雷达后向散射模型及其在雷达图像地形影响纠正中的应用

在从雷达估测森林生物量时，经常遇到的一个问题是地形对雷达信号的影响。地形使得雷达波的入射角度改变，使每个雷达图像像元所包含的地表面积改变，由于地面的起伏，植被本身的结构也不同，纠正这种由地形而不是植被类型引起的雷达图像的变化是一个很复杂的问题，除了需要高质量的地形数据外，还必须理解植被雷达信号随地形变化的规律。提出一种可用来模拟森林及其它植被处于山坡上的雷达后向散射模型。结合DEM数据，模拟的结果可用来进行雷达图像的地形影响纠正，如果多极化或多波段图像存在，通过雷达模型可用从一种极化推导出的地形信息来纠正其它极化的图像数据。     

An unsupervised segmentation method based on MPM for SAR images

An unsupervised segmentation method for synthetic aperture radar (SAR) images is proposed. It alternately approximates the maximization of the posterior marginals estimate of the pixel class labels and estimates all model parameters except the number of classes during segmentation. In this method, a multilevel logistic (MLL) model for the pixel class labels and Gamma distribution for the marginal distribution of each class in the observed SAR image are employed. In our implementation, the expectation-maximization algorithm is used to estimate parameters of the Gamma distributions, and the iterative conditional estimation algorithm is used to estimate the MLL model parameters. The segmentation results for synthetic and real SAR images show that the proposed method has a good performance.     

邻近辐射的精确计算研究

简化或积分算法计算邻近辐射,对山区或处于近红外波段的植被等情况产生较大误差。尽管近似离散求和算法考虑了所有因子,然而由于算法的复杂性,未做出更加详尽细致的分析。针对上述问题,本文在考虑每个邻近像素地表单元的辐射对目标像素影响的基础上,重点分析了在一定的范围内所有与目标地表单元通视且朝向目标地表单元的邻近像素,同时考虑搜索半径的问题。试验证明,该算法获得比传统算法更加精确的邻近辐射计算结果。     

Potential of ASAR-ENVISAT for Estimating Near Surface Soil Moisture in a Sloping Terrain of a Himalayan Watershed

Soil moisture estimation is considered to be one of the important parameters in hydrological studies. The extraction of information on near surface soil moisture from the synthetic aperture radar is well established. The available Advanced Synthetic Aperture Radar (ASAR) data onboard ENVISAT with multi-incidence and multi-polarization mode for soil moisture estimation on sloping terrain was investigated. Empirical models were developed to estimate near surface soil moisture in the fallow agricultural fields by incorporating the effects of surface roughness using multi-incidence angle ASAR data. Medium incidence angle (IS-4) with VV polarization of ASAR data had higher correlation coefficient to volumetric soil moisture content. The ratio of medium (IS-4) to high incidence (IS-6) angle could further reduce the effect of surface roughness. The effect of topography on the radar data is taken care by calculating local incidence angle derived from ASTER DEM data. The VV polarization in the sloping terrain provided better results in comparison to VH polarization.     

Ocean Internal Wave Observations Using Space Shuttle and Satellite Imagery

Abstract

Space shuttle photographs and satellite radar (SAR) images provide an excellent view of high‐contrast ocean features such as internal waves, fronts, eddies, oil slicks, and cloud patterns which contain the signatures of atmospheric processes. Since ocean internal waves generate local currents which modulate surface wavelets and slicks, we have been able to detect packets of internal waves in space shuttle photographs and radar imagery of the Atlantic, Pacific, and Indian Oceans. A global database on internal waves has been developed at our center with support from ONR and NASA, and is accessible on the Internet. The database includes visible and radar imagery. To test the database, digitally orthorectified images were used for dynamic and statistical analysis of internal waves. In the deep ocean we found the wavelength distribution to be Gaussian while in the coastal ocean it is Rayleigh. Results have also been applied to non‐linear evolution studies of ocean internal waves, atmospheric solitary waves and to estimate ocean currents.     

