An empirical model for interpreting the relationship betweenbackscattering and arid land surface roughness as seen with the SAR

The nature of the surface in a flat arid zone located in the western Sahara desert was studied and its roughness parameters defined. The main geological units were compared with backscattering data from ERS-1 images. Results showed a close correlation between the backscattering coefficient, σ₀, and the maximum height, h_max, of the rocks making up the arid landscape. The simplified relationship σ₀=4.63.1og h_max-15.29 (with h_max in cm and σ₀ in dB) was established from field measurements, giving an r² of about 88%. This empirical relationship is confirmed by another close relation between σ₀ and the statistical parameter s, the height standard deviation. The advantage of h_max is that it can be extracted for any site, whatever its roughness may be. The spatial distribution of the rock facets and the geometric characteristics of the incidence angle of the radar signal seem to explain the results     

A sensitivity analysis of soil moisture retrieval from the tau-omega microwave emission model

The potential of the /spl tau/--/spl omega/ model for retrieving the volumetric moisture content of bare and vegetated soil from dual-polarization passive microwave data acquired at single and multiple angles is tested. Measurement error and several additional sources of uncertainty will affect the theoretical retrieval accuracy. These include uncertainty in the soil temperature, the vegetation structure, and consequently its microwave single-scattering albedo, and uncertainty in soil microwave emissivity based on its roughness. To test the effects of these uncertainties for simple homogeneous scenes, we attempt to retrieve soil moisture from a number of simulated microwave brightness temperature datasets generated using the /spl tau/--/spl omega/ model. The uncertainties for each influence are estimated and applied to curves generated for typical scenarios, and an inverse model used to retrieve the soil moisture content, vegetation optical depth, and soil temperature. The effect of each influence on the theoretical soil moisture retrieval limit is explored, the likelihood of each sensor configuration meeting user requirements is assessed, and the most effective means of improving moisture retrieval indicated.     

A nonrectangular patch model for scattering from surfaces

A model is derived for treatment of surfaces using nonrectangular patches. Tests show good agreement, where comparison is possible, with results published elsewhere. The results are shown to be reasonable even in cases where some of the quadrilaterals degenerate to triangles.     

A model for edge scattering from dichroic surfaces

This paper discusses experimental and theoretical results for edge scattering from dichroic (frequency selective) surfaces. Measurements presented for half planes show the nature of the scattering from the edge for both reflection and transmission frequencies. An impedance boundary condition approach is used to calculate the diffraction from dichroic edges. In the analysis the dichroic surface is replaced with an equivalent surface impedance and the total scattered field calculated. Measurements support the theory demonstrating the usefulness of the approach. The method is also useful for calculating diffraction from dielectric edges.     

A physical model to determine snowfall over land by microwave radiometry

Falling snow is an important component of global precipitation in extratropical regions. This paper describes the methodology and results of physically based retrievals of snow falling over land surfaces. Because microwave brightness temperatures emitted by snow-covered surfaces are highly variable, precipitating snow above such surfaces is difficult to observe using window channels that occur at low frequencies (/spl nu/<100 GHz). Furthermore, at frequencies /spl nu//spl les/37 GHz, sensitivity to liquid hydrometeors is dominant. These problems are mitigated at high frequencies (/spl nu/>100 GHz) where water vapor screens the surface emission, and sensitivity to frozen hydrometeors is significant. However, the scattering effect of snowfall in the atmosphere at those higher frequencies is also impacted by water vapor in the upper atmosphere. The theory of scattering by randomly oriented dry snow particles at high microwave frequencies appears to be better described by regarding snow as a concatenation of "equivalent" ice spheres rather than as a sphere with the effective dielectric constant of an air-ice mixture. An equivalent sphere snow scattering model was validated against high-frequency attenuation measurements. Satellite-based high-frequency observations from an Advanced Microwave Sounding Unit (AMSU-B) instrument during the March 5-6, 2001 New England blizzard were used to retrieve snowfall over land. Vertical distributions of snow, temperature, and relative humidity profiles were derived from the Mesoscale Model (MM5) cloud model. Those data were applied and modified in a radiative transfer model that derived brightness temperatures consistent with the AMSU-B observations. The retrieved snowfall distribution was validated with radar reflectivity measurements obtained from a ground-based radar network.     

