Simulation of interferometric SAR response for characterizing thescattering phase center statistics of forest canopies

A coherent scattering model for tree canopies is employed in order to characterize the sensitivity of an interferometric SAR (INSAR) response to the physical parameters of forest stands. The concept of an equivalent scatterer for a collection of scatterers within a pixel, representing the vegetation particles of tree structures, is used for identifying the scattering phase center of the pixel whose height is measured by an INSAR. Combining the recently developed coherent scattering model for tree canopies and the INSAR Δk-radar-equivalence algorithm, accurate statistics of the scattering phase-center location of forest stands are obtained numerically for the first time. The scattering model is based on a Monte Carlo simulation of scattering from fractal-generated tree structures, and therefore is capable of preserving the absolute phase of the backscatter. The model can also account for coherent effects due to the relative position of individual scatterers and the inhomogeneous extinction experienced by a coherent wave propagating through the random collection of vegetation particles. The location of the scattering phase center and the correlation coefficient are computed using the Δk-radar equivalence simply by simulating the backscatter response at two slightly different frequencies. The model is successfully validated using the measured data acquired by JPL TOPSAR over a selected pine stand in Raco, MI. A sensitivity analysis is performed to characterize the response of coniferous and deciduous forest stands to a multifrequency and multipolarization INSAR in order to determine an optimum system configuration for remote sensing of forest parameters     

Directional Reflectance Distributions of a Hardwood and Pine Forest Canopy

The directional reflectance distributions for both a hardwood and pine forest canopy at Beltsville, Maryland, were measured in June as a function of sun angle from a helicopter platform using a hand-held radiometer with AVHRR band 1 (0.58-0.68 ?m) and band 2 (0.73-1.1 ?m). Canopy characteristics were measured on the ground. The reflectance distributions are reported and compared to the scattering behavior of agricultural and natural grassland canopies. In addition, the three-dimensional radiative transfer model of Kimes was used to document the unique radiant transfers that take place in forest canopies due to their special geometric structure. Measurements and model simulations showed that the scattering behavior of relatively dense forest canopies is similar to the scattering behavior of agricultural crops and natural grasslands. Only in more sparse forest canopies with significant spacing between the tree crowns (or clumps of tree crowns) does the scattering behavior deviate from homogeneous agricultural and natural grassland canopies. This clumping of vegetation material has two effects on the radiant transfers within the canopy: A) it increases the probability of gap to the understory and/or soil layers that increases the influence of the scattering properties of these lower layers; and B) it increases the number of low transmitting clumps of vegetation within the scene causing increased backscatter and decreased forward scatter to occur relative to the homogeneous case. Both effects, referred to as phenomenon A and B, respectively, tend to increase backscatter relative to forward scatter.     

Solar zenith angle effects on forest canopy hemisphericalreflectances calculated with a geometric-optical bidirectionalreflectance model

The bidirectional reflectance distribution function (BRDF) provided by the Li-Strahler geometric-optical forest canopy model has been integrated to provide spectral instantaneous hemispherical reflectances of sparsely vegetated surfaces. Further integration over the Sun's zenith angles can yield daily or longer interval hemispherical reflectances as well. A variety of simulated canopies were modeled with varying solar angles. In all cases, as the geometric-optical model introduced increased shadowing of the surface with increased solar zenith angle, the direct-beam hemispherical surface reflectance gradually decreased. The hemispherical reflectance values are direct beam calculations and do not directly include canopy multiple scattering and leaf specularity or consider the impact of diffuse irradiance. These limitations are acceptable for sparse canopies, in which 3D shadowing effects are large. However, radiative transfer calculations have shown that these phenomena must be incorporated before truly realistic modeling of hemispherical surface reflectances can be achieved for dense canopies     

Coherent effects in microwave backscattering models for forestcanopies

In modeling forest canopies, several scattering mechanisms are taken into account, (1) volume scattering; (2) surface-volume interaction; (3) surface scattering from forest floor. Depending on the structural and dielectric characteristics of forest canopies, the relative contribution of each mechanism in the total backscatter signal of an imaging radar can vary. In this paper, two commonly used first-order discrete scattering models, distorted Born approximation (DBA) and radiative transfer (RT) are used to simulate the backscattered power received by polarimetric radars at P-, L-, and C-bands over coniferous and deciduous forests. The difference between the two models resides on the coherent effect in the surface-volume interaction terms. To demonstrate this point, the models are first compared based on their underlying theoretical assumptions and then according to simulation results over coniferous and deciduous forests. It is shown that by using the same scattering functions for various components of trees (i.e. leaf, branch, stem), the radiative transfer and distorted Born models are equivalent, except in low frequencies, where surface-volume interaction terms may become important, and the coherent contribution may be significant. In this case, the difference between the two models can reach up to 3 dB in both co-polarized and cross-polarized channels, which can influence the performance of retrieval algorithms     

