Microwave Attenuation Properties of Vegetation Canopies

A major impediment to the understanding and modeling of propagation through and backscattering and emission from vegetation canopies has been the lack of canopy attenuation data as a function of frequency, incidence angle, and polarization configuration. This paper presents the results of attenuation experiments conducted for canopies of winter wheat and soybeans in the late spring and early summer of 1984. Attenuation data were acquired at 1.55, 4.75, and 10.2 GHz for horizontal and vertical polarizations at incidence angles near 20° and 50°. In addition, wheat decapitation and soybean defoliation experiments were conducted to evaluate the relative importance of different canopy constituents (such as heads, leaves, and stalks) to the total canopy attenuation. The measured data were compared to calculations based on a model that treats the stalks as parallel elements of a uniaxial crystal and the leaves and branches as randomly oriented disks and needles, respectively. Very good agreement was obtained between theory and experimental observations for the soybean canopy for both polarizations and for the wheat canopy for vertical polarization; however, the model consistently underestimated wheat attenuation (relative to the data) for horizontal polarization. This deficiency of the model is attributed to the fact that it considers all the stalks to be vertically oriented, whereas in reality the stalks exhibit an orientation distribution, although it is centered around the vertical direction.     

Exploring the potential for multipatch soil-moisture retrievalsusing multiparameter optimization techniques

This paper explores the potential to retrieve surface soil moisture and optical depth simultaneously for several different patches of land cover in a single pixel from dual polarization, multiangle microwave brightness temperature observations such as will be provided by, for instance, the Soil Moisture and Ocean Salinity (SMOS) mission. MICRO-SWEAT, a coupled land-surface and microwave emission model, was used in a. year-long simulation to define the patch-specific soil moisture, optical depth, and synthetic, pixel-average microwave brightness temperatures similar to those that will be provided by SMOS. The microwave emission component of MICRO-SWEAT also forms the basis of an exploratory retrieval algorithm in which the difference between (synthetic) observations of microwave brightness temperatures and modeled, pixel-average microwave brightness temperatures for different input values of soil moisture and optical depth is minimized using the shuffled complex evolution (SCE) optimization procedure. Results are presented for two synthetic pixels, one with eight patches, where only the soil moisture is retrieved, and one with five patches, where both the soil moisture and the optical depth are retrieved     

Microwave polarization index for monitoring vegetation growth

It is known that microwave emission for soil is partially polarized, whereas the polarization degree of radiation from crops is either very small or equal to zero. The effect of vegetation on the polarized emission from the soil is analyzed by means of a model based on the radiative transfer theory. A comparison between the values predicted by the model and the experimental data obtained at 10 and 36 GHz shows that the model is particularly adequate for corn at 10 GHz, in which case different values of leaf area index can be estimated. At 36 GHz, the polarization degree changes very rapidly as vegetation grows and it is possible to distinguish only bare soil from vegetated soil     

Parameter sensitivity of soil moisture retrievals from airborne C- and X-band radiometer measurements in SMEX02

Among passive microwave frequencies, sensors operating at C- and X-band frequencies have been used with some success to estimate near-surface soil moisture from aircraft and satellite platforms. The objective of this paper is to quantify the sensitivities of soil moisture retrieved via a single-channel single-polarization algorithm to the observed brightness temperature and to retrieval algorithm parameters of surface roughness, vegetation B parameter, and single-scattering albedo. Examination of the regions within the parameter space that produce accurate soil moisture retrievals reveals that reasonably accurate retrievals can be made over a range of conditions using a fixed set of input parameters. Retrievals with horizontally polarized brightness temperature observations are more consistent than with vertically polarized observations. At horizontal polarization, sensitivity to the input parameters is much greater for wet soils than for dry soils, whereas for vertical polarization the moisture dependence is much weaker. At vertical polarization, sensitivities to variations in all parameters are much lower. To ensure that retrieval accuracy specifications are consistently met, high soil moisture conditions should be used in defining parameter accuracy requirements. Given the spatial and temporal variability of vegetation and soil conditions, it seems unlikely that, for regions with substantial rapidly growing vegetation, the accuracy requirements for model parameters in a single-frequency, single-polarization retrieval algorithm can be met with current satellite products. For such conditions, any soil moisture retrieval algorithm using parameterizations similar to those of this study may require multiple frequencies, polarizations, or look angles to produce stable, reliable soil moisture estimates.     