Using Multibaseline InSAR to Recover Layovered Terrain Considering Wideband Array Problem

This letter deals with the problems of retrieving height and synthetic aperture radar (SAR) reflectivity of layovered terrain using multibaseline SAR interferometry (InSAR). In particular, we focus on the wideband array problem caused by practically large InSAR arrays and high-resolution SAR images, i.e., the problem of signal envelope misalignment, which is neglected in the work by Gini We propose two methods to eliminate or mitigate the effect of envelope misalignment, one called the aligning method and the other called the joint range cell processing method. In the aligning method, we align each signal envelope for each searched height of the layovered components (i.e., ground resolution cells with different altitudes) during searching procedure. The joint range cell processing method jointly processes the neighboring cells in range to estimate the parameters of layovered components so that the effect of the envelope misalignment can be mitigated. Theoretical analysis and computer simulation results show that both methods have the ability to provide accurate estimation of the heights and radar reflectivities of multiple layovered resolution cells in the presence of large envelope misalignments.      

大坡度水下地形的SAR遥感模拟仿真

根据星载合成孔径雷达(SAR)的成像机理,建立了浅海水下地形的数学物理模型,提出了利用该模型进行水下地形计算的数值方法.利用该模型和方法对大坡度水下地形进行数值模拟,分析了大坡度水下地形的地形坡度与SAR探测程度的关系,发现在水下地形坡度较大的情况下,SAR可测量水深达100～200 m.通过对曾文溪附近海域水下地形的数值模拟,证明了SAR对大坡度水下地形的探测深度.     

极化信息在水下地形SAR探测中的应用

现有的水下地形SAR(Synthetic Aperture Radar)探测模型尚未综合应用SAR的极化信息。本文基于Valenzuela的雷达散射截面表达式与袁业立提出的海波高频谱表达式,并结合全极化SAR相干斑抑制的极化白化滤波法,将极化信息有效地运用到水下地形SAR探测中去。最后,以中国台湾浅滩为例,分别利用Radarsat-2单一极化和全极化影像开展水下地形SAR探测研究,探测结果表明充分利用全极化信息的探测精度明显优于单一极化信息的探测精度,利用极化信息可以有效地提高水下地形SAR探测精度。     

星载SAR水下地形和水深遥感的最佳雷达系统参数模拟

根据星载合成孔径雷达 (SAR)浅海水下地形和水深成像机理 ,建立了浅海水下地形和水深雷达后向散射截面仿真模型。该模型包括奈维 斯托克斯方程、谱作用量平衡方程和雷达后向散射模式。利用该模型仿真结果 ,探讨了不同波段 (P、L、C和X)、不同极化 (VV和HH)和不同入射角 (2 0°— 70°)的星载SAR测量浅海水下地形和水深的能力。研究结果表明 ,浅海水下地形和水深遥感的最佳波段为P波段 ,L波段次之 ,C波段比X波段要好一些。VV极化SAR的测量能力要强于HH极化。 2 0°— 40°是星载SAR测量浅海水下地形和水深的最佳入射角范围。     

基于DEM的遥感数据复原方法研究

介绍了一种基于数字高程模型（DEM）的遥感数据复原新方法。此方法将地形因子作为最主要的作用因子，不考虑卫星传感过程中的随机影响。首先，根据基础地理数据，按其等高线层生成DEM； 然后，利用DEM，通过实测样点、DEM和经过纠正的遥感数据的信息融合，进行遥感数据中像元样点的坡度、坡向分析，建立DEM与遥感信息的相关关系模型，以数学统计方法描述地形因子对遥感数据的作用机理； 最后，进行逐像元的遥感信息复原(归一化)。结果表明，该方法具有较好的信息复原效果，可消除或减少地形对遥感数据的影响，增强遥感技术在山区复杂地形下的实用性。     

