Soil Moisture Information And Thermal Microwave Emission

This paper presents theoretical and experimental results that demonstrate the depth to which soil moisture can be directly measured using microwave radiometers. The experimental results also document the effect of uniform surface roughness on the response of thermal microwave emission to soil moisture. Experimental measurements were executed in July 1980 at the Texas A&M University Research Farm near College Station, TX. Thermal microwave emission measurements were made at 1.4, 4.9, and 10.7 GHz at both vertical and horizontal polarization at off nadir angles from 0 to 50°. It has been demonstrated that passive microwave measurements at frequencies down to 1.4 GHz can only measure soil moisture directly to very shallow soil depths, approximately 2 cm. This is due to the fact that the soil moisture dependence of the transmission coefficient across the air-soil interface predominates over the soil moisture dependence of the total energy originating within the soil volume. It also has been demonstrated that the combination of low incident angle and measurement frequency in the C-band range does not minimize the effect of surface roughness for passive microwave measurements. This result is significant in view of the fact that this combination of frequency and incident angle has been described as the optimum combination for minimizing the effect of surface roughness on the response of radar-backscatter measurements to soil moisture.     

Microwave Emission and Plant Water Content: A Comparison between Field Measurements and Theory

Microwave radiation from a canopy cover depends primarily on the vegetation's thermal and dielectric properties; the latter are dependent on plant biometrical parameters and water content. Emission measurements carried out by means of ground-based X-and Ka-band radiometers have shown that crop coverage of soil can be detected through the spectral signatures of bare soil and vegetation. Moreover, measured brightness temperature and the radiative transfer theory for a scattering medium allow estimation of the scattering and absorption properties of the canopy. These parameters have been computed for corn and alfalfa using experimental data and a simple model in which anisotropic scattering is considered by means of transformed parameters w' and r'. We found that the single scattering albedo w' is always lower than 0.1, whereas the optical depth T' is very high. The latter has been correlated to plant water content by means of a logarithmic function.     

Microwave Thermal Emission from Periodic Surfaces

The emissivity of a periodic surface is calculated from one minus the reflectivity by using the reciprocity principle. The reflectivity consists of the sum of all scattered power as determined from the modal theory which obeys both the principle of reciprocity and the principle of energy conservation. The theoretical results are matched to experimental data obtained from brightness temperature measurements as functions of viewing angle for soil moisture in plowed fields. The threshold phenomenon with regard to the appearing and disappearing of modes in their contributions to the scattered field amplitudes is discussed in connection with the theoretical results. It is shown that this approach for calculating the emissivity greatly reduces computational efforts by requiring substantially smaller matrix sizes.     

Microwave Emission from Smooth Bare Fields and Soil Moisture Sampling Depth

This paper studies the depth to which soil moisture can be directly estimated with microwave measurements over smooth bare fields. The analyses are based on both theoretical and experimental considerations at the frequencies of 1.4, 5.0, and 10.7 GHz. Radiative transfer calculations of microwave emissivities at these frequencies are performed with a number of moisture profiles measured for two soils. The calculated emissivities are compared with those derived from the Fresnel equation to deduce the microwave sampling depth in soils. The data acquired from the ground-level radiometric measurements during the summers of 1979-1981 are examined and compared with the theoretical retical analysis. Both theoretical and experimental analyses lead to the conclusion that the microwave sampling depth in soils is about one tenth of the wavelength of observation. It is shown that the moisture content at any depth near the surface of a smooth soil can be estimated, in principle, by a combination of a radiometric measurement and a curve generated by the Fresnel equation at an appropriate frequency, provided that the texture of the soil is known.     