A simple parameterization of the L-band microwave emission fromrough agricultural soils

A simple model for simulating the L-band microwave emission from bare soils is developed. The model is calibrated on a large set of measurements obtained during a three-month period over seven plots covering a wide range of surface roughness (representing the total range which can be expected on agricultural fields), soil moisture, and temperature conditions. The approach is based on the parameterization of an effective roughness parameter as a function of surface characteristics: surface roughness (standard deviation of height and correlation length) and the surface soil moisture. The parameterizations that are developed are independent of incidence angle and polarization and are valid over a large range in surface roughness conditions, representative of most of typical agricultural bare fields, from very smooth (rolled field after sowing) to very rough surfaces (deeply plowed soil). This approach will enable the use of microwave radiometric observations for soil moisture retrieval over agricultural areas     

基于目标边缘轮廓偏振特征的姿态分析初探

通过微面元理论的函数模型,基于红外偏振辐射传输方程,推导分析红外偏振角与目标表面折射率、反射率及探测角度等关系的数学模型,通过合理简化模型仿真得出偏振角随入射角的单调变化曲线;开展红外偏振成像试验,试验数据分析与仿真结果一致,这表明:目标边缘轮廓的偏振角特征与探测波长的相关性可以忽略,在目标与背景辐射相差较小的情况下,采用光谱辐射亮度对比度的方法无法区分目标,而通过偏振角对比度可以明显区分-目标边缘特征,并提出通过基于目标几何特征以及探测位置,反演解算目标的姿态信息的新方法.     

L-band radiometer measurements of soil water under growing clover grass

A field experiment with an L-band radiometer at 1.4 GHz was performed from May-July 2004 at an experimental site near Zurich, Switzerland. Before the experiment started, clover grass was seeded. Thermal infrared, in situ temperature, and time-domain reflectometer (TDR) measurements were taken simultaneously with hourly radiometer measurements. This setup allowed for investigation of the microwave optical depths and mode opacities (parallel and perpendicular to the soil surface) of the clover grass canopy. Optical depths and opacities were determined by in situ analysis and remotely sensed measurements using a nonscattering radiative transfer model. Due to the canopy structure, optical depth and opacity depend on the polarization and radiometer direction, respectively. A linear relation between vegetation water-mass equivalent and polarization-averaged optical depth was observed. Furthermore, measured and modeled radiative transfer properties of the canopy were compared. The model is based on an effective-medium approach considering the vegetation components as ellipsoidal inclusions. The effect of the canopy structure on the opacities was simulated by assuming an anisotropic orientation of the vegetation components. The observed effect of modified canopy structure due to a hail event was successfully reproduced by the model. It is demonstrated that anisotropic vegetation models should be used to represent the emission properties of vegetation. The sensitivity of radiometer measurements to soil water content was investigated in terms of the fractional contribution of radiation emitted from the soil to total radiation. The fraction of soil-emitted radiation was reduced to approximately 0.3 at the most developed vegetation state. The results presented contribute toward a better understanding of the interaction between L-band radiation and vegetation canopies. Such knowledge is important for evaluating data generated from future satellite measurements.     

Dynamical control of directional nonlinear scattering from metallic nanoantennas by three-dimensional focal polarization orientation

We demonstrate that the directionality of far-field second harmonic(SH) emission generated from individual gold nanosphere can be flexibly engineered by manipulating three-dimensional(3D) focal polarization orientation of the excitation field, which is explained by the coherent interference between SH dipolar and quadrupolar emission modes. The SH dipolar emission mode is independent of the polarization direction of the fundamental field whereas the evolution of the focal polarization orientation can dramatically modify the quadrupolar emission pattern. Therefore, the resultant SH emission pattern has a polarization-dependent behavior and side scattering almost perpendicular to the propagation direction of the incident light can be observed under a specific condition. Our findings provide a novel degree of freedom for all-optical control of directional nonlinear scattering from single nanoantenna, thereby opening new possibilities for future potential applications.     

