹面的母线,根据曲线为曲面曲率线以及曲面可展的充要条件,得到直纹面的母线需要满足的关系式。并引入控制函数控制曲面的形状。结果 给出了以给定曲线为曲率线的直纹面可展的具体表达式,根据可展曲面分类分析了设计曲面为柱面、锥面和空间曲线切线面的充要条件,并给出了两个代表性的实例验证该方法的有效性,实例结果表明,该方法不仅适用于一般参数曲线,对分段参数曲线也是有效的。结论 利用构造性的方法给出了插值曲率线的可展曲面的具体表达形式,并通过具体实例验证了该方法的有效性。     

匹配滤波在SAR目标单个散射部件检测中的应用

目的目标部分缺失是SAR(synthetic aperture radar)ATR(automatic target recognition)中的难点问题,实现SAR目标散射部件级别的检测可以有效解决这一问题。方法基于目标的部件级别3维电磁散射模型提出了一种基于匹配滤波的单个散射部件检测方法。该方法通过模型预测单个散射部件的图像构造匹配滤波器,并通过在模型3维参数域的调整实现了匹配滤波器在3维空间的滑动,得到最佳的部件检测位置和最大相关系数。为克服方位角估计不准的限制,采用了领域搜索寻找最佳检测结果。按照散射部件的强度由强至弱序惯检测各个散射部件。采用CLEAN算法去除部件之间的相互干扰。结果在简易坦克部件级3维电磁散射模型基础上,利用电磁仿真数据、暗室数据以及MSTAR(moving and stationary target acquisition and recognition)干扰数据进行了单个散射部件检测实验,实验结果表明,本文方法可以很好地检测出实测图像中存在的目标散射部件,并且可以检测出缺失的散射部件。结论本文提出了基于匹配滤波器的散射部件检测方法,实验证明了该方法的有效性。     

Modelling of radar backscattering from low-salinity ice with ice ridges

Abstract

Radar backscattering from a snow-free ice surface characterized by an exponential correlation function is studied for conditions typical in the Baltic Sea. The C-band backscattering coefficient from first-year ice is normally found to be characterized primarily by the air-ice surface roughness. However, if the salinity and temperature are low, as in the Baltic Sea, both volume scattering and ice-water surface scattering might contribute. Even more important than scattering from the level ice is scattering from the ice ridges, and a simple approach is taken to characterize the properties of ice ridges and the corresponding scattering.     

Aircraft identification using a bilinear surface representation of radar data

Low frequency radar scattering data is used for the identification of aircraft. It is shown that such radar data lies on two-dimensional surfaces in n-space. A bilinear approximation for these surfaces is described. Surface intersections using this approximation can be found simply and directly without solving a system of n simultaneous nonlinear equations. This intersection information can be used to show separability and effect feature reduction. The approximation is utilized to construct a modified nearest neighbor algorithm, which is evaluated by computer simulation experiments. These experiments showed a phenomenon of “bias”, where one aircraft data surface is more susceptible to misclassification in the presence of noise than the surface corresponding to another aircraft. This “bias” observed is shown to be related to the surface characteristics of the data surfaces involved, specifically proximity and relative curvature of corresponding points on the two surfaces.     

A coordinate transformation approach for efficient repeated solution of Helmholtz equation pertaining to obstacle scattering by shape deformations

A computational model is developed for efficient solutions of electromagnetic scattering from obstacles having random surface deformations or irregularities (such as roughness or randomly-positioned bump on the surface), by combining the Monte Carlo method with the principles of transformation electromagnetics in the context of finite element method. In conventional implementation of the Monte Carlo technique in such problems, a set of random rough surfaces is defined from a given probability distribution; a mesh is generated anew for each surface realization; and the problem is solved for each surface. Hence, this repeated mesh generation process places a heavy burden on CPU time. In the proposed approach, a single mesh is created assuming smooth surface, and a transformation medium is designed on the smooth surface of the object. Constitutive parameters of the medium are obtained by the coordinate transformation technique combined with the form-invariance property of Maxwell’s equations. At each surface realization, only the material parameters are modified according to the geometry of the deformed surface, thereby avoiding repeated mesh generation process. In this way, a simple, single and uniform mesh is employed; and CPU time is reduced to a great extent. The technique is demonstrated via various finite element simulations for the solution of two-dimensional, Helmholtz-type and transverse magnetic scattering problems.     

