Search

One of the important current problems is imaging and detection of objects hidden by obscuring obstacles. There have been extensive works reported on “Through-Wall Imaging” in recent years. This paper extends the previous works to the case where the wall is non-penetrating and the diffraction around the edge becomes important. This hard-wall imaging is related to the historical problem of “Poisson Spot” and “Anti-Podal point.” This paper makes use of the modified time-reversal technique and GTD, and shows the formulations of the multistatic data matrix, eigenvectors and steering vectors to form the imaging function. Signal processing includes the multiplicative modified time reversal technique to eliminate the unwanted extraneous images. Numerical examples include comparisons between the modified and the conventional time-reversal imaging to show the effectiveness of the hard-wall imaging method.

A space-fed X-band phased array antenna (PAA) is presented. The space-fed PAA uses physically autonomous individually addressable unit cells as the building blocks of an arbitrarily large array antenna. The X-band PAA unit cell consists of both a feed facing printed antenna element and a target facing printed antenna element. These elements are related by an RF phase shifter placed in the RF signal path between them which introduces relative phase delay and enables analog beam forming. Unit cell autonomy is realized by wirelessly transferring power and command signals required to operate a microcontroller (MCU) and the RF phase shifter at each unit cell. Each unit cell includes RF to DC conversion circuitry as well as an MCU designed to interpret a common control signal and command unit cell phase delay. The wireless phased array unit cell can be replicated in configurations of size and shape that are independent of RF, power, and array module control distribution networks. To demonstrate feasibility a four-element wireless PAA is constructed and antenna patterns for various control states are shown.

This paper considers the imaging of objects located close to rough surfaces such as ocean or terrain. If transmitters and receivers are also located close to rough surfaces, incident wave is no longer a plane wave nor a spherical wave in free space and it is necessary to consider Green’s functions with the point source located close to the surface, similar to the Sommerfeld dipole problem. This paper considers the near-surface imaging by making use of time-reversal imaging and surface flattening transform. Surface flattening transform converts the rough surface problem into flat surface with inhomogeneous random medium. Mutual coherence function is obtained and used to obtain imaging of point target near rough surface, making use of the multi-static data matrix, time-reversal matrix, the eigenvectors, and the steering matrix. Numerical examples are given. An important point is that integration of stochastic wave propagation and signal processing is necessary to obtain imaging through complex clutter envir...

Wireless power transfer (WPT) is a popular research field in recent years and can be categorized into three approaches: inductive coupling, laser beaming, and microwave power transmission (MPT). MPT system operates at the microwave and transfers the energy over more than a few wavelengths. It has its unique advantages of supplying power to non-accessible and mobile receivers. The overall efficiency, which is the ratio between available DC power at the receiver and supplied DC power at the transmitter, depends on both circuit design and wave propagation. As a comprehensive theory of MPT system is not available, this chapter starts with the study of MPT system from the perspectives of mathematical formulation and the experiment in the indoor environment, in Sect. 3.1. The preliminary study leads to the conclusion that highly directional wireless transmitter is very useful in the MPT system for achieving high transmission efficiency. For this reason, phased array antennas with beamforming functionality are usually used to direct the electromagnetic wave toward mobile receivers, and adaptive array algorithms are implemented to enable wireless power focusing in the complex environment. Section 3.3 presents a novel beamforming algorithm, which is proven to give the optimal transmission efficiency and applies to the arbitrarily positioned unequal array based on our problem formulation. To verify this algorithm, Sect. 3.4 validates it with numerical electromagnetic simulation in different cases. The numerical comparison in these examples shows that this algorithm gives higher transmission efficiency over other optimal beamforming algorithms discussed in Sect. 3.2.

Imaging of objects through complex environment is important in several applications, including imaging of hidden objects in obscuring media such as atmospheric and ocean turbulence, rough ocean surfaces, rain, fog, snow, and biological tissues. These media are often randomly varying in space and time, and statistical treatments are necessary to obtain images with useful spatial and temporal resolutions. In recent years, there has been increasing interest in using signal processing and correlation techniques to improve resolutions and to distinguish images from clutter. This paper presents several imaging techniques for objects in the presence of random media. Time-reversal MUSIC (multiple signal classification) imaging has excellent resolution when multiple scattering is small or moderate. Modified beamformer imaging has moderately high resolution even at large multiple scattering. We also include time reversal (TR) and synthetic aperture radar (SAR) imaging for comparison. The technique involves stochastic Green's function and mutual coherence function (MCF), eigenvectors of time-reversal matrix and pseudo spectrum. Numerical examples are given to show the effectiveness of these imaging techniques.