CdSe量子棒的线偏振光学性质

用有效质量包络函数理论研究了CdSe量子棒的线偏振光学性质,考虑了形状和磁场的影响.发现CdSe量子球具有负的线偏振因子(xy-平面内的线偏振发射),而小半径长量子棒具有正的线偏振因子(z-方向的线偏振发射).z-方向就是晶格c-轴方向.因六角晶格对称性和晶格场劈裂能的影响,大半径长量子棒具有负的线偏振因子.线偏振因子随着z-方向磁场的增加而减小,可能由正变负,即z-方向的线偏振发射相对xy-平面内的线偏振发射减小了.     

CdSe量子棒的线偏振光学性质

用有效质量包络函数理论研究了CdSe量子棒的线偏振光学性质,考虑了形状和磁场的影响.发现CdSe量子球具有负的线偏振因子(xy-平面内的线偏振发射),而小半径长量子棒具有正的线偏振因子(z-方向的线偏振发射).z-方向就是晶格c-轴方向.因六角晶格对称性和晶格场劈裂能的影响,大半径长量子棒具有负的线偏振因子.线偏振因子随着z-方向磁场的增加而减小,可能由正变负,即z-方向的线偏振发射相对xy-平面内的线偏振发射减小了.     

锥面共形阵列天线盲极化DOA估计算法

 由于共形载体曲率的影响,共形阵列天线中各阵元单元方向图具有不同的指向,使得共形阵列天线具有了多极化特性(Polarization Diversity),为了描述共形阵列天线的多极化特性,通常在共形阵列天线的快拍数据模型中引入阵列入射信号的极化参数,因此共形阵列天线的DOA(Direction-Of-Arrival)估计需要与阵列入射信号极化参数联合估计.本文提出了一种盲极化DOA估计算法,通过在锥面共形阵列天线中设置三对特殊子阵,利用ESPRIT(Estimation of Signal Parameters via Rotational Invariance Techniques)算法,将入射信号极化参数与二维角参数去耦合,在入射信号极化参数未知条件下实现了高分辨DOA估计,并对估计性能进行了理论分析与推导,给出了参数估计的CRB(Cramer-Rao Bound),通过Monte Carlo仿真实验验证了DOA估计算法的有效性.     

Linearly polarized emission from PTCDI-C8 one-dimensional microstructures

Linearly polarized emission has been observed from a crystalline one-dimensional (1D) microstructure fabricated from N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C₈) molecules via solution phase self-assembly. Rotating microscopy imaging of a 1D microstructure under crossed polarization was performed for the investigation of polarized emission. The anisotropy birefringence was maximum only when the 1D microstructure was aligned 45° to the direction of the polarizer and it was minimum when aligned parallel to the polarizer implying that the transmission axis of the 1D microstructure is perpendicular to its π–π stacking direction. A model has been proposed to explain linearly polarized emission from the microstructure.     

Polarization control in strained T-bar VCSELs

A vertical-cavity surface-emitting laser device with polarization stable emission at a predefined angle is investigated. The polarization flip probability is reduced from 32% to 2% when compared to circular symmetric devices and the polarization angle follows the direction of the deliberately introduced strain.     

Microwave emission of vegetation: sensitivity to leafcharacteristics

The effects of leaf characteristics on the microwave emission of land surfaces are analyzed. In order to simulate these effects, a radiative transfer model is presented. The medium consists of a vegetated layer containing randomly oriented leaves, modeled as elliptic-shaped scatterers, over the ground surface. Radiative transfer equations are solved with a discrete-ordinate-eigenanalysis method. The calculation of the phase matrix of the elliptic scatterers is based on the generalized Rayleigh-Gans approximation, which increases the frequency range of the modeling. The sensitivity of brightness temperature and polarization ratio to leaf characteristics, volume fraction, gravimetric moisture, size, shape, and inclination distribution is investigated at C-, and X-band. The behavior of the simulated emission of a soybean canopy versus frequency and incidence angle is studied for different soil moisture levels. Up to 10 GHz the microwave emission appears to contain significant information on underlying soil moisture     

Understanding the relationships between radar response patterns andthe bio- and geophysical parameters of urban areas