Radiative transfer model for microwave bistatic scattering from forest canopies

A bistatic forest scattering model is developed to simulate scattering coefficients from forest canopies. The model is based on the Michigan Microwave Canopy Scattering (MIMICS) model (hence called Bi-MIMICS) and uses radiative transfer theory, where the first-order fully polarimetric transformation matrix is used. Bistatic radar systems offer advantages over monostatic radar systems because of the additional information provided by the diversity of the geometry. By simulating the forest canopy scattering from multiple viewpoints, we can better understand how the forest scatterers' shape, orientation, density, and permittivity affect the canopy scattering. Bi-MIMICS is parametrized using selected forest stands with different canopy compositions and structure. The simulation results show that bistatic scattering is more sensitive to forest biomass changes than backscattering. Analyzing scattering contributions from different parts of the canopy gives us a better understanding of the microwave's interaction with the tree components. The ground effects can also be studied. Knowledge of the canopy's bistatic scattering behavior combined with additional synthetic aperture radar measurements can be used to improve forest parameter retrievals. The simulation results of the model provide the required information for the design of future bistatic radar systems for forest sensing applications.     

Three-dimensional forest light interaction model using a MonteCarlo method

A model for light interaction with forest canopies is presented, based on Monte Carlo simulation of photon transport. A hybrid representation is used to model the discontinuous nature of the forest canopy. Large scale structure is represented by geometric primitives defining shapes and positions of the tree crowns and trunks. Foliage is represented within crowns by volume-averaged parameters describing the structural and optical properties of the scattering elements. Simulation of three-dimensional photon trajectories allows accurate evaluation of multiple scattering within crowns, and between distinct crowns, trunks and the ground surface. The sky radiance field is treated as anisotropic and decoupled from bidirectional reflectance calculation. Validation has been performed on an example of dense spruce forest. Results show close agreement between model predictions and field measurements of bidirectional reflectance, high-resolution spectra and hemispherical albedo     

Numerical Studies of Scattering Properties of Leaves and Leaf Moisture Influences on the Scattering at Microwave Wavelengths

This paper uses a 3-D finite-difference time-domain method to accurately calculate the single-scattering properties of randomly oriented leaves and evaluate the influence of vegetation water content (VWC) on these properties at frequencies of 19.35 and 37.0 GHz. The studied leaves are assumed to be thin elliptical disks with two different sizes and have various VWC values. Although leaf moisture causes considerable absorption in the scattering process, the effective efficiencies of extinction and scattering of leaves essentially linearly increase with VWC, which is critical for forest remote sensing. Calculated asymmetry factors and phase functions also indicate that there is a significant amount of scattered energy at large scattering angles at microwave wavelengths. This paper can improve the modeling of the radiative transfer by vegetation canopies at the higher frequencies of the microwave spectrum, which is important for passive microwave remote sensing.     

Horizontal propagation through periodic vegetation canopies

Experimental data suggest that a semideterministic technique is needed to model certain man-made vegetation canopies such as orchards, plantations, and row crops. A two-dimensional model has been developed to explain wave propagation through such canopies. The model is intended for media containing vertical cylinders, representing the stalks, and randomly oriented disks, representing the leaves. The formulation treats the canopy as a one-dimensional array of parallel rows, with each row comprising an array of parallel stalks and a random distribution of leaves. The quasi-static approximation used for computing scattering by the leaves is valid only when the dimensions of the leaves are smaller than the wavelength. The model is a field approach accounting for all coherent, multiple interactions occurring in the canopy. The experimental component of this study includes measurements of the attenuation and phase shift patterns for horizontally and vertically polarized waves transmitted through a fully grown canopy of corn plants observed at 1.5 GHz. The model has good agreement with the experimental results     