A Comparison between Active and Passive Sensing of Soil Moisture from Vegetated Terrains

A comparison between active and passive sensing of soil moisture over vegetated areas is studied via scattering models. In active sensing, three contributing terms to radar backscattering can be identified: 1a) the ground surface scatter term; 2a) the volume scatter term representing scattering from the vegetation layer; and 3a) the surfacevolume. scatter term accounting for scattering from both surface and volume. In emission, three sources of contribution can also be identified: 1b) surface emission, 2b) upward volume emission from the vegetation layer, and 3b) downward volume emission scattered upward by the ground surface. As ground moisture increases, terms 1a) and 3a) increase due to increase in permittivity in the active case. However, in passive sensing, term 1b) decreases but term 3b} increases for the same reason. This self-compensating effect produces a loss in sensitivity to change in ground moisture. Furthermore, emission from vegetation may be larger than that from the ground. Hence, the presence of vegetation layer causes a much greater loss of sensitivity to passive than active sensing of soil moisture.     

Thermal Microwave Emission from a Three-Layer Random Medium with Three-Dimensional Variations

The radiative transfer theory has been used to study the thermal microwave emission from a three-layer random medium. The random medium is characterized by correlation functions that are Gaussian in the horizontal direction and exponential in the vertical direction. Gaussian quadrature methods are used to solve for the brightness temperatures. The spectral and angular dependence of the brightness temperature are illustrated numerically. In particular we compare the case of a two-layer random medium whose brightness temperature decreases with frequency with the case of a three-layer random medium whose brightness temperature increases with frequency.     

扩展的肖特基热发射理论研究

通过对阴极零场发射模型和肖特基热发射模型的理论分析,可以发现这两种模型都不能充分地考虑到实际阴极发射时存在高电场强度的情形,鉴于这种局限性,提出一种介于场发射和热发射之间的模型,既扩展的肖特基热发射模型,并用MATLAB软件模拟并分析了三种模型之间的连续性,重点阐述了阳极电压对阴极发射的电流密度影响,结果揭示了在高电场强度存在时扩展的肖特基热发射模型的相对合理性,从而总结了新模型的应用条件。     

Microwave Emission from Row Crops

In order to examine the emission properties of vegetation without taking into consideration the effects of variations in the soil background, strips of metal screening were used to cover the soil surface between adjacent rows of plants. Temporal measurements were made at 2.7 and 5.1 GHz for soybean, wheat, and corn canopies. Several special experiments were conducted to evaluate the sensitivity of brightness temperature to look direction (relative to row direction), polarization configuration, and incidence angle, and to evaluate the emission contributions of defoliated stalks. In general, the results show that the canopy is highly anisotropic, the emission exhibits a strong dependence on polarization and look direction, and the scattering albedo is typically less than 0.1. Canopy transmissivity was estimated from the radiometric observations and then related empirically to the canopy's integrated water content. Using this relation in a zero-order radiative transfer model led to good agreement between the experimental observations and the model predictions.     

Microwave Emission and Scattering From Deserts: Theory Compared With Satellite Measurements

The emission and scattering from desert surfaces are analyzed using simulations and measurements from the Special Sensor Microwave/Imager (SSM/I) and the Advanced Microwave Sounding Unit (AMSU) microwave satellite instruments. Deserts are virtually free of vegetation, so the satellite radiometers are able to observe the emissivities of different minerals, such as limestone and quartz. Moreover, since deserts contain little moisture, the thermal emission originates below the surface at a depth of many wavelengths. At high frequencies, where the penetration depth of radiation is smallest, the radiometric measurements display the large diurnal variation in surface temperature, which reaches its maximum at around 1 P.M. Conversely, at low frequencies, where the penetration depth is largest, the radiation measurements display the small diurnal variation of subsurface temperature, which reaches a minimum at around 6 A.M. In addition to these emission signals, sand particles also scatter microwave radiation. Volume scattering causes the measurements to decrease as the frequency increases; although compared to other scattering media (snow cover and precipitation), the larger absorption and fractional volume (i.e., solidity) of sand reduce the scattering. Although the scattering effect is small, SSM/I measurements between 19 and 85 GHz show that deserts scatter the upwelling microwave radiation in a manner similar to light precipitation, which makes it difficult to uniquely identify precipitation over arid regions. Interestingly, the higher frequency AMSU measurement at 150 GHz is nearly the same as at 89 GHz for deserts, whereas the 150-GHz measurement is much lower than at 89 GHz for precipitation. These different spectral features at high frequencies can provide a means of separating the scattering from desert surfaces from that of precipitation.     