基于间断自适应MRF的相位解缠算法

合成孔径干涉雷达（InSAR）技术已被广泛应用于地面高程信息的获取。相位解缠作为其关键步骤直接关系到地表高程的提取精度。然而,相位解缠受相位连续性假设的限制无法识别间断相位,影响在陡峭地形区域的高程反演精度。针对上述问题,本文提出了一种基于间断自适应马尔可夫随机场模型（DA-MRF）的相位解缠算法处理间断相位。首先构建DA-MRF模型的能量函数能够自适应地在连续相位处保持像素连接,同时在间断相位处阻断像素连接;然后再利用图割算法优化能量函数,完成相位解缠。仿真和实测数据试验证明,本文方法与常用的相位解缠算法相比,具有更强的间断相位保持能力,更适用于地形变化剧烈场景中的地形重建。     

SAR辐射定标中角反射器RCS的快速求解

角反射器雷达截面积的快速准确解算是顺利开展合成孔径雷达辐射定标工作的重要基础。用于合成孔径雷达辐射定标的角反射器为电大尺寸,因此,宜采用高频近似法进行雷达截面积的仿真计算。本文利用几何光学对入射波和反射波进行射线追迹以确定每次入射场及其相对应的照明区域,然后采用Gordan面元积分法对每个照明区域求散射场并累加得到角反射器的雷达截面积,通过与文献、电磁仿真软件FEKO(FEldberechnung für körper mit beliebiger oberfläche)的仿真结果比较,证明该方法有效且简单快捷,尤其适用于电大尺寸平板角反射器雷达截面积的快速求解。     

Evaluation and integration of ERS-1-SAR and optical sensor data (TM and IRS) for geological investigations

The C-band imaging radar of ERS-1, due to its high sensitivity to terrain surface features, holds tremendous potential in topographic terrain mapping for various applications. This is being examined for geological applications, mainly structural and lithological mapping in a mineral belt of Bihar and Orissa, India. The high image contrast that facilitates structural interpretation and highlights topography on the SAR images, reflects the high sensitivity of the ERS-1-SAR to change in terrain slope in the study area. Extensive lineaments, fold structure and major lithological contacts are easily mappable from the SAR imagery. Many of the lineaments, lithological contacts and fold pattern are mapped equally from optical data (Landsat-TM and IRS-1B FCC). The close association of fold pattern and mineral deposits in the region has necessitated the study of those structures carefully from various remote sensing data products. Synergism between SAR and TM provided useful results regarding structure and lithology of the region. The advantage of SAR in highlighting topography and detecting lineaments are affected to a great extent by the speckle noise and low pixel resolution. The present study shows that future geologic interpretation demands high spatial resolution and efficient data processing technique which reduces the speckle noise more significantly.     

一种改进的CFAR船只探测方法

提出了一种改进的CFAR船只探测算法。该方法采用PNN模型来估计海面雷达后向散射的概率分布模型,利用CFAR技术来确定整体阈值,采用基于交叉验证技术的黄金分割搜索法估算高斯分布的形状参数,使用区域生长法去除虚警。使用Radarsat图像对该方法进行了检验,并与改进前的算法进行了比较,结果显示该文的探测算法在探测精度和探测速度上均明显优于改进前的算法。     

Low-angle reflectivity modeling of land clutter

In this letter, a practical mean reflectivity model of radar land clutter (LC) for the complex system design of ground-based radars involved in low-angle targets on some typical terrains is studied using the inductive reasoning method. The functional relationships between the radar parameters and radar surface clutter backscattering are analyzed. Following the recent research work in the area of reflectivity modeling of LC in the literature, the least squares method is employed to estimate the model parameters. The model is validated using reliable practical data and shown to outperform other models in accuracy.     