Multiple input-multiple output systems have received considerable attention because of their potential to achieve high channel capacity. This paper presents a study of the effects of a random scattering medium on channel capacity. Formulations are given including stochastic Green's functions and mutual coherence functions. Transmitter and receiver characteristics are included and analytical formulation for eigenvalues and channel capacities are given in terms of the medium scattering characteristics, optical depth, frequency, number of transmitter and receiver elements, transmitting power, and noise spectral power. As an example, we show 500 m link at 60 GHz through rain. The eigenvalues and the channel capacity are calculated in terms of SNR and the rain rate representing the optical depth. It is shown that as the rain rate increases, the correlation of waves at antennas decreases and the capacity increases. However, at high rain rate, the capacity tends to decrease due to the absorption and scattering.

In order to improve the performance of terrestrial free-space optical communication systems in adverse visibility conditions, we present a method for estimation of the atmospheric channel impulse response function which governs the optical intensity propagation. This method reduces run-time computational demands and system complexity in comparison to our previously proposed dual-wavelength channel estimation technique. We consider propagation of optical wavelengths in fog, where the droplet diameters are close to the wavelength and thus scattering and absorption effects are significant. A method for rapid calculation of a channel response function based on estimating the effective optical depth of the channel and curve-fitting is described. The channel response estimate can then be used to design a receiver-side equalizer (minimum mean-squared error linear equalizer) to correct the signal distortion due to propagation through the dispersive channel. The channel estimates are based on parametric curve-fitting functions which have been developed using the modified-vector radiative transfer theory to model the channel response. The optimal fit parameters are found using particle-swarm optimization to minimize the simulated bit-error rate of the received signal.

A general formulation of a theory of imaging through random obscuring layers is described. Previously we presented a theory for the temporal behavior of a short pulse scattered from a random medium and from a point target. In this paper, we generalize our study to include the imaging of objects of finite size and the actual imaging pattern at the receiver. This involves the study of two-frequency fourth order moments. Numerical examples are given to illustrate several important features, including the optical depth, backscattering enhancement, shower curtain effects, aperture size, bandwidth and target size.

A simple steerable array antenna is designed and developed using a movable dielectric phase shifter. The change of effective dielectric constant at different dielectric slab positions on a coplanar waveguide is used as the phase shifter. The impedance matching and desired phase shift conditions are satisfied at two slab heights, and the reflection is designed to be minimized at these slab positions. The low-loss dielectric material is used as the dielectric slab and is placed close to a coplanar transmission line with airgap. The 4times4 steerable array antenna with the phase shifters is designed and fabricated at 20 GHz. The H-plane radiation patterns are measured at different phase shift positions and compared with the expected results

A steerable antenna using a movable dielectric phase shifter is designed and developed. The change of the effective dielectric constant at different dielectric slab positions on a CPW is used as the phase shifter. The impedance matching and desired phase shift conditions are satisfied at two slab heights. Additionally, the reflection is designed to be minimized at these slab positions and at the designed frequency. A low-loss dielectric material is used as the dielectric slab and is placed close to a coplanar transmission line with an airgap. A tapered 8 × 7 steerable array antenna with phase shifters is designed and fabricated at 24 GHz. The E- and H-plane radiation patterns are measured at different phase shift positions and compared with the expected results. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 3118–3122, 2007; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.22906

It has been noted that angular and frequency correlations of the scattered wave from multiple scattering or rough surfaces exhibit the characteristics indicating that the scattered wave remembers the angular and frequency characteristics of incident wave under certain conditions. This is called the “Memory Effects” and was discussed by Feng in 1988. In July 2014 at URSI meeting in Memphis, we presented a preliminary study of the use of Memory Effects to reduce the clutter from rough surface and to improve the resolution of images of objects located near rough surface.

“Through-Wall Radar Imaging” (TWRI) has been studied extensively to reconstruct the image from the backscattered waves through the opaque dielectric wall. When the objects are located in the shadow region of the metallic wall (hard wall) where the waves can only bypass the wall via diffraction, a new problem arises as “Hard-Wall Radar Imaging” (HWRI), which has been proposed and discussed in our previous papers.