Fowler-Nordheim emission from non-planar surfaces

A mathematically closed form expression for the electric field between arbitrarily shaped surfaces is derived utilizing the techniques of differential geometry. This solution is then applied to the problem of Fowler-Nordheim emission from non-planar surfaces. Knowledge of the field everywhere on the emitting surface allows calculation of the current emitted. A comparison of these calculated values, with measurements made on textured polysilicon emission test structures, is made. The agreement is within 10% across eight orders of magnitude in current. The topological features of the emitting surface predicted by the computations are in agreement with those observed in scanning electron micrographs (SEM's).     

Semi-empirical model of the ensemble-averaged differential Mueller matrix for microwave backscattering from bare soil surfaces

A semi-empirical model of the ensemble-averaged differential Mueller matrix for microwave backscattering from bare soil surfaces is presented. Based on existing scattering models and data sets measured by polarimetric scatterometers and the JPL AirSAR, the parameters of the co-polarized phase-difference probability density function, namely the degree of correlation /spl alpha/ and the co-polarized phase-difference /spl sigmav/, in addition to the backscattering coefficients /spl sigma//sub /spl nu//spl nu///sup 0/,/spl sigma//sub hh//sup 0/ and /spl sigma//sub /spl nu/h//sup 0/, are modeled empirically in terms of the volumetric soil moisture content m/sub /spl nu// and the surface roughness parameters ks and kl, where k=2/spl pi/f/c, s is the rms height and l is the correlation length. Consequently, the ensemble-averaged differential Mueller matrix (or the differential Stokes scattering operator) is specified completely by /spl sigma//sub /spl nu//spl nu///sup 0/,/spl sigma//sub hh//sup 0/,/spl sigma//sub /spl nu/h//sup 0/,/spl alpha/, and /spl zeta/.     

Nonthermal microwave emission from frog muscles

Emission has been detected from electrically stimulated frog gastrocnemius muscles. It had wavenumbers certainly below 50cm^?1, and arguably less than 5cm^?1. The radiation has been shown to be non thermal, thus originating in some subsystem rather than in the muscle as a whole. This is in line with a pumped phonon prediction by Fröhlich.     

A statistical model for interpreting computerized dynamic posturography data

Computerized dynamic posturography (CDP) is widely used for assessment of altered balance control. CDP trials are quantified using the equilibrium score (ES), which ranges from zero to 100, as a decreasing function of peak sway angle. The problem of how best to model and analyze ESs from a controlled study is considered. The ES often exhibits a skewed distribution in repeated trials, which can lead to incorrect inference when applying standard regression or analysis of variance models. Furthermore, CDP trials are terminated when a patient loses balance. In these situations, the ES is not observable, but is assigned the lowest possible score--zero. As a result, the response variable has a mixed discrete-continuous distribution, further compromising inference obtained by standard statistical methods. Here, we develop alternative methodology for analyzing ESs under a stochastic model extending the ES to a continuous latent random variable that always exists, but is unobserved in the event of a fall. Loss of balance occurs conditionally, with probability depending on the realized latent ES. After fitting the model by a form of quasi-maximum-likelihood, one may perform statistical inference to assess the effects of explanatory variables. An example is provided, using data from the NIH/NIA Baltimore Longitudinal Study on Aging.     

A field-enhanced generation model for field emission from p-typesilicon

Field penetration into p-type silicon emitter creates a depletion region and causes the field emission current to he limited by the supply of electrons in the presence of a high electric field. A model is presented that takes field-enhanced generation within the depletion region into account, which may explain the nonlinear phenomenon in Fowler-Nordheim (F-N) plots of p-type silicon     

New mode of microwave emission from GaAs

Observations of emission from GaAs are reported. These differ from earlier observations in that they are produced by amplification of an electromagnetic wave within a sample of GaAs. The specimen dimensions perpendicular to the applied d.c bias field form a microwave cavity and determine the frequency of emission.     