利用X波段导航雷达探测海洋表面流速的方法

针对X波段导航雷达对海杂波的成像,研究了利用多谱勒频移和布拉格散射来探测海表面流的方法。提出了一种结合几何滤波模型的X波段雷达海流反演预处理算法,此算法不需要已知的最大海流流速进行带通滤波处理,结合线性波理论色散方程以及最小二乘算法,可以得到精确的海表面流速。与传统的X波段导航雷达测流方法相比,该算法简单高效,占用资源少,并且不仅适用于岸基X波段导航雷达,还适用于船载X波段导航雷达。与附近站点高频地波雷达表面流数据进行对比实验,二者结果的一致性表明了本文方法是可行的。     

Cloud removal for remotely sensed images by similar pixel replacement guided with a spatio-temporal MRF model

Cloud cover is generally present in remotely sensed images, which limits the potential of the images for ground information extraction. Therefore, removing the clouds and recovering the ground information for the cloud-contaminated images is often necessary in many applications. In this paper, an effective method based on similar pixel replacement is developed to solve this task. A missing pixel is filled using an appropriate similar pixel within the remaining region of the target image. A multitemporal image is used as the guidance to locate the similar pixels. A pixel-offset based spatio-temporal Markov random fields (MRF) global function is built to find the most suitable similar pixel. The proposed method was tested on MODIS and Landsat images and their land surface temperature products, and the experiments verify that the proposed method can achieve highly accurate results and is effective at dealing with the obvious atmospheric and seasonal differences between multitemporal images.     

Wave directional spectrum from SAR imagery

Gaussian smoothed SAR image spectra have been evaluated from 512 x 512 pixel subscenes of image mode ERS-1 SAR scenes off Goa, Visakhapatnam, Paradeep and Portugal. The two recently acquired scenes off Portugal showed the signature of swell of wavelength ≈ 200m and internal waves of wavelength > 400m. Only internal waves of wavelength > 400m were seen in the scene off Goa observed on 11 March 1992. The scenes off Visakhapatnam and Paradeep did not show any wave like features, the latter appearing to be of “white noise” nature. There exists a 180° ambiguity in wave direction observed from radar imageries. Based on the method of Atanassov et al (1985), a computer program has been developed for the removal of the 180° ambiguity by using two images of the same area separated by a time interval which is small compared to the period of the dominant waves. The computer program has been successfully tested with computer simulated images.     

Accurate estimation of correlation in InSAR observations

Interferometric synthetic aperture radar (InSAR) correlation, a measure of the similarity of two radar echoes, provides a quantitative measure of surface and subsurface scattering properties and hence surface composition and structure. Correlation is observed by comparing the radar return across several nearby radar image pixels, but estimates of correlation are biased by finite data sample size and any underlying interferometer fringe pattern. We present a method for correcting bias in InSAR correlation measurements resulting in significantly more accurate estimates, so that inverse models of surface properties are more useful. We demonstrate the value of the approach using data collected over Antarctica by the Radarsat spacecraft.     

Potential to estimate the canting angle of tilted structures in clouds from microwave radiances around 183 GHz

The effects of cloud structures on microwave radiances at frequencies from 89-190 GHz are investigated by simulations using the Goddard cumulus ensemble model data as input for a radiative transfer model. It was found that the brightness temperatures at these frequencies have different sensitivities to clouds with a tilted structure. The different sensitivities to altitude and amount of hydrometeors allow the estimation of the canting angle and tilt direction of tilted clouds using brightness temperatures at the water vapor channels at 183.3 /spl plusmn/ 1 and 183.3 /spl plusmn/ 7 GHz. The estimated canting angle and tilt direction are in agreement with the model situation. This method provides a potential to estimate tilted convective structures from microwave radiometric observations at 183.3 /spl plusmn/ 1 and 183.3 /spl plusmn/ 7 GHz. It is applied to a tilted storm observed from the National Aeronautics and Space Administration's ER-2 aircraft flying at about 20 km on August 26, 1998 during the third Convection and Moisture Experiment using the observed downlooking brightness temperatures at the water vapor channels of a Millimeter-wave Imaging Radiometer. The estimated results are in good agreement with the realistic storm situation obtained from the simultaneous observations of the ER-2 Doppler radar. This method also provides information about the vertical displacement of cloud structure and thereby to estimate the accurate location of surface rainfall. This is important when validating precipitation retrieval based on observations of the ice scattering above surface rainfall against surface rain observations using the microwave frequencies sensitive to high altitudes.