微波硫灯局部热平衡理论研究

目前国内对于微波硫灯的仿真研究集中于对工程结构的验证及优化,采用的方式是：在冷状态下,对整灯结构的电磁模型进行仿真或转化为等效电磁模型进行仿真。本文则是将关注点放在了发光机理中很重要的能量平衡部分,在热状态下采用自洽的局部热平衡理论,详细阐述了其用于仿真的迭代过程,并推导了其中各参量的计算公式。     

Effect of Texture on Microwave Emission from Soils

The intensity brightness temperature (TB) of the microwave emission from the soil is determined primarily by its dielectric properties. The large difference between the dielectric constant of water (?80) and that of dry soil (3-5) produces a strong dependence of the soil's dielectric constant on its moisture content. This dependence is effected by the texture of the soil because the water molecules close to the particle surface are tightly bound and do not contribute significantly to the dielectric properties. Since this surface area is a function of the particle size distribution (soil texture), being larger for clay soils with small particles, and smaller for sandy soils with larger particles; the dielectric properties will depend on soil texture. This dependence has been demonstrated by laboratory measurements of the dielectric constant for soils which are briefly summarized in this paper. The dependence of the microwave emission on texture is demonstrated by measurements of TB from an aircraft platform for a wide range of soil textures. The main conclusion of the paper is that the effect of soil texture differences on the observed TB values can be normalized by expressing the soil moisture values as a percentage of field capacity (FC) for the soil.     

A Simple Relation between Active And Passive Microwave Remote Sensing Measurements of Earth Terrain

A simple approximate relation between backscattering coefficient and emissivity measurements is derived. The relation is applicable to active and passive microwave remote sensing of earth terrain where volume scattering plays a dominant role. From the relation, one can obtain a rough estimate of the backscattering coefficient from the emissivity and vice versa. Such estimation is useful in checking experimental measurements and also helps to ascertain the validity of theoretical models. It also safeguards against obtaining arbitrarily high values for both backscattering coefficients and emissivities.     

用TEMCELL测量辐射发射的理论与实验

本文首次从矢势和洛仑兹互易定理出发，通过矢量和张量运算得出横电磁波室中包括电四极子辐射模型被测物的辐射发射的更一般结论。通过实验验证，结果较好。     

Microwave Radiation from Ferrimagnetically Coupled Electrons in Transient Magnetic Fields

Under certain restrictive conditions it appears that ferrimagnetically coupled electron spins are capable of coupling energy from a transient magnetic field and giving it up in the form of microwave radiation. This paper analyzes the behavior of the uniform precession of motion in ferromagnetic insulators under the influence of transient magnetic fields of changing amplitude and direction. The expected radiation power and efficiency are calculated for such an oscillator employing yttrium iron garnet.     

热辐射型光纤高温传感器的理论与实验研究

从理论上分析了热辐射型光纤高温传感器的特性,给出了涉及探测器波长响应和光纤损耗谱响应的普朗克积分的数值计算。这种计算方法简单、适用并且精度很高,与所做的实验相比较,变化趋势一致。     

Observations of Oceanic Surface-Wind Fields from the Nimbus-7 Microwave Radiometer

Brightness temperatures from the fiIve-frequency (6.6, 10.7, 18, 21, and 37 GHz) dual-polarized scanning multichannel microwave radiometer (SMMR) on Nimbus 7 have been used to obtain surface wind fields over the ocean. The sateilite-derived wind field for 1200Z, February 19, 1979, in the eastern North Pacific has been compared with an operationally generated surface-wind analysis field. Previous point comparisons at selected locations have indicated that satellite winds are accurate to 3 ms-1. The results here, although of a preliminary nature, indicate that SMMR-derived winds may be used to determine large-scale wind fields over the ocean, particularly in areas of strong wind gradients such as found in cyclonic systems.