Microwave Dielectric Properties of Plant Materials

Three waveguide transmission systems covering the 1-2, 3.5-6.5, and 7.5-8.5 GHz bands were used to measure the dielectric properties of vegetation material as a function of moisture content and microwave frequency. The materials measured included, primarily, the leaves and stalks of corn and wheat. Dielectric measurements also were made of the liquid included in the vegetation material after it was extracted from the vegetation by mechanical means. The extracted liquids were found to have an equivalent NaCl salinity of about 10 per mil, which can have a significant effect on the dielectric loss at frequencies below 5 GHz. The results of attempts to model the dielectric constant of the vegetation-water mixture in terms of the dielectric constants and volume fractions of its constituent parts (i. e., bulk vegetation, air, bound water, and free water) are discussed. Additionally, measurements of the temporal variations in the total attenuation at 10.2 GHz are presented for a corn canopy and a soybean canopy.     

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.     

Improvement of Moisture Estimation Accuracy of Vegetation-Covered Soil by Combined Active/Passive Microwave Remote Sensing

The measured effects of vegetation canopies on radar and radiometric sensitivity to soil moisture are compared to first-order emission and scattering models. The models are found to predict the measured emission and backscattering with reasonable accuracy for various crop canopies at frequencies between 1.4 and 5.0 GHz, especially at angles of incidence less than 30°. The vegetation loss factor L (?) increases with frequency and is found to be dependent upon canopy type and water content. In addition, the effective radiometric power absorption coefficient of a mature corn canopy is roughly 1.75 times that calculated for the radar at the same frequency. Comparison of an L-band radiometer with a C-band radar shows the two systems to be complementary in terms of accurate soil moisture sensing over the extreme range of naturally occurring soil-moisture conditions. The combination of both an L-band radiometer and a C-band radar is expected to yield soil-moisture estimates that are accurate to better than +/-30 percent of true soil moisture, even for a soil under a lossy crop canopy such as mature corn. This is true even without any other ancillary information.     

L-Band Radar Estimation of Forest Attenuation for Active/Passive Soil Moisture Inversion

In the radiometric sensing of soil moisture through a forest canopy, knowledge of canopy attenuation is required. Active sensors have the potential of providing this information since the backscatter signals are more sensitive to forest structure. In this paper, a new radar technique is presented for estimating canopy attenuation. The technique employs details found in a transient solution where the canopy (volume-scattering) and the tree–ground (double-interaction) effects appear at different times in the return signal. The influence that these effects have on the expected time-domain response of a forest stand is characterized through numerical simulations. A coherent forest scattering model, based on a Monte Carlo simulation, is developed to calculate the transient response from distributed scatterers over a rough surface. The forest transient-response model for linear copolarized cases is validated with the microwave deciduous tree data acquired by the Combined Radar/Radiometer (ComRAD) system. The attenuation algorithm is applicable when the forest height is sufficient to separate the components of the radar backscatter transient response. The frequency correlation functions of double-interaction and volume-scattering returns are normalized after being separated in the time domain. This ratio simply provides a physically based system of equations with reduced parameterizations for the forest canopy. Finally, the technique is used with ComRAD L-band stepped-frequency data to evaluate its performance under various physical conditions.      

方向自适应的星载光子计数激光测高植被冠层高度估算

星载光子计数激光测高系统具有较高的沿轨距离分辨率,能够探测得到植被冠层和地表的连续高程信息。然而星载植被点云的低点云密度和低信噪比,对植被相对冠层高度的估算方法提出了新的要求。本文提出了一种方向自适应的星载光子计数激光测高植被点云冠高估算方法。首先通过寻找点云高程统计直方图中代表冠层和地面位置的极值进行粗去噪,大致得到信号高程所在的范围,并估算出冠层,地面和噪声点云的平均密度以及地表坡度。随后对粗去噪后的点云进行方向自适应的密度聚类精去噪,其邻域的方向为地表坡度,与密度有关的阈值均根据估算出的点云密度自适应的做出调整。在滤波后,结合点云的密度和高程百分比分别找出地面与树冠顶端的初始点,并通过三角网方法(TIN)扩展初始点以进行分类,最终确定地表与树冠顶端的高程。采用ATLAS星载激光测高仪的植被点云对算法进行了验证,结果表明算法能够正确估算植被冠高,十分适用于坡度较大和叶面积指数较低的地区,其中冠顶与地面的高程和机载LIDAR数据高程的决定系数R~2分别为0.99与0.77,均方根误差RMSE为0.28 m与2.6 m。     

A model to estimate the path loss in areas with foliage of trees

This paper presents an empirical model to predict attenuation in forest environments considering parameters related to vegetation. Typically, environmental parameters are only included in theoretical models, but they are more difficult to apply. The developed model uses tree density, average tree canopy diameter and foliage density as input parameters. The foliage density is very difficult to determine since it depends on the characteristics of trees. A simple metric of this parameter was obtained by measuring the background light silhouetted by the canopy. The model was developed with measurements obtained in different forest environments for two frequencies within the UHF band (Ultra High Frequency). A procedure was also applied to extend the operating frequency range of the model.     