Statistical Electromagnetic Theories have been developed over many years and applied to a wide range of practical problems in remote sensing of geophysical media, biological media, medical optics, ultrasound imaging and object detection and communication in clutter. This paper gives a review of recent developments in applications of statistical wave theories. The super resolution of images occurs in random media due to the multiple scattering and the increase of apparent aperture size of the transmitter. Another interesting effect is that the scattered wave from multiple scattering remembers the direction of the incident wave and strong correlations can be observed under certain conditions. This is called the “Memory Effect”. Recent study shows that under certain conditions, the angular and frequency correlations of the scattered wave can be enhanced or reduced and this effect can be used to reduce the clutter from the rough surface. It is also noted that the coherence in multiple scattering causes the increase of radar cross sections in turbulence due to the interference between the forward and backward waves, called the “double passage effect”. This is related to the Anderson localization and the coherent backscattering.

This paper presents a theory of imaging objects behind layers of scattering media. The transmitter is a focused array or an aperture emitting a short pulse. The scattered pulse is received by a focused array or aperture. The received signal consists of two components: the pulse scattered from a random medium and from the target, and these two components can be distinguished by the use of ultra wide band (UWB) pulse. The second moment of the received signal includes the fourth-order moments of stochastic Green's functions, which are reduced to the second moments by the use of the circular complex Gaussian assumption, and of the generalized two-frequency mutual coherence function. This imaging theory is a generalization of optical coherence tomography (OCT), SAR and confocal imaging. It clarifies the relationships among resolution, coherence length, shower curtain effects and backscattering enhancement.

A simple steerable array antenna is designed using a movable dielectric phase shifter. The change of the effective dielectric constant at different dielectric slab positions on a coplanar waveguide is used as the phase shifter. The impedance matching and desired phase shift conditions are satisfied at two slab heights, and reflection is designed to be minimized at all slab positions. A four-element steerable array antenna at 5.8 GHz is fabricated and measured for testing purposes. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2222–2227, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21920

Optical surface plasmon resonance sensors have been known for a long time. In this paper, we discuss the use of metamaterials to construct a surface plasmon sensor which can be used at microwave frequencies. We review the conditions for the existence of surface plasmon and the use of the forward and backward surface waves. A sharp dip in the reflection coefficient occurs when the propagation constant of the incident wave along the surface is nearly equal to the propagation constant of the plasmon surface wave and may be used to probe bulk material characteristics or to determine metamaterial characteristics. Numerical examples are given to illustrate the basic characteristics.

This paper presents a quasi-static analysis of a metama- terial consisting of a three-dimensional array of small tunable stacked split ring resonators (SSRRs). The resonance frequency of the pro- posed resonator structure can be controlled by adjusting the auxiliary lumped elements which are inserted between the split of each ring. In addition, the size of the ring resonator can be reduced to an order of 0.01λ that is one tenth that of the split ring resonator (SRR) by choosing the proper lumped elements. The analysis is based on the quasi-static Lorentz theory, and the generalized matrix representation of the macroscopic constitutive relations of the composite medium is calculated.

This paper presents a theory of the radar cross section (RCS) of objects in multiple scattering random media. The general formulation includes the fourthorder moments including the correlation between the forward and the backward waves. The fourth moments are reduced to the second-order moments by using the circular complex Gaussian assumption. The stochastic Green’s functions are expressed in parabolic approximation, and the objects are assumed to be large in terms of wavelength; therefore, Kirchhoff approximations are applicable. This theory includes the backscattering enhancement and the shower curtain effects, which are not normally considered in conventional theory. Numerical examples of a conducting object in a random medium characterized by the Gaussian and Henyey–Greenstein phase functions are shown to highlight the difference between the multiple scattering RCS and the conventional RCS in terms of optical depth, medium location and angular dependence. It shows the enhanced backscattering due to multiple scattering and the increased RCS if a random medium is closer to the transmitter.