形状可调插值曲线曲面的参数选择

目的 因大多数插值基函数中的参数都是全局参数,从而导致插值曲线曲面的形状无法进行局部调整。另外,当插值曲线曲面形状可调时,也存在如何选择参数才能获得形状较为理想的曲线曲面的问题,为此给出一种无需反求控制顶点、包含局部形状调整参数、具有显式表达式、能重构部分二次曲线曲面的插值曲线曲面构造方法,同时给出易于使用的形状参数确定方案。方法 基于经典3次Hermite插值曲线的Bernstein基函数表达形式,将其中的Bernstein基换成已证明具有全正性的一组三角基函数,根据三角基的端点性质调整曲线表达式以保证其插值性,然后设定插值数据点处的导向量,在其中引入参数,并保证相邻曲线段之间的连续性,得到了一种新的三角基插值曲线。结果 新曲线可以整理成以待插值数据点为控制顶点与一组插值基函数的线性组合形式,插值基表达式简单,插值曲线含一组局部形状调整参数,一个参数的改变只影响一条曲线段的形状,相邻曲线段之间G¹连续,曲线可以重构椭圆。根据不同目标给出了3种用于确定曲线中形状参数的准则,每种准则都提供了可以直接使用的公式。相应的插值曲面具有与插值曲线类似的性质。结论 形状参数选取准则的给出使含参数插值曲线曲面的设计由随意变为确定,这使得采用本文方法更易于得到满意的结果。本文所给插值基函数的构造方法具有一般性,可以采用相同的思路构造其他函数空间上性质类似的插值基。     

The impact of spectral emissivity on the measurement of land surface temperature from a satellite

Abstract

The split-window method is successfully used to infer sea surface temperature from satellite radiances, principally because sea surface temperature is not very different from the air temperature near the surface and because the emissivity of the sea is constant over large areas and is not very different from one in the spectral channels of interest. This is not true for land surfaces and the split-window method has to be re-examined for such a case. This is the aim of this paper. In order to relate land surface temperature to the two brightness temperatures measured from space in the two channels of interest (namely, AVHRR 4 and AVHRR 5), several formulae are derived and their accuracies are discussed. Assuming that the emissivities ε₁ and ε₂ in the two channels considered, and therefore their average $ are unity, it is shown that the error ΔT generated on the land surface temperature by correcting atmospheric effects using the split-window method in most situations studied is of the order of

$

This error may be quite significant, except for the sea surface where it is shown to be negligible. In order to infer land surface temperature from space, it is therefore necessary to know the surface spectral emissivity to good accuracy. Possible methods to determine it are then proposed and discussed.     

Search for MC in modified networks

In this paper, first we develop an intuitive algorithm using the shortest path based upon the reformation of all MCs in the original network. Next, the computational complexity of the proposed algorithm is analyzed and compared with the existing methods. One computer example is illustrated to show how all MCs are generated in a modified network based upon reformation of all of the MCs of the corresponding original network.Scope and purposeA method is proposed to search for all minimal cuts (MCs) in a network (modified networks) obtained by modifying the original network. It can be used for reliability analysis of various modifications in an existing network for network expansion or reinforcement evaluation and planning. Analysis of our algorithm and comparison to existing best-known algorithms shows that our proposed method has the following advantages: (1) it can be used to search for all MCs without knowing all minimal paths (MPs) and MCs in advance. (2) it is simple and more effective in finding and verifying MC candidates in modified networks than the existing methods. and (3) our method is easier to understand and implement.     

高精度曲面建模优化方案

目的 为了进一步提高高精度曲面建模（HASM）方法的模拟精度和计算速度,进而拓宽该模型的应用领域,提出了新的HASM模型算法。方法 采用新的差分格式计算HASM高斯方程中的一阶偏导数,以HASM预处理共轭梯度算法为例分析改进的差分格式对HASM的优化效果。结果 数值实验表明：在计算耗时及内存需求不变的情况下,采用新的差分格式的HASM算法可以显著提高单次迭代的模拟精度,同时能够降低关键采样点缺失对模拟结果精度的影响。进一步研究发现,当HASM采用新差分格式与原始差分格式（中心差分）交替迭代时,能够快速降低模拟结果的误差。结论 本文算法当达到指定的精度条时能够显著减小计算耗时,同时还能降低关键采样点缺失对模拟结果的影响。     