Radar backscattering model for multilayer mixed-species forests

A multilayer canopy scattering model is developed for mixed-species forests. The multilayer model provides a significantly enhanced representation of actual complex forest structures compared to the conventional canopy-trunk layer models. Multilayer Michigan Microwave Canopy Scattering model (Multi-MIMICS) allows overlapping layer configuration and a tapered trunk model applicable to forests of mixed species and/or mixed growth stages. The model is the first-order solution to a set of radiative transfer equations and includes layer interactions between overlapping layers. It simulates SAR backscattering coefficients based on input dimensional, geometrical, and dielectric variables of forest canopies. The Multi-MIMICS is an efficient realization of actual forest structures and can be shaped for specific interest of forest parameters. We present the model's application and validation in the paper. The model is parameterized using data collected from a 220,000-ha area of forests in central Queensland, Australia. Fifteen 50/spl times/50 m test sites representing the general forest diversity and growth stages are chosen as ground truth. Polarimetric backscattering airborne SAR (AIRSAR) data of the same area are acquired to validate the model simulations. The model predicts SAR backscattering coefficients of the test areas. Simulation results show a good agreement with AIRSAR data at most frequencies and polarizations. The simulated backscattering coefficient from the multilayer model and the standard MIMICS are also compared and significant improvements are observed.     

Side-Face Effect of a Dielectric Strip on Its Optical Properties

Light scattering by horizontally oriented platelike particles under normal incidence, such as ice plates or tree leaves under spaceborne lidar or radar waves, needs to be investigated for remote sensing of cirrus clouds or vegetation canopies. The solutions from the conventional geometrical ray tracing method for the scattering of electromagnetic waves by these particles are quite inaccurate because of the singularity problem that is inherent to this method. The scattering properties of large horizontally oriented platelike particles are usually approximated by using physical optics or electromagnetic wave theory while ignoring the side-face effect of the plates. In this paper, to examine the effect of side faces on light scattering by platelike particles, a 2-D finite-difference time-domain technique is applied to calculate light scattering by horizontally oriented ice and leaf strips under normal or quasi-normal incidence. It is found that for moderate-sized strips, the side faces of the particles scatter a significant amount of energy, resulting in strong maxima in the scattering phase function at certain scattering angles. By ignoring the effect of side faces, the scattering phase functions derived from electromagnetic wave theory have significant errors for small or moderate-sized strips. However, the ratio of the amount of energy scattered by the side faces to the total scattered energy decreases with the increase of strip width. When the size parameter of the strip is in the limit of geometric optics, the side-face effect is reduced to a negligible amount. However, even in this case, the polarization degrees from the approximation solutions of physical optics or electromagnetic wave theory ignoring the side-face effect still have large errors.      

Measurements of the propagation parameters of tree canopies at MMWfrequencies

The presence of trees in a given scene can hamper detection of nearby targets by millimeter-wave (MMW) radars especially at near grazing incidence. Proper characterization of scattering and attenuation in tree canopies is important for optimal detection algorithms. In this paper, a new technique for determining the extinction and volume backscattering coefficients in tree canopies using the measured radar backscatter response is proposed and verified experimentally. The technique, which can be applied to already available wideband radar backscatter data, is used to compute the extinction and volume backscattering coefficients of different tree canopies under various physical conditions. The dynamic range of these coefficients are presented and results at 35 GHz are compared with results at 95 GHz     

Radiophysical investigations of sea roughness (radiooceanography) at the Ukrainian Academy of Sciences

Experiments on the scattering of radio waves in the range 200 m to 3 cm from a rough sea surface are described. Amplitude, frequency, and space-time characteristics of scattered radio signals at different states of the sea surface are presented. It is shown that the problem of the short and medium wave scattering from the sea can be solved by the perturbance method. In this case the mechanism of scattering is of "resonant" character. The intensity of the backscatter signals is proportional to the density of the spatial spectrum on the half-length of the radio waves. The high frequency radio wave scattering is well described by a two-scale model of the scattering surface, "ripple on the large wave." The intensity of scattered radio signals is also proportional to the spectrum density of "ripples" whose length is approximately equal to half a radio wave. The effect of the large waves is to modulate the amplitude of a scattered radio signal and to broaden its frequency spectrum. Methods of solution of the reverse problem were considered. This allowed determination of parameters of sea roughness by characteristics of scattered radio signals. The principles of design of the corresponding equipment are described.     