A semiempirical model for interpreting microwave emission fromsemiarid land surfaces as seen from space

A radiative transfer model for simulating microwave brightness temperatures over land surfaces is described. The model takes into account sensor viewing conditions (spacecraft altitude, viewing angle, frequency, polarization) and atmospheric parameters over a soil surface characterized by its moisture, roughness, and temperature and covered with a layer of vegetation characterized by its temperature, water content, single scattering albedo, structure and percent coverage. In order to reduce the influence of atmospheric and surface temperature effects, the brightness temperatures are expressed as polarization ratios that depend primarily on the soil moisture and roughness, canopy water content, and percentage of cover. The approach used is described, and the sensitivity of the polarization ratio to these parameters is investigated. Simulation of the temporal evolution of the microwave signal over semiarid areas in the African Sahel is presented and compared to actual satellite data from the SMMR instrument on Nimbus-7     

Plant water content and temperature of the Amazon forest fromsatellite microwave radiometry

An attempt is made to derive the evolution of the temperature and the water status of the Amazon forest canopy from satellite microwave radiometry. The Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) temperature-corrected tapes data are analyzed for the 6.6, 10.7, 18, and 37 GHz frequencies, at daytime and nighttime, over a zone near Manaus (3°S, 60°W), Brazil. Two periods are investigated: the wet (April-May) and dry (July-August) seasons of 1985. After separating forest- from river-contaminated pixels, atmospheric corrections are performed for water vapor, clouds, and rain, using surface and satellite data. Algorithms are developed to model the microwave thermal emission of vegetation following a continuous approach and a discrete approach. A sensitivity study is performed in order to determine which frequencies are relevant to retrieve land surface parameters. The models are then used along with an optimization procedure so as to carry out the inversion of the canopy structure parameters. The vegetation temperature and water content are retrieved through the continuous model     

Normalized Differential Spectral Attenuation (NDSA) Measurements Between Two LEO Satellites: Performance Analysis in the Ku/K-Bands

Normalized differential spectral attenuation (NDSA) is a novel differential measurement method to estimate the total content of water vapor [integrated water vapor (IWV)] along a tropospheric propagation path between two low Earth orbit (LEO) satellites. A transmitter onboard the first LEO satellite and a receiver onboard the second one are required. The NDSA approach is based on the simultaneous estimates of the total attenuation at two relatively close frequencies in the Ku/K-bands and of a “spectral sensitivity parameter” that can be directly converted into IWV. The spectral sensitivity has the potential to determine the water vapor contribution, to cancel out all spectrally flat unwanted contributions, and to limit the impairments due to tropospheric scintillation. In this paper, we focus on the measurement accuracy of the spectral sensitivity parameter. Specifically, we examine this accuracy at three different frequencies and for two models of atmospheric structure. We first provide an approximate expression of the accuracy and then validate this expression through Monte Carlo simulations based on microwave propagation models.      

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.     

A feedback based modification of the NDVI to minimize canopybackground and atmospheric noise

The Normalized Difference Vegetation Index (NDVI) equation has a simple, open loop structure (no feedback), which renders it susceptible to large sources of error and uncertainty over variable atmospheric and canopy background conditions. In this study, a systems analysis approach is used to examine noise sources in existing vegetation indices (VIs) and to develop a stable, modified NDVI (MNDVI) equation. The MNDVI, a closed-loop version of the NDVI, was constructed by adding 1) a soil and atmospheric noise feedback loop, and 2) an atmospheric noise compensation forward loop. The coefficients developed for the MNDVI are physically-based and are empirically related to the expected range of atmospheric and background “boundary” conditions. The MNDVI can be used with data uncorrected for atmosphere, as well as with Rayleigh corrected and atmospherically corrected data. In the field observational and simulated data sets tested, the MNDVI was found to considerably reduce noise for any complex soil and atmospheric situation. The resulting uncertainty, expressed as vegetation equivalent noise, was ±0.11 leaf area index (LAI) units, which was 7 times less than encountered with the NDVI (±0.8 LAI). These results indicate that the MNDVI may be satisfactory in meeting the need for accurate, long term vegetation measurements for the Earth Observing System (EOS) program     