This paper reviews the current understanding of the relationships between radar response patterns and the bio- and geophysical parameters of urban areas. Specifically, it examines the effects of radar system, ground target, and environmental factors on the intensity and pattern of radar returns from urban features. System parameters considered include radar signal wavelength, polarization, incident angle, and look direction. Ground target factors are the dielectric properties and surface roughness of urban features. The environmental variables examined entail terrain relief, street and structure configuration, soil and vegetation types, and the composition, fragmentation, and variation of urban infrastructure elements. The effects of radar data processing techniques on the detectability of settlements and the accuracy of urban land use/land cover mapping are also described     

Source localization and polarization determination in low frequency satellite radio astronomy

The authors review the techniques available for finding out the location and polarization of a radio source using antennas with low directivity, that is, electric or magnetic antennas with dimensions less than the wavelength. The use of such antennas is imposed by the combination of the low frequency of observation and of the dynamical constraints upon the antenna length particularly in the case of planetary probes. First they summarize the relationship of an arbitrary antenna to the parameters describing the radiation field, emphasizing the importance of the antenna polarization vector. The application of these results defines precisely the limited information available using short electric antennas. In principle, it is possible to determine the direction (except for the sense) and also the polarization of a point source, using either three such antennas on a three-axis stabilized spacecraft or two antennas on a spinning spacecraft. With a single antenna which is neither parallel nor perpendicular to the spin axis it is possible to determine the source direction with a πambiguity but not its complete polarization. If the single antenna is perpendicular to the spin axis (as is usually the case) only the great circle containing the source direction and the spin axis can be obtained, and this at the expense of assumptions about the source polarization. Source localization requires, in addition to the direction, either observations from another spacecraft or some assumption about the source mechanism and the spatial distribution of the relevant plasma parameters. The application of these methods to the observations of solar radio bursts and terrestrial kilometric radiations is presented.     

Controlled polarization switching in VCSELs by means of asymmetric current injection

We have investigated the potential of asymmetric current injection for polarization switching in GaAs-based intracavity contacted vertical-cavity surface-emitting lasers using two sets of p- and n-type contacts per device. When using the contacts aligned along the [11~0] crystal direction, the observed laser polarization is parallel to [110], whereas, using the contacts along the [110] crystal direction, the polarization of the laser emission switches to a direction making an angle of 25/spl deg/-90/spl deg/ towards [110]. To overcome this peculiar result, a careful design of the contact layers in the intracavity structure is required.     

Microwave Dielectric Model for Aggregated Soils

In order to understand the interactions of soil properties and microwave emission better, a series of field experiments were conducted in 1984. Small plots were measured with a truck-mounted passive microwave radiometer operating at 1.4 GHz. The microwave data were collected concurrently with ground observations of soil moisture and bulk density. Treatment effects included different soil moisture contents and bulk densities. Evaluations of the data showed that commonly used models of the dielectric properties of wet soils could not explain the observations. This conclusion was based on the fact that the roughness parameters determined through optimization were significantly larger than those observed in similar other investigations. These discrepancies are most likely due to inadequate characterization of the effects of soil structure. Commonly used models assume a homogeneous three phase mixture of soil solids, air, and water. Under tilled conditions the soil is actually a two phase mixture of aggregates and voids. Appropriate dielectric models for this tilled condition were evaluated and found to match the observed data. These results indicate that previous conclusions concerning the effects of surface roughness in tilled fields may be incorrect, and the improved model may explain some of the inconsistencies encountered in roughness modeling.     

极化情况下高功率微波稀布阵计算模型

建立了考虑极化情况下高功率微波稀布阵合成场的计算模型,分析了阵列合成场大小与极化及阵元初相位关系。极化方向相同时,二元线阵场分布为明暗相间的条纹,三元面阵场分布为网状结构。阵元极化方向及阵元初相位共同影响合成场的叠加效果,阵元的极化决定阵列合成场的最大值,阵元初相位在此基础上对阵列叠加效果进一步调节。对于二元线阵,极化方向夹角越小,合成叠加效率越高;对三元以上的多元面阵,其极化方向组合方式复杂,可通过阵元位置和目标位置模拟阵列合成场的叠加效率,三元稀布面阵的合成场强归一化最大值在2.22~2.43之间,与理想合成场最大值3有一定的差距。该模型提高了稀布阵列合成场强的计算准确度,对阵列高功率微波进行天线阵列设计,进一步提升目标区域的功率密度是一种比较有效的方法。