Low VHF-band backscatter from coniferous forests on sloping terrain

Low-frequency synthetic aperture radar (SAR) is a promising technique for stem volume retrieval, particularly for dense forests, due to the good penetration of forest canopies. However, it is well known that the dominant scattering mechanism, the trunk-ground dihedral interaction, decreases rapidly on sloping terrain. In this paper, we use low VHF-band SAR data, collected with CARABAS over dense coniferous forests in Sweden, to examine the effect of topography. Using flight passes with different headings, the effect of slope and aspect angle on backscatter is characterized. For tall trees (/spl sim/30 m), on the steepest slopes in the test-site (up to /spl sim/12/spl deg/), differences of up to 8 dB are observed between images acquired with different look directions relative to the slope. A physical model is developed to investigate the different scattering mechanisms and their sensitivity to terrain slopes. The model shows that the trunk-ground scattering still dominates the response for large trees on moderate slopes, and a semiempirical model for the effect of topography on backscatter is proposed. The model shows good agreement with measurements, indicating the possibility of using it to compensate for the effects of sloping terrain when retrieving stem volume in coniferous forest.     

Modeling the Radiant Transfers of Sparse Vegetation Canopies

The three-dimensional radiative transfer model of Kimes [2] was used to extend our understanding of the physical principles causing the scattering dynamics in sparse vegetation canopies (? 50-percent ground cover). The model was upgraded by including an aniotropic scattering algorithm for soil developed by Walthall et al. [7]. The model was validated using measured directional reflectance data that covered the entire exitance hemisphere. Two canopies were chosen to present in this study-an orchard grass canopy (50-percent ground cover) and a hard wheat canopy (11-percent ground cover). These canopies showed the typical scattering behavior of canopies with low and intermediate vegetation density. A red wavelength (0.58-0.68 pm) band was used throughout the study. A number of phenomena contributed to the directional reflectance distributions observed in the field. These include: 1) the strong anisotropic scattering properties of the soil, 2) the geometric effect of the vegetation probability of gap function on the soil anisotropy and solar irradiance, and 3) the anisotropic scattering of vegetation which is controlled by the phase function (for an infinitely small volume of representative leaves) and geometric Effect 1 (cause by layering of leaves). These phenomena as identified in this paper account for the major scattering behavior of observed data sets of directional reflectance distributions. Such knowledge provides an intelligent basis for defining specifications of earth-observing sensor systems and for inferring important aspects of physical and biological processes of the plant system.     

Polarimetric channel characterization of foliage for performanceassessment of GPS receivers under tree canopies

The attenuation, depolarization, and fluctuation of a microwave signal going through a tree canopy are investigated by developing a Monte Carlo based coherent scattering model. In particular, the model is used to analyze the performance of Global Positioning System (GPS) receivers under tree canopies. Also the frequency and time-domain channel characteristics of a forest are investigated when a transmitter is outside and a receiver is inside a forest. A fractal algorithm (Lindenmayer system) is used to generate the structure of coniferous or deciduous trees whose basic building blocks are arbitrarily oriented finite cylinders, thin dielectric needles, and thin dielectric disks. Attenuation and phase change of the mean field through foliage is accounted for using Foldy's approximation. Scattering of the mean field from individual tree components and their images in the underlying ground plane are computed analytically and added coherently. Since tree trunks and some branches are large compared to the wavelength and may be in the close proximity of the receiver, a closed-form and uniform expression for the scattered near-field from dielectric cylinders is also developed. Monte Carlo simulation of field calculation is applied to a cluster of trees in order to estimate the statistics of the channel parameters, such as the probability density function (pdf) of the polarization state of the transmitted field, path loss, and the incoherent scattered power (the second moment of the scattered field), as a function of the observation point above the ground     

海洋粗糙面全极化电磁散射特性研究

针对基于几何光学-微扰法(GO-SPM)的传统双尺度模型对截断波数敏感的问题,该文建立了一种基于几何光学-小斜率近似(GO-SSA)的改进双尺度模型。该模型将小尺度部分的微扰法替换为一阶小斜率近似,并改进镜向分量的几何光学解。仿真表明该方法在获取传统双尺度精度的同时,不需要考虑截断波数的选取问题。针对Elfouhaily海浪谱模型的特点,简化GO-SSA积分形式。最后采用GO-SSA对以Elfouhaily海浪谱建模的海表面的单双站极化散射特性进行了仿真分析。发现双站极化散射中,粗糙表面的斜率调制导致交叉极化出现了异于传统模型的分布。同时在全空域的双站极化散射仿真中,发现所有的极化方式在方位向内均存在极小值。该极小值的大小与环境参数密切相关,在环境参数反演方面具有应用潜力。     