Generic rules to evaluate system-failure frequency

Frequency of failure of a system with s-independent components can be obtained from the system availability (unavailability) expression and failure and repair rates of the components. Although, Grouped Variable Inversion is an efficient technique to find the system availability, there is no convenient method to convert the “availability expression obtained by this technique” into an “expression for system-failure frequency.” This paper present generic rules to find system-failure frequency, particularly, when the availability or unavailability expression of a system is obtained using this technique. The rules are straightforward, and produce appreciably shorter expressions for system-failure frequency. Examples illustrate the simplicity and efficiency of the proposed rules     

多角度成像解析大豆冠层的二向反射特征

植被冠层二向性反射特征是定量遥感必须关注的一个问题。论文借助自主研发的多角度成像系统,在不同观测时间对不同种植密度下的大豆冠层进行多角度成像数据采集,通过对图谱合一的高光谱影像中大豆植株、土壤背景和阴影叶片进行逐步分离,对比分析纯大豆植被与植被-土壤混合冠层的二向反射（Bidirectional Reflectance, BR）变化特征,研究发现:在主平面观测时,土壤光谱去除后,即纯植被冠层反射率在前向观测时,随着天顶角的减小而增大,这不同于植被和土壤同时存在时的研究结果（BR 随着天顶角的增加而增大）;当观测方向由主平面的前向朝后向变动时,可见光和近红外波段的纯植被冠层反射率表现为逐步增大的趋势,这和土壤光谱去除前的变化趋势也不同;在垂直主平面观测时,去除土壤背景后的纯植被冠层反射率与混合植被反射率特征有相同的趋势,但在垂直主平面方向的对称性更强。上述结果在不同密度、不同观测时间的大豆冠层BR 特征有相近的趋势,这为多角度遥感的发展提供了必要的基础研究。     

AlN-TiC复相微波衰减材料性能的研究

采用热压烧结工艺制备了AlN-TiC复相微波衰减材料。通过XRD、SEM和网络分析仪,研究了TiC含量对材料的微波衰减性能的影响。结果表明,当w(TiC)低于10%时,材料呈选频衰减且衰减非常小;当w(TiC)为25%~50%时,材料呈现良好的多点选频衰减,且其衰减量随w(TiC)的增加而增加,最大达–18 dB。中心谐振频率有向高频漂移的趋势。初步探讨了AlN-TiC复相材料微波衰减曲线的频谱特性与衰减机理。     

The b-factor as a function of frequency and canopy type at H-polarization

For anticipated synergistic approaches of the L-band radiometer on the Soil Moisture and Ocean Salinity (SMOS) mission with higher frequency microwave radiometers such as the Advanced Microwave Scanning Radiometer (AMSR) (C-band), a reanalysis has been performed on the frequency dependence of the linear relationship between vegetation optical depth (/spl tau//sub o/) and vegetation water content (W), given by /spl tau//sub o/=b/spl middot/W. Insight into the frequency dependence of the b-factor is important for the retrieval of surface moisture from dual- or multifrequency microwave brightness temperature observations from space over vegetation-covered regions using model inversion techniques. The b-values presented in the literature are based on different methods and approaches. Therefore, a direct comparison is not straightforward and requires a critical analysis. This paper confirms that when a large frequency domain is considered, the b-factor is inversely proportional to the power of the wavelength b=c/(/spl lambda/)/sup x/, which is in line with theoretical considerations. It was found that different canopy types could be separated into different groups, each with a different combination of values for log(c) and x, which characterize the linearized relationship log(b)=log(c)-x/spl middot/log(/spl lambda/). A comparison of ratios b/sub C//b/sub L/ (with C and L denoting C- and L-band, respectively) also resulted in basically the same groups.     

Sensitivity to soil moisture by active and passive microwavesensors

The backscatter measured by radar and the emission measured by a radiometer are both very sensitive to the moisture content m_υ of bare-soil surfaces. Vegetation cover complicates the scattering and emission processes, and it has been presumed that the addition of vegetation masks the soil surface, thereby reducing the radiometric and radar soil-moisture sensitivities. Even though researchers working in the field of microwave remote sensing of soil moisture are all likely to agree with the preceding two statements, numerous claims and counterclaims have been voiced, primarily at symposia and workshops, espousing the superiority of the radiometric technique over the radar, or vice versa. The discussion is often reduced to disagreements over the answer to the following question “Which of the two sensing techniques is less impacted by vegetation cover?” This paper is an attempt to answer that question. Using realistic radiative-transfer models for the emission and backscatter, calculations were performed for three types of canopies, all at 1.5 GHz. The results lead to two major conclusions. First, the accepted presumption that vegetation cover reduces the soil-moisture sensitivity is not always true. Over certain ranges of the optical depth τ of the vegetation canopy and the roughness of the soil surface, vegetation cover can enhance, not reduce, the radar sensitivity to soil moisture. The second conclusion is that under most vegetation and soil-surface conditions, the radiometric and radar soil-moisture sensitivities decrease with increasing τ, and the rates are approximately the same for both sensors, suggesting that at least as far as vegetation effects are concerned, neither sensor can claim superiority over the other     