The problem of polarized light propagating through scattering media can be explained using the vector radiative transfer equation. This equation is an integro-differential equation and is well-known to be unsolvable analytically. One of the approximate solutions is discrete ordinates method which is based on the discretization of the Stokes parameters and the Mueller matrix. Although it produces accurate results, it requires a lot of computational resources. In addition, there are limitations on the calculation for angles that are very close to the optical axis. The solutions at these angles are necessary for some applications such as atmospheric imaging. First-order scattering approximation has been applied to mitigate the computational resource situation. It can also be used to calculate the solution at the angles that are very close to optical axis. However, it lacks information about the cross-polarization and it is inaccurate when light encounters more scattering events. Second-order scattering approximation provides more accurate solutions and offers some information about cross-polarization. We develop the first-order and second-order scattering approximations and their solutions for the pulse wave case. We investigate the second-order approximation solutions and compare them to the solution from the complete vector radiative transfer equation in several cases.

A CW-mode optically controlled microwave switch (CW- mode OMS) on a coplanar waveguide (CPW) for both a standard and a new design with carrier-confinement structure is investigated. We exper- imentally show that it may not be possible to obtain less than 7 dB of insertion loss with the standard OMS, and the carrier diffusion limits the improvement of insertion loss in CW-mode operation. We present a new design with a carrier-confinement structure with silicon (Si) sub- strate etching to confine the optically generated free-carriers. Less than 2 dB of insertion loss is obtained with the new carrier-confined OMS. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 36: 257-262, 2003; Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/mop.10737

The passage of a photon density wave through random media has been investigated extensively for medical imaging based on the diffusion approximation. In this paper, the photon density wave is studied based on the exact time-dependent vector radiative transfer theory. Both continuous and pulse photon density waves are analysed in a plane parallel medium using Mie scattering and the discrete ordinates method. The photon density wave shows superior properties over regular waves in several aspects. It has a narrower angular spectrum and maintains the original pulse shape. It also preserves the degree of polarization and increases the cross-polarization discrimination. These properties of a photon density wave suggest its potential for improving imaging. Thus, we apply the photon density wave to an imaging problem and show that it improves the quality of the images compared to other conventional imaging techniques.

The Applied Physics Laboratory at the University of Washington (APL-UW) has developed a miniaturized along-track interferometry synthetic aperture radar for remote sensing of nearshore ocean and river surface current velocities. The system, which is operated on a Cessna 172 aircraft, consists of two radar transceivers, an inertial navigation system (INS), and six antennas. The antennas are mounted on the outside of the aircraft underneath the fuselage. They are regular flat-panel broadside antennas, and as such they are mechanically oriented in the proper directions. This introduces aerodynamic drag resulting in reduced fuel economy of the aircraft. In addition, the antennas and the related mounting structure weigh two thirds of the entire radar system. Substantial savings in operating costs could be achieved if the antennas were reduced in size and weight and mounted parallel to the fuselage. In addition, this would make it possible to move the entire radar system onto an Unmanned Aerial Vehicle (UAV) resulting in further cost savings.

The technology of “Through-the-Wall Radar imaging” (TWRI) enables us to see through the walls by reconstructing the image based on the backscattered waves passing through the opaque dielectric wall. However, when the electromagnetic waves cannot penetrate the walls (such as metallic walls), a new problem called “Hard-Wall Radar Imaging” (HWRI) emerges as we have discussed in our previous papers. Because in this problem the objects are located in the shadow region, where only diffracted field around edges can be reflected back, the conventional imaging techniques are not applicable.

In recent years, researchers have done extensive studies on “Through Wall Imaging(TWI)”, which reconstructs the image based on the backscattered waves passing through the dielectric wall. However, in some cases,the objects are completely obscured by non-penetrating obstacles such as “hard wall ”and the signals cannot pass through the wall as it is in TWI case.Therefore, conventional techniques are not applicable to “Hard Wall Imaging(HWI)”. In this paper, a new imaging technique is proposed based on time-reversal(TR) imaging technique and diffraction theory to resolve this problem.

The techniques of wireless power transfer can be divided into three categories: inductive coupling, laser beaming and microwave power transfer (MPT). The MPT has its unique advantages of supplying power to devices which are far away from transmitter and moving. However, the power transmission efficiency is seriously constrained due to the inherent radiation of microwave. For this reason, the array antenna has to be employed at both transmitting end and receiving end in order to maximize the transmission efficiency so the weigh distribution on array elements is the key problem to achieve optimal transmission efficiency.

