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Imaging and detection of objects at optical wavelengths offers better resolution than at microwave or millimeter wavelengths. However, the imaging is severely affected by scattering from fog and clouds. This paper presents a study of optical imaging through clouds by using the point-source vector radiative transfer theory. The point-spread function including complete polarization characteristics is presented with numerical examples at 1 /spl mu/m wavelength showing the effects of aperture size and optical depth on the shower curtain effects.

The polarimetric angular memory effect is applied to obtain the average topographic height of a rough surface. This novel effect improves the height sensitivity while maintaining a good degree of correlation between the sensors. By using a reference flat surface, the interferometric phase is linearly related to the mean topographic height. The combination of angular memory and wideband frequency interferometry (AMFI) is realized and offers a means to design a robust interferometric system. Extension of the technique to the pulse scattering problem is studied through the two-frequency mutual coherence function, and its time-domain transform provides an equivalent way to obtain the mean topographic height by combining both spatial and temporal diversity. Millimeter-wave (MMW) experiments are conducted with rough surfaces of different statistics and scattering media of different types (gravel, sand, and rough surfaces) to prove the effectiveness of the technique.

An experimental study was conducted to investigate the angular memory effect of millimeter-wave scattering from two-dimensional conducting random rough surfaces. The surfaces under investigation were machine-fabricated with known Gaussian roughness statistics, and the copolarized and cross-polarized angular correlation functions (ACFs) of scattering amplitudes were measured. It was found that for the case of reference antenna positions located bistatically in a backward direction, the measured ACF exhibits broad response when single scattering dominates but two peaks when multiple scatteringdominates. These observations are in good agreement with the second-order Kirchhoff approximation (KA2). Specifically, the observed broad and peak responses are analytically identified to be due to the first-order and second-order (ladder and cyclical) scattering components, respectively, in KA2.

The product branching ratio was studied in relation to the radical scrambling mechanism of laser isotope separation of uranium. A mixture of UF6/CH4/Ar was irradiated either using a low-intensity cw-UV source (high-pressure Hg lamp, 250 nm ∼ 400 nm) or using high-intensity pulsed UV lasers (4th-harmonic YAG laser at 266 nm or KrF laser at 248 nm) in a supersonic nozzle reactor. While the latter gave a higher C2H6/CH3F ratio in the photoproduct than the former, the C2H6/CH3F branching ratio was found to disagree with the results reported in a static cell experiment. This anomaly in the results obtained using a supersonic nozzle reactor was well represented by a model proposed in this study which assumed no mixing before a shock wave zone and sufficient mixing after it, leading to complete suppression of radical recombination.

The effect of a random medium on the microwave imaging process is experimentally examined by positioning randomly distributed glass beads embedded in thin styrofoam sheets in various densities between the target and the antennas. The main effect of the random medium is a reduction in the signal-to-noise ratio for the tested medium with a small size parameter. If the medium consists of densely distributed particles, it is shown that the image quality is less than that of the sparsely distributed particle medium of the same optical distance. >

A mechanically steerable antenna was designed and developed using an adjustable microwave phase shifter. The dielectric material /spl epsiv//sub /spl tau// = 10 (alumina) was used as the dielectric slab and was placed close to a coplanar TL. Numerical simulations using Ansoft HFSS and Designer were conducted at 20 GHz. For the testing purpose, a 4-element steerable array antenna at 5.8 GHz was fabricated and measured.

A truck mounted, fully polarimetric radar system was used for measuring the backscattering from snow-covered terrain at 35 and 94 GHz. The radar system is capable of obtaining the Mueller matrix using the coherent-on-receive method. In this technique, the radar transmits 6 different polarizations and measures magnitude and phase of V and H channels simultaneously. The Mueller matrix and the degree of polarization were obtained as a function of incidence angle and the terrain roughness. The diurnal measurement showed that the degree of polarization was sensitive to the snow liquid water content. polarization was approximately 0.5 for the circular incidence polarization. During the warm daytime, on the other hand, the degree of polarization increased to about 0.8.

(1996). Experimental studies of millimeter-wave scattering in discrete random media and from rough surfaces - Summary. Journal of Electromagnetic Waves and Applications: Vol. 10, No. 3, pp. 451-453.

We investigate experimentally an adaptive imaging approach, namely that of a synthetic wideband, stepped CW transmitter and a multichannel adaptive array for receiving backscattered energy from short range objects in a full +/-90 degree field of view. The method is a hybrid one, using conventional Fourier processing for range and is adaptive in angle. The 12-element linear array that was constructed is able to form an image in range and angle without Doppler information, and was tested with targets embedded in natural background clutter. The results show how background clutter affects the angular resolution of the array due to the increase in rank of the signal-plus-clutter covariance matrix, whereas at the same time the rank of this matrix is reduced for closely spaced scatterers due to signal coherence. In addition to investigating some known angular enhancement methods, we propose a method to enhance angular resolution in the presence of clutter by a projection, which seeks to reduce the received signal to a lower rank approximation, without using eigendecomposition, thus having an implementation advantage. This method allows more control over the angular resolution and the background clutter level. Computer simulations as well as the experimental results are presented.

In this work we have investigated the problem of angle of arrival (AOA) estimation in the indoor environment. Coherent, multi-path signals must be resolved. The approach is based upon measurement of the AOA using a band of frequencies, approximately 10%-20% of center, and utilizes order statistic filtering to estimate the true angle of arrival from the set of individual AOAs. A plane wave model is assumed for the propagating waves and a random arrangement of scatterers between the source and receiver array is assumed. We have constructed a four-element dipole array and associated receiver-acquisition system to test the approach. The results show good AOA estimates can be obtained in a building that would otherwise yield completely inconsistent estimates using narrowband methods. In addition we have simulated EM (electromagnetic) wave propagation characteristics inside buildings using 3-D computer simulation codes based on ray tracing. The model includes the physical and electrical description of the walls and the patterns of the antennas used. The EM modeling is used for evaluating the base level performance of indoor AOA algorithms.

This paper is concerned with the numerical simulation for the electromagnetic (EM) scattering by a rough surface at a low grazing angle. First scattered electromagnetic fields are computed by the finite volume time domain (FVTD) method, second the field data in the time domain are converted to those in the spectral domain by the discrete Fourier transform (DFT), and finally far fields are computed by use of the Kirchhoff-Huygense theorem. The numerical results are compared with those obtained by other methods as well as with the experimental data.