Time-domain low frequency approximation for the off-diagonal termsof the ground impedance matrix

The ground impedance is one of the parameters needed for field-to-transmission line coupling calculations. In the case of multiconductor lines this ground impedance is a full matrix with diagonal (self-impedance) and off-diagonal (mutual impedance) terms, The expression for the mutual ground impedance between two conductors i and j has been derived by Sunde (1968) and a low frequency approximation by Carson (1926). The general expression for the ground impedance matrix terms in the frequency-domain does nor have an analytical inverse Fourier transform. Therefore, the elements of the “transient ground resistance” matrix in time-domain should be, in general, determined using an inverse Fourier transform algorithm. In low-frequency approximation, however, Timotin (1967) and Mok and Costache (1992) has found an analytical inverse Fourier transform for the ground self-impedance. We extend the Timotin formula to the off-diagonal terms of the ground resistance matrix     

An Analysis of Conditional Site Diversity: A Study at Ka-Band

The use of EHF and SHF frequencies above 20 GHz is becoming increasingly important for high-capacity communication systems. Whether these systems are slant-path links, terrestrial fixed links, or deep-space links, the high bandwidths available and the relatively low spectral congestion are very attractive. One of the main disadvantages of these frequency bands is that the attenuation caused by meteorological effects can become significant, and the attenuation caused by clouds, rain, and atmospheric gases becomes very large. The largest attenuation events are caused by rain and clouds with a high liquid water content. In order to provide high-availability links, it is possible to use site diversity, by providing two spatially independent terminals. The spatial separation of the terminals reduces the probability of both terminals being faded. In this paper, we present an analysis of two spatially unique measurements of a satellite-based 20.7-GHz beacon. The results show that even at modest separations there is still the opportunity for significant availability improvements using site diversity. The probability density functions (pdfs) conditioned on the single-site attenuation level are presented. These demonstrate a characteristic shape and could form the basis of future modeling approaches.     

Millimeter wave investigation of dielectric cylinders absorption cross-section by resonant method

A new millimeter wave resonant method for determination of absorption cross-section of vegetation stalks is given. The method is developed for study of vegetation stalks absorption cross-section as a function of their moisture content, orkR parameter (k is a wave number, R is a stalk radius) in millimeter wavelengths band. ModesE _01p of a cylinder resonator are employed for such measurements. Dielectric permittivity and loss tangent of a sample can be found by measuring a frequency shift andQ- factor changes of the resonator. Absorption cross-section of a sample is then calculated using a formula obtained in our theoretical study. This formula can be written analytically as a result of rigorous solution of the related diffraction problem (scattering of a plane electromagnetic wave by cylinder with complex dielectric permittivity if stalk radius is less then 0.5 mm for the wavelength of 8 mm). Results of this investigation may be used in creating electrodynamical models of media containing cylindrical stalks of plants, in direct measurements of some canopy parameters, transmission properties measurements of vegetation canopies etc.     

Using a modeling approach to predict soil hydraulic properties frompassive microwave measurements

A soil water and energy budget model coupled with a microwave emission model (MICRO-SWEAT) was used to predict the diurnal courses of soil surface water content and microwave brightness temperatures during a number of drying cycles on soils of contrasting texture that were either cropped or bare. The parameters describing the soil water retention and conductivity characteristics [saturated hydraulic conductivity, air entry potential, bulk density, and the exponent (b) describing the slope of the water release curve] had a strong influence on the modeled bare-soil microwave brightness temperatures. These parameters were varied until the error between the remotely sensed and modeled brightness temperatures was minimized, leading to their predicted values. These predictions agreed with the measured values to within the experimental error. The modeled brightness temperature for a soybean-covered soil was sensitive to some of the vegetation parameters (particularly to the optical depth), in addition to the soil hydraulic properties. Preliminary findings suggest that, given an independent estimate of the vegetation parameters, it may still be possible to estimate the soil hydraulic properties under a moderate vegetation canopy