A new type of optically controllable, millimeter-wave (MMW) attenuator based on high-resistivity (high-R) silicon (Si) wafers and a layered structure is developed. A high-R float-zone Si wafer is a lossless dielectric material at microwave frequency without optical excitation. When an Si wafer is optically excited, free carriers are generated, and the Si wafer becomes a lossy dielectric. This property is combined with a layered structure to develop a simple optically tunable MMW attenuator. A more than 20 dB attenuation with a 10% bandwidth of the center frequency is obtained at W-band. The proposed structure is useful for developing low-cost attenuators and switches in the MMW region. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 9–13, 2000.

Recently, there has been increasing interest in the use of spaceborne synthetic aperture radar (SAR) for measuring forest biomass. However, it is noted that conventional SAR using C-band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavelengths, typically P-band (500 MHz). It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses. The variance of the refractive index fluctuations due to turbulence is approximately proportional toƒ−4. In addition, the Faraday rotation due to the geomagnetic field in the ionosphere becomes significant. This paper presents an analysis with numerical examples of the following effects in the frequency range from 100 MHz to 2 GHz in order to show the frequency dependence and the effects of total electron content (TEC) of the ionosphere. First, the ionospheric turbulence can reduce the coherent length below the equivalent aperture size, and the azimuthal resolution becomes greater than D/2 where D is the antenna aperture size. Second, the ionospheric dispersion causes a shift of the imagery due to the group velocity. Third, the dispersion also creates broadening of the pulse. In addition, multiple scattering due to ionospheric turbulence gives rise to pulse broadening. Fourth, we consider the Faraday rotation effect and show that the ellipticity change is negligible, but the orientation angle changes significantly at P-band. Numerical examples are shown using typical ionospheric parameters, turbulence spectrum, and TEC values.

The correlation effect of waves scattered from random media has attracted considerable interest in recent years. In particular, the angular correlation function (ACF) of waves scattered from rough surfaces and volumes has been studied extensively, and the characteristics of the angular memory effect (AME), which was first described by Feng et al. [1988], were reported for different surface conditions. Applications of AME, including height profiling in interferometric synthetic aperture radar (INSAR) and detection of buried objects, have been proposed and studied by researchers using numerical and experimental techniques. However, most of the recent studies of AME have been confined to the area of waves scattered from rough surfaces. In this paper we will investigate the angular memory signature of volume scatterers. Numerical and experimental studies on the ACF of waves scattered from randomly distributed cylinders are presented at millimeter-wave frequencies (80–105 GHz). It has been found that the angular width of ACF for volume scattering is narrower and shorter than that for random rough surfaces.

This paper presents a theory and its experimental demonstration of an imaging technique based on three-dimensional (3D) space-time confocal imaging and circular synthetic aperture radar (SAR). The theory is an extension of the conventional straight-path SAR-to-SAR on an arbitrary curved path. Next, a general formulation for the curved SAR is applied to circular SAR geometry, which has two important features. First, it allows the maximum attainable resolution to be an the order of a wavelength. Second, it makes 3D confocal imaging possible, X-band (7-13 GHz) imaging experiments are conducted to demonstrate this technique.

A new technique is proposed for subsurface detection of buried objects using the angular correlation function (ACF) measurement of scattered waves. Compared with the traditional detection technique which relies on radar cross section (intensity) measurement, this new ACF-based technique results in better signal-to-clutter ratio and thus higher target visibility. Laboratory experiments were conducted at millimetre-wave (80–105 GHz) and X-band (7–13 GHz) frequencies to illustrate the potential effectiveness of this new correlation approach over the traditional cross section approach.

The discrimination of the scattered wave from an object buried in shallow ground from that of the rough surface is a difficult task with present ground penetrating radar (GPR) systems. Recently, a new approach for this classical problem has been proposed and its effectiveness has been verified. This new method is based on the angular correlation function (ACF) of the scattered wave observed at two or more different incident and scattered angle combinations. It has been shown that the angular memory signatures of rough surfaces are substantially different from those of typical man-made targets and by choosing the appropriate incident and scattered angles, the surface scattering can be minimized whereas the scattering from the target is almost unchanged. The authors present detailed numerical studies of the ACF of the scattered wave from rough surfaces with and without a buried object. To obtain the ACF, the three averaging methods: realization, frequency and angular averaging, are tested numerically. It is shown that a single random rough surface of moderate extent can exhibit memory effect by using frequency averaging. Frequency averaging with a wide bandwidth is also effective for suppressing fluctuation in ACF and is most useful for practical applications. Numerical simulations indicate that even when the ratio of scattered intensities with and without the buried object is close to unity, the corresponding ratio of ACF magnitude can be more than 10 dB. Thus, using the ACF is superior to using the radar cross section (RCS) in the detection of buried objects.