Two types of microwave emission from InSb

Two types of microwave emission from InSb plasma subjected to the crossed electric and magnetic fields were investigated experimentally. From the simultaneous measurements of Hall effect and microwave emission, the threshold condition of the one type of emission was obtained as(omega_{ce} tau_{e})^{2} cdot Delta n geq 3 times 10^{14}cm^-3, whereomega_{ce}, tau_{e}, andDelta nare electron cyclotron frequency, relaxation time, and electron-hole pair density, respectively. It was also found that the "Hall" mobility showed anomalous decrease above the threshold. The emission power of the other type of emission showed maximum value at several values of the applied magnetic field lower than the threshold of the noise-type emission. These characteristic magnetic fields were not influenced by current density except by slight change in the low-current density region.     

A semiempirical line shape model of GaAs MQW structures

We have developed a semiempirical model to describe the line shape in the vicinity of the exciton absorption lines in GaAs multiple quantum well (MQW) structures. This model is based on a conventional line-broadening analysis similar in spirit to that used in atomic and plasma physics. The absorption spectrum is determined through the line positions, oscillator strengths, homogeneous and inhomogeneous broadening, and continuum shifts. Values for these parameters are derived from a combination of theoretical models, some existing in the literature and other developed by the authors, and experimental data where theory is lacking. Our results are in good agreement with the absorption measurements and the nonlinear coefficients available in the literature. This model shows improvements in the nonlinearity at low temperature and small inhomogeneous linewidth, which is as expected     

A microwave scattering model for layered vegetation

A microwave scattering model was developed for layered vegetation based on an iterative solution of the radiative transfer equation up to the second order to account for multiple scattering within the canopy and between the ground and the canopy. The model is designed to operate over a wide frequency range for both deciduous and coniferous forest and to account for the branch size distribution, leaf orientation distribution, and branch orientation distribution for each size. The canopy is modeled as a two-layered medium above a rough interface. The upper layer is the crown, containing leaves, stems, and branches. The lower layer is the trunk region, modeled as randomly positioned cylinders with a preferred orientation distribution above an irregular soil surface. Comparisons of results obtained using this model with measurements from deciduous and coniferous forests show good agreement at several frequencies for both like and cross polarizations     

Theory for microwave thermal emission from a layer of cloud or rain

Microwave thermal emission from a layer of cloud or rain consisting of spherical particles has been investigated. Scattering effects are studied in great detail with both numerical and analytical approaches. In the absence of ground emission, it is found that scattering induces brightening for optically thin layers and vice versa for optically thick layers. As a function of observation angle brightening occurs near nadir while darkening occurs at large angles in the case of small optical thickness. For large optical thickness, darkening occurs at all angles because of backscattering effects. When the layer of cloud or rain is above an air layer and an ocean surface at a higher temperature, it is found that the darkening effect at large optical thickness is much more pronounced. The darkening effect is also larger for vertical polarizations because the ocean emits more vertically polarized components. The effect of thermal emission and molecular absorption by atmospheric gases is also taken into account. Results obtained from analytical formulas under single scatteirng assumptions are compared and illustrated.     

A simplified model for interpreting the Doppler spectrum of forward-scatter radar signals

An approximate form of Birkemeier's anisotropic scattering function is developed. This Gaussian scattering model is meant to be used for interpreting the Doppler Spctra of signals taken from a forward scatter radar system equipped with a Rake receiver. The model offers advantages in extracting the anisotropy coefficient from the Doppler spectrum of the received signal.     

A microwave model for high electron mobility transistors

In this paper, the authors present a high-frequency model for the high electron mobility transistor (HEMT). The model includes the distributed effects in the channel of the device through two newly developed wave equations in the linear and saturation regimes. The equations are solved taking into account the electric fields along and perpendicular to the flow of the current. The Y and S parameters are derived and the theoretical predictions of the model are compared with the experimental data and shown to be in good agreement over a wide range of frequencies     

A physical simulation model for field emission triode

A simple but accurate physical model, which can be incorporated into circuit simulation programs such as SPICE for the field emission triode (FET), is developed. The model is based on the Fowler-Nordheim (F-N) current density-electric field (J-E) relationship. An electric field form is adopted to calculate the current density distribution along the surface of the sphere-shape tip. The cathode current is obtained by integration of the current density over the emission surface. The gate current is derived by the same integration, but over part of the emission area. A procedure to extract the values for the parameters of the model is also given. The model and the procedure has been applied to experimental devices to demonstrate its accuracy