Ground truth requirements for radar observations over land and seat†

Abstract

At a first stage of development, ground truth is gathered in conjunction with radar data in order to establish statistical correlations between observed backscatter and properties of the irradiated surface. Significant correlations have been shown to exist between backscatter and many geophysical quantities; for example scatterometer measurements over the ocean can be used to infer wind speed, synthetic aperture radar (SAR) measurements over woodland can allow the separation of tree species, etc. Once statistical relationships are established, the next stage is to try to understand the physics underlying the relationship, both because of its intrinsic interest and because of the need to optimize system performance for particular applications. This places a second role on ground truth, since it must be used to validate the physical theories stemming from it. Microwave scattering from land surfaces is a topic of growing interest as we look towards land observation missions within the coming decade. Several theories have been proposed to account for scattering from surfaces of a vegetation layer, but none of these has been rigorously tested. If they are to contribute effectively to future land missions, it is essential that such testing be carried out. The first section of this paper describes the requirements such testing will put on ground measurements, and tries to identify whether such measurements are feasible. Microwave scattering mechanisms from the sea surface, and particularly their effects on SAR images of waves, and clutter-statistics in SAR sea-surface images are still a matter of some debate. The second section of this paper outlines the problems inherent in sea imaging with SAR and describes experiments that need to be carried out in order that these questions can be resolved.     

高质量快速全景场景再现技术

目的 色彩纠正和图像融合是生成高质量全景场景图像的关键技术。色彩纠正中参考图像的选择以及图像融合算法,决定着所生成全景图像的质量和速度。现有方法在确定一幅图像是否适合作为参考图像时,需要遍历所有其他图像,计算其作为参考图像进行色彩纠正的效果,复杂度高,速度慢;在图像融合时存在融合质量与融合速度之间的矛盾。因此,如何快速生成高质量的全景图像就成为全景场景再现的主要诉求。为此本文提出优化的参考图像自动选择的色彩纠正方法和基于重叠区域划分的分区融合方法。方法 针对参考图像选择算法复杂度高的问题,根据图像质量与稳定性通常呈反比关系的事实,采用贪婪策略,选择质量最差的图像在色彩纠正前后的相似度,作为是否选择当前图像作为参考图像的评价指标,在保证参考图像满足色彩纠正需求的前提下,大幅降低计算复杂度。针对融合质量与融合速度之间的矛盾,提出分区融合：将重叠区域划分为接缝区域和非接缝区域,利用泊松融合的接缝不可见性和线性融合实现速度快的特性分别对接缝区域和非接缝区域进行融合,既保证融合的质量,又加快融合速度。在此基础上,加入简单点光源,解决上述过程产生的光线一致性问题,进一步提高图像质量。结果 采用主观和客观相结合的方法对结果进行评估。主观方面,本文算法生成的全景图像色彩基本实现平滑过渡且图像原始信息保留完整。客观方面,色彩纠正前后图像的结构相似度（SSIM）控制在0.850.99之间,时间复杂度由原来的O（n²）降低到O（n）;分区融合后图像信息熵接近于泊松融合,但时间消耗降低72%。采用基于PC端的问卷调查法和OG-IQA算法将本文算法与PTGui、OpenCV、Xiong方法生成的全景图质量进行对比,在大多数情况下本文算法均优于上述算法。结论 实验表明,本文算法适用于多种场景,在保证目视效果良好的前提下,时间消耗降低,可广泛应用于医学、数字旅游、遥感等领域。     

Correction of the effect of Sun-sensor-target geometry in NOAA AVHRR data

Models of determining the effects of the bidirectional reflectance distribution function (BRDF) of different surfaces and of eliminating the effect of Sun-sensor-target geometry from the remotely sensed satellite data are actual. The objective of this study is to develop a simple relation between the Sun-sensor-target geometry and the remotely sensed vegetation index. In this investigation 238 National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) images were used over Hungary during the period 1996-98. The greenness vegetation index (the difference between the reflectance values of near-infrared and visible channels) was used between days of the year 140-200, because the greenness values can be considered as constant in this period over the agricultural areas. The so-called 'hot spot effect' can be observed in the variation of reflectance values with different viewing zenith angles of the sensor. A simple quadratic relation was found between the raw AVHRR greenness values and the angle enclosed by the Sun-target and target-sensor directions over the agricultural areas, forests and grasslands. A correction method was developed to eliminate the effect of the Sun-sensor-target geometry, which it is hoped will improve the accuracy of yield forecasting and estimation procedures using NOAA AVHRR data.     

A simple geometrical model for analysing the spectral response of a citrus canopy using satellite images †

Abstract

A simple geometrical model has been proposed for a citrus canopy. We assume the citrus orchard to be a lattice structure, with the trees positioned at its points and where the composite-scene reflectance is the sum of the reflectance of its individual components as weighted by their respective surfaces within a unit area. The model has been used to analyse the citrus spectral response obtained from Landsat-5 TM images for winter and summer, where the status of the orchard is different. The correlations between spectral and geometrical data show the influence of per cent crop cover, shadows and background in the composite scene reflectance. We conclude that the summer images could be more useful than the winter ones for parcel classification according to per cent crop cover.     