Radar detection of a target embedded in geophysical media can be obscured by clutter. In this article the angular correlation function ( ) ACF is studied for wa! e scattering by an object embedded in random discrete scatterers under a rough surface. The ACF is calculated by using realization a! eraging, frequency a! eraging, and angular a! eraging. The results of the ACF for ! olume scattering, combined ! olume, and rough surface scattering and with a buried object are shown and compared. It is found that the ACF can be used to distinguish scattering by random discrete scatterers, random rough surface, and a deterministic buried object. The clutter return can be suppressed and the target becomes conspicuous. The ratio of ACF of the scattered wa! e with a target and that without a target can be 10 dB larger than the ratio of intensities. ! 1996 John Wiley & Sons, Inc. Microwave Opt Technol Lett 14, 144! 151, 1997.

One of the important current problems is imaging and detection of objects hidden by obscuring non-penetrating obstacles. There have been extensive research on “through wall imaging”, but our interest is focused on imaging objects which are obscured by non-penetrating obstacles such as “hard wall”. The hard wall imaging may have potential applications in many problems of practical interest such as objects behind hard walls near terrain.

Imaging and detection of objects located near rough ocean surface from transmitters to receivers which are also located on ocean surface are important practical problem. This problem is different from usual rough surface scattering including LGA (Low Grazing Angle) scattering, because the incident wave on the surface is neither a beam wave nor a spherical wave, but it is a mixture of the coherent and incoherent waves. Therefore, this needs to be studied on the basis of stochastic rough surface Green Function. Rough Surface Green Function has been obtained for perturbation solution, but it has been difficult for large rough surface heights. We have been considering this problem using the surface flattening transform. Using this transform, Green's function for rough surface is transformed to Green's function for flat surfaces with inhomogeneous random medium. Solutions for the transformed functions have been obtained using path integral and other method for several cases. We made use of the modified Rytov Method, which conserves the power, and obtained rough surface MCF (Mutual Coherence Function). The transformed equation contains the rough surface slope, the RMS height, and the correlation distances. The integral representations in the Rytov solution have been evaluated using asymptotic form and include the surface spectrum.

Despite the recent development of analytical and numerical techniques for problems of scattering from two-dimensional rough surfaces, very few experimental studies were available for verification. The authors present the results of millimeter-wave experiments on scattering from two-dimensional conducting random rough surfaces with Gaussian surface roughness statistics. Machine-fabricated rough surfaces with controlled roughness statistics were examined. Special attention was paid to surfaces with large rms slopes (ranging from 0.35 to 1.00) for which enhanced backscattering is expected to take place. Experimentally, such enhancement was indeed observed in both the copolarized and cross-polarized returns. In addition, it was noticed that at moderate angles of incidence, the scattering profile as a function of observation angle is fairly independent of the incident polarization and operating frequency. This independence justifies the use of the geometric optics approximation embodied in the Kirchhoff formulation for surfaces with large surface radius of curvature. When compared with the experimental data, this analytical technique demonstrates good agreement with the experimental data.

It is known that a change in the direction of an incident wave on a random medium is "remembered" by the angular correlation characteristics of the scattered waves. This "memory effect" is studied for rough-surface scattering by means of theoretical [second-order Kirchhoff approximation (KA)], numerical (Monte Carlo simulations), and experimental (millimeter-wave range) approaches. The second-order KA has been found to be effective for wave scattering from very rough surfaces with large radii of curvature and high slopes (0.5-1.5). Although the second-order KA is based on a number of approximations including the geometrical optics approximation and the approximate forms of the shadowing functions, excellent agreement with Monte Carlo simulations and millimeter-wave experiments was achieved. The results are presented in a form of memory signatures which clearly exhibit the important features of this effect.