An enhanced millimeter-wave foliage propagation model

In this paper, the behavior of wave propagation through coniferous forest stands at millimeter-wave frequencies is characterized both theoretically and experimentally. A coherent wave propagation model is used to simulate the propagation through foliage. The coherent model is composed of two components: a forest stand generator that makes use of a stochastic fractal model, and an electromagnetic model that makes use of Foldy's approximation and single scattering. An outdoor measurement system is designed and used for characterizing the channel behavior for a pine tree stand at Ka-band (35 GHz). In this experiment, 84 independent spatial samples of transmitted signal through the pine stand were collected to obtain the path-loss statistics. The comparison between measurement and simulation results showed that single scattering theory overestimates the wave attenuation through foliage. To improve the accuracy of the coherent model, partial multiple scattering occurred among the needles of highly dense leaf clusters must be included for the estimation of the coherent attenuation. Distorted Born approximation is used to macromodel the scattering pattern from needle clusters. This technique has comparable accuracy and requires much less computational resources than a full-wave solution, such as method of moment. By including multiple scattering effects of needle clusters in the simulation model, much better agreement is obtained for both mean and standard deviation of the path-loss.     

一种新的植被热红外辐射模型

以植被的能量平衡方程为基础,根据植被内的湍流交换模式及辐射能的传输过程,考虑了风速、气温和水汽压廓线等影响,建立了一个适合于林冠的3层植被热红外辐射模型.用了新的计算方法,大大地提高了模型收敛速度.实验数据检验结果表明:所建的模型能正确地模拟植被各层温度随不同气象条件、不同植被结构等参数的变化.     

风轮机雷达散射特性仿真及微多普勒特征分析

风轮机复杂的电磁散射特性,会对其附近的空管通信、导航和监视等电子设备产生严重影响.研究风轮机的电磁散射特性,可为风轮机杂波检测和抑制提供理论依据,对保证空中交通安全具有重要的意义.论文首先基于风轮机散射点叠加的理论,考虑了雷达入射波到风轮机叶片和桅杆的初始相位以及入射波方位角和俯仰角对回波的影响,将单基地回波模型扩展到双基地模型.同时,在散射点叠加模型的基础上,提出了基于混合模型的风轮机散射特性分析.混合模型结合了散射点叠加模型和电磁仿真软件FEKO的优点,考虑了电磁波在叶片和桅杆上的反射系数等因素对回波的影响,可以实现任意观测点处的电磁散射特性计算及其微多普勒特征的分析.最后,分别对散射点叠加模型、FEKO以及混合模型的风轮机电磁散射特性分析方法进行了对比分析,给出了各自的优缺点及其适用场合.     

Quantitative analysis of RADARSAT SAR data over a sparse forest canopy

This article studies the behavior of the backscattering coefficient of a sparse forest canopy composed of relatively short black spruce trees. Qualitative analysis of the multiangular data measured by the RADARSAT synthetic aperture radar (SAR) sensor shows a good agreement with surface and vegetation volume scattering fundamental behaviors. For a quantitative analysis, allometric equations and measurements of tree components collected within the framework of the Extended Collaboration to Link Ecophysiology and Forest Productivity (ECOLEAP) project are used, in an existing multilayer radiative transfer model for forest canopies, to simulate the RADARSAT SAR data. In our approach, the fractional cover of trees estimated from aerial photographs is used as a weighting parameter to adapt the closed-canopy backscattering model to the sparse forest under study. Our objective is to analyze the sensitivity of the backscattering coefficient as a function of sensor configuration, soil wetness, forest cover, and forest structural properties in order to determine the suitable soil, vegetation, and sensor parameters for a given thematic application. For the entire incidence angle domain (20/spl deg/ to 50/spl deg/) of the sensor, simulations show that over a sparse forest composed of mature trees the monitoring of the ground surface is possible only under very wet soil conditions. Therefore, this article informs about the ability of the RADARSAT SAR sensor in monitoring wetlands.