高阶条件随机场引导的多分支极化SAR图像分类

目的 针对极化合成孔径雷达（polarimetric synthetic aperture radar,PolSAR）小样本分类问题,基于充分挖掘有限样本的极化、空间特征考虑,提出一种由高阶条件随机场（conditional random field,CRF）引导的多分支分类网络模型。方法 利用Yamaguchi非相干目标分解方法,构建每个像素的极化特征向量。设计了由高阶CRF能量函数引导的多卷积分支特征提取网络,将像素点极化特征向量作为输入,分别提取像素点的像素特征、邻域特征和位置特征信息。将以上特征进行加和融合,并输入到softmax分类器中得到预分类结果。利用超像素方法对预分类结果图进行进一步修正和调优,平滑相邻像素之间的特异性和相似性。结果 采用1%的采样率对两组真实的极化SAR数据进行测试。同时,为了更好地模拟实际应用中训练样本位置分布不均匀的情况,考虑了空间不相交采样方法作为对比实验。综合两种采样策略的实验结果表明,相较于只利用像素级特征或简单利用空间特征的方法,本文方法总分类精度平均提升7%～10%,不同地物类别的分类精准度均在90%以上,运行速度相比于支持向量机（support vector machine,SVM）提高了2.5倍以上。结论 通过构建高阶CRF引导的卷积神经网络,将像素特征信息、同质区域特征和地理位置信息进行融合,有效建立了像素级和对象级数据之间的尺度关联,进一步扩充了像素点之间的空间依赖性,提取到了更强大更准确的表征特征,显著提高了标记样本数量较少情况下的卷积网络模型的分类性能,进一步保证了地物目标散射机制表征的全面性和可靠性。     

Method of Direct Texture Synthesis on Arbitrary Surfaces

A direct texture synthesis method on arbitrary surfaces is proposed in this paper. The idea is to recursively map triangles on surface to texture space until the surface is completely mapped. First, the surface is simplified and a tangential vector field is created over the simplified mesh. Then, mapping process searches for the most optimal texture coordinates in texture sample for each triangle, and the textures of neighboring triangles are blended on the mesh. All synthesized texture triangles are compressed to an atlas. Finally, the simplified mesh is subdivided to approach the initial surface. The algorithm has several advantages over former methods: it synthesizes texture on surface without local parameterization; it does not need partitioning surface to patches; and it does not need a particular texture sample. The results demonstrate that the new algorithm is applicable to a wide variety of texture samples and any triangulated surfaces.     

The Rayleigh lookup tables for the SeaWiFS data processing: Accounting for the effects of ocean surface roughness

In the atmospheric correction of ocean colour remote sensing, it is important to account for the effects of ocean surface roughness (wind speed) in the computation of Rayleigh radiance lookup tables, in particular, for the large solar and/or sensor zenith angles (>～60°). In the paper, both simulated and the SeaWiFS-retrieved results that demonstrate the effects of the ocean surface wind speed on the Rayleigh radiance computations for the various solar and sensor-viewing geometries as well as on the performance of atmospheric corrections are presented and discussed. An improved set of Rayleigh lookup tables, in which the Rayleigh scattering radiance is also a function of the sea surface wind speed, were generated and implemented into the SeaWiFS data processing system in May 2000.     

随机场中运动一致性的多线索目标跟踪

目的 通过建立各线索间的关联,提高多线索目标跟踪方法的鲁棒性,利用简单而有效的模型使多线索目标跟踪方法的表达和实现变得容易.方法 在不同线索描述下的目标对象间引入运动一致性约束,利用链状结构随机场模型表达不同线索描述下的目标对象及其约束关系,将多线索目标跟踪问题转化为随机场目标函数的简单优化求解.实验中结合亮度直方图、方向梯度直方图和局部二进制模式描述目标对象.结果 15组公测视频序列上的实验结果表明,所提方法相对于多种优秀的目标跟踪方法,在目标受到遮挡、运动模糊、光照变化、背景杂乱等因素干扰时,获得了较低中心位置误差和较高的精度值,反映了所提方法的有效性.结论 运动一致性约束能够较好地增强各线索间的关联,通过链状结构的随机场模型表达该约束关系和各线索描述下的目标对象,在提高跟踪鲁棒性的同时,使跟踪方法的实现变得简单.     

A simulation of bidirectional reflectance distributions for various surface microstructures

Abstract

A simple model was developed to allow the determination of the bidirectional reflectance distribution function along the Sun's azimuthai plane for surfaces with roughness described by a repetition or microstructures. A modular surface unit is utilized representing a wide range of microstructures. The simulation is based on equations accounting for the shadowing and obstruction geometries which determine the reflectance properties of the surface. This Letter presents some of the equations developed and typical directional reflectance curves produced by the simulation.