In recent years there has been an increasing amount of interest in bistatic scattering characteristics of various kinds of media with regards to volume and rough surface scattering. The scattering of spheres has been studied analytically, numerically, and experimentally by many scientists. The scattering of cylinders has also been studied. The focus of this paper is on the experimental studies of millimeter-wave scattering from dense concentrations of cylinders when the size parameter ka is on the order of 1. Solving for the case of a sparse distribution of scatterers, where the fractional volume S is less than 5 to 10%, can be done with radiative transfer. This is true because for small fractional volumes, each scatterer is independent and thus the total scattered field is simply the sum of the individual scattered fields due to each scatterer. When f>10%, hereafter referred to as the dense case, each scatterer is no longer independent and the suppositions of radiative transfer theory no longer apply. >

Wideband millimeter wave experiments from 75–100 GHz on the scattering from two-dimensional very rough conducting surfaces are presented. The two-dimensional very rough surfaces are manufactured using a computer-numerical-controlled milling machine so that the surface statistics are precisely controlled. The surfaces have both Gaussian roughness statistics and Gaussian surface correlation functions. Bistatic scattering experiments on surfaces with either isotropic or anisotropic correlation functions are performed. Copolarized and cross-polarized bistatic scattering cross sections are measured for both transverse electric and transverse magnetic incidence at 20°. For isotropic surfaces, backscattering enhancement exists for both copolarized and cross-polarized returns and is found to be a function of the surface rms slope. In addition, a strong frequency dependence is observed across the 25-GHz bandwidth in the cross-polarized returns. To investigate the effect of surface correlation anisotropy, scattering experiments on anisotropic rough surfaces are also performed. It is found that the bistatic scattering cross section for an anisotropic surface is a function of the effective correlation length projected along the plane of scattering. Results on the bistatic scattering experiments presented here serve as a motivation to further pursue more elaborate and complete scattering experiments in order to advance research on scattering from very rough surfaces.

Using an 850-nanometer-wavelength free-space optical (FSO)communications system of our own design, we acquired field data for the transmitted and received signals in fog at Point Loma, CA for a range of optical depths within the multiple-scattering regime. Statistical estimators for the atmospheric channel transfer function and the related coherency function were computed directly from the experimental data. We interpret the resulting channel transfer function estimates in terms of the physics of the atmospheric propagation channel and fog aerosol particle distributions. We investigate the behavior of the estimators using both real field-test data and simulated propagation data. We compare the field-data channel transfer function estimates against the outputs from a computationally-intensive radiative-transfer theory model-based approach, which we also developed previously for the FSO multiple-scattering atmospheric channel. Our results show that the data-driven channel transfer function estimates are in close agreement with the radiative transfer modeling, and provide comparable receiver signal detection performance improvements while being significantly less time and computationally-intensive.

The detection of ground moving targets, such as humans and ground vehicles is important for border security. Using the data received from a passive radar system, an algorithm was developed to pinpoint the location of ground moving objects over a fixed area. The algorithm combines the data from cross-correlation range-Doppler techniques with the data from angle of arrival Multiple Signal Classification (MUSIC) techniques. The extracted time-delay values from the derived ambiguity function are converted into ellipse ranges and then joined with MUSIC'S derived target angles; the intersection point between the range ellipses and angle of arrival (AOA) lines provides estimation of the target location. By combining the derived intersection points from all the various still images of a moving target, a target tracking plot was generated. These algorithms were tested theoretically in both free space and in the presence of artificial background noise particles. The system and algorithm was also tested experimentally using a 1-GHz cell phone antenna (emitting a BPSK signal) as the source, dipole antennas as the receivers, and a conducting cylinder as the target. This experiment was conducted in an outdoor environment to simulate noisy and realistic conditions.

In this paper we present the findings of an effort to create a millimeter-wave radar system to image the internal structure of a kraft recovery boiler as well as measure the dielectric coating on the pipes. A kraft boiler is used by the paper industry to reclaim inorganic salts used in the pulping of wood. The bio-mass and salt structure of the fuel combusted in the boiler causes rapid deposition on the boiler pipes reducing the efficiency of the heat-transfer mechanism. A large portion of the generated steam is used to mitigate the build-up of the salts (using sootblowers), commonly referred to as saltcake. The steam used for maintenance consumes a substantial portion of the energy generated in the boiler. A millimeter-wave radar system capable of measuring the saltcake could add a feedback path for an intelligent sootblower mechanism. We investigate the optimal center frequency as well as required bandwidth for an effective imaging/measuring device.

Frequency and time-domain experiments are conducted to present additional confirmation of the slow speed of electromagnetic waves in a dense strongly scattering medium. A wide-band microwave signal is propagated through randomly distributed glass spheres, and the speed is defined as the time at which 50% of the pulse energy arrives. Thus, the speed was measured directly without the use of the transport mean free path and the diffusion coefficient. The speed of an incoherent pulse in the Mie resonance scattering region is found to be less than 40% of the vacuum speed for a fractional volume of 11% consistent with the energy velocity. Since the incoherent wave in a dense medium is the diffuse wave, the velocity presented in this paper corresponds to the velocity of the diffuse wave and it is different from the ballistic velocity. The speed of coherent waves is found to be almost constant and is quite different from the energy velocity.

We present experimental results on the scattering of electromagnetic waves at millimeter-wave frequencies from one-dimensional very rough conducting surfaces with controlled surface roughness statistics. Very rough surfaces are defined as surfaces with rms height and correlation length of the order of a wavelength such that the rms slope is at least unity. It is expected that scattering experiments using these surfaces can provide useful insights since their statistics lie outside the range of validity of the present theories, namely, the Kirchhoff and perturbation theories. Strong backscattering enhancement at different incident angles, both in the transverse electric and transverse magnetic polarizations, are observed experimentally. Numerical calculations based on the exact integral equation method for cylindrical beam wave illumination compare favorably with the experimental results. The agreement between measurements and numerical calculations is good over a wide range of incident angles and for all scattering angles. The close agreement between the experimental results and numerical simulations indicates that this controlled experimental setup can be used to study scattering phenomena from one-dimensional very rough surfaces with different roughness statistics as well as from two-dimensional rough surfaces.

High spectral components contained in surfaces with non-Gaussian correlation functions are believed to increase the magnitude of backscattering enhancement. In this paper, we present experimental and numerical studies of bistatic scattering from one-dimensional surfaces with Gaussian and power-law roughness spectral densities. Rough surfaces with an rms height of σ = 3 mm and correlation lengths of l = 3, 6 and 9 mm were fabricated and measured with a millimetre-wave scatterometer. The corresponding rms slopes were m = 1.41, 0.707 and 0.47, respectively. Both experimental and numerical simulation data show that surfaces obeying the power-law roughness spectral density have stronger backscattering enhancement than those with a Gaussian roughness spectral density for the same surface rms slope. Good agreement between experiments and numerical simulations was obtained. In addition, the numerical studies were conducted to obtain the scattering characteristics of surfaces with Gaussian and power-law roughness ...

We present some preliminary results from our recent free-space optical communications field test experiments in the foggy littoral environment along the coast of Point Loma, San Diego, conducted between October 2009 and June 2010. Our custom-built 850nm lasercomm system uses on-off keyed non-return-to-zero intensity-modulation and direct-detection to transmit pseudo-random bit sequences (PN-11 codes) at 250 Mbps over a 300m horizontal atmospheric path. We investigate improvements offered by using the latest Advanced Navy Aerosol Model to calculate the aerosol size-distribution function, a fundamental input to the radiative transfer code which we use to generate an estimate of the channel frequency response/impulse-response function. The estimated channel response function is used to design an equalization filter to correct signal distortion due to multiple-scattering effects and additive noise. We compare the performance of the Advanced Navy Aerosol Model against the more simplistic log-normal, Gaussian, and Mooradian Pt. Loma distributions. In this conference proceeding manuscript we are presenting only preliminary findings of our work in progress. Additional analysis, verification, and study is required before any final results can be posited from these preliminary findings.

A steerable array of patch antennas is designed for wireless communication at 60GHz. The antenna utilizes a novel dielectric phase shifting technique, previously presented by J. Cha et al. [1]. The propagation constant of a fixed portion of 60GHz coplanar waveguide transmission line is altered by the presence of ceramic slabs. A mechanism for introducing relative phase delay is designed based on this effect and both discrete and continuous slab height range is explored. The electrical characteristics and far field patterns are numerically approximated using Ansoft HFSS. A prototype antenna is fabricated and preliminary test results are gathered.

The small-angle approximation for the radiative transfer theory is based on the assumption that the scattered wave is confined within a small angle in the forward direction. This assumption is generally valid for the turbulence case in which the turbulence size is much greater than the wavelength and the index of refraction is close to 1. However, the validity of this assumption for a discrete random medium with an index of refraction much greater than the surrounding medium is not well known. In spite of this, the small-angle approximation has sometimes been used for discrete random media because of its mathematical simplicity.