Numerical simulation of low-grazing-angle ocean microwavebackscatter and its relation to sea spikes

This paper presents the results of numerically simulating microwave backscatter from a deep-water breaking wave profile. Enhanced microwave backscatter from the crests of breaking waves has been hypothesized as the source of bright short-lived microwave radar echoes that are observed at low-grazing angles (LGAs). The characteristics of these “sea spikes” are distinctly different from the Bragg-scatter echoes that dominate measurements made at moderate grazing angles. Of particular interest is the high contrast that sea spikes present against ocean background backscatter when observed with horizontally polarized transmit/receive configurations [horizontal (HH) versus vertical (VV)]. This HH/VV contrast disparity has been attributed to polarization-selective cancellation of the direct reflection from the wave crest by the surface reflection. This hypothesis is reinforced first by showing evidence that VV polarization is suppressed in the intensity range that would normally be populated by the brightest scatterers. Histograms of unaveraged Doppler-centroid measurements show further that the depleted VV backscatter population is responding to scatterers that are moving much more slowly than the HH scatterers. The Doppler-centroid histograms provide a sharper delination between the two scattering populations than do the unconditionally averaged Doppler spectra that are more commonly reported. Finally, our numerical simulations show evidence of an interference mechanism that selectively suppresses VV backscatter. In our simulations, the polarization selectivity comes from the phase dependence of the backscatter from the wave crest. A Brewster phenomenon at the surface reflection point is not necessary     

Classification of wave phenomena and its relation to coherence theory

Various types of problems that arise within the framework of wave propagation through a medium are classified according to whether the medium or the source is random or nonrandom. When both medium and source are nonrandom, the applicable differential equation is of a deterministic nature; otherwise, it is stochastic. A representative case of a random medium and nonrandom source is the multiple scattering of a monochromatic wave by a randomly perturbed medium or by random scatterers. A case in which the medium is nonrandom and the source random is exemplified by the theory of partial coherence. Finally, when both medium and source are random we have the general case in which waves generated by random sources propagate through random media. The last case, which reduces under appropriate conditions to each one of the remaining types, is discussed in detail and its formal solution is obtained by the perturbation method.     

Backscattering enhancement and clustering effects of randomlydistributed dielectric cylinders overlying a dielectric half space basedon Monte-Carlo simulations

Backscattering enhancement can exist in volume-surface interactions where a double bounce can arise from one volume scattering and one surface scattering. An important quantity to be determined in backscattering enhancement is the angular width. The authors study backscattering enhancement of the volume-surface interaction by performing Monte-Carlo simulations of scattering by vertical dielectric cylinders overlying a dielectric half space using the Foldy-Lax multiple-scattering equations. The results indicate that the angular width of backscattering enhancement for a scattering layer with a small optical thickness is of the order of the wavelength divided by the layer thickness giving an appreciable angular width of the order of 10-30 degrees that can be important for remote sensing applications. The effects of clustering of scatterers leading to collective scattering and absorption effects are also studied     

Hybrid methods for analysis of complex scatterers

Depending on the angle of illumination, electrically large scatterers can support a variety of electromagnetic (EM) phenomena, such as traveling waves, creeping waves, and edge/surface diffraction effects. The electrical size of a body limits the tractability of numerical methods such as the method of moments (MM), and the geometric complexity of an object circumscribes the applicability of optics-derived methods. Hybrid methods incorporating both numerical and high-frequency asymptotic techniques have the potential to substantially enlarge the class of EM scattering problems that can be treated. In this discussion, the current-based hybrid formulation is summarized for classes of two- and three-dimensional scatterers. The use of Ansatz solutions derived from physical optics, the physical theory of diffraction, and the Fock theory is illustrated for perfectly conducting, partially penetrable, and totally coated bodies. For the latter, a generalization rooted in the impedance boundary (Leontovich) condition is used. Complementing these Ansatz solutions, the Galerkin representation is used for regions where the foregoing are computationally or physically intractable. The above cases are illustrated by representative solutions explicating the approach     

Surface waves of printed antennas on planar artificial periodicdielectric structures

The characteristics of surface waves from a Hertzian (elementary) dipole on a multilayer structure with planar periodic material elements are investigated. An integral-equation moment method in conjunction with an analytical array scanning scheme is applied to the boundary-value problem associated with source interaction with infinite periodic structures. A pole-extraction technique and the saddle-point method are applied to find the far-zone periodic surface waves due to a current source. The investigation provides a fundamental study of surface wave properties of printed circuit antennas on planar artificial periodic (photonic bandgap) structures. It is found from the power patterns that surface waves are suppressed in the directions with wave bandgap and greatly enhanced in the directions just outside the bandgap zones. The surface wave pattern may be highly directive and the beam angle varies with frequencies. The finding suggests possible frequency-space selection devices. Experiments are carried out to validate the surface wave bandgap phenomenon and the beam angle frequency-selection property     

Electromagnetic scattering for oblique incidence on impedancebodies of revolution

Combined field integral equations for the surface currents induced by an obliquely incident wave on a rotationally symmetric body are considered. The relative surface impedance is independent of the azimuthal angle but may vary along the profile of the scatterer in any plane containing the axis of symmetry; the currents are conveniently expressed in terms of Fourier series of uncoupled terms in the azimuthal angle. Simple integral expressions for the far field are given and a computer code is described and tested on a variety of scatterers. Geometry of scatterer, surface impedance and Fourier harmonics of induced currents are described by splines. The results are in agreement with physical interpretation     

A complete physics-based channel parameter simulation for wavepropagation in a forest environment

At HF through UHF frequencies, wave propagation in a forest environment is mainly attributed to a lateral wave which propagates at the canopy-air interface. Due to the existence of tree trunks, significant multiple scattering also occurs which is the dominant source of field fluctuations. Basically, the current induced in the tree trunks by the source and the lateral wave reradiate and generate higher order lateral waves and direct scattered waves. Using a full-wave analysis based on the method of moments in conjunction with Monte Carlo simulations, the effect of multiple scattering among a very large number of tree trunks is studied. It is shown that only scatterers near the source and the observation points contribute to the field fluctuations significantly. This result drastically simplifies the numerical complexity of the problem. Keeping about 200 tree trunks in the vicinity of the transmitter dipole and the receiver point, a Monte Carlo simulation is used to evaluate the statistics of the spatial and spectral behavior of the field at the receiver. Using a wide-band simulation, the temporal behavior (impulse response) is also studied as is performance of antenna arrays and the effects of different spatial diversity combining schemes in such a multipath environment     

Complex waves

Plotting the frequency versus the wave number provides a unifying way of permitting visualization of the linear dynamics of distributed systems. A series of simple examples is presented in order to illustrate the physical significance of ordinary and evanescent waves, absolute instability, waves on moving media, and convective instability or wave amplification. The examples give a rare opportunity to develop a complete physical and mathematical picture of the dynamics, since the presentation can serve as a guide to two films?produced by the Education Development Center for the National Committee on Electrical Engineering Films?in which the dispersion relation evolves by computer animation on the bottom half screen in synchronism with changes in the physical phenomenon in the top half screen.     

Resolution of a controversy surrounding the focusing mechanisms ofsynthetic aperture radar images of ocean waves

This paper addresses key problems regarding the focusing of synthetic aperture radar (SAR) images of ocean surface waves, explaining why applying a processor defocus will generally yield an enhanced image, why the same defocus applies to both image modulations brought about by the radar cross section and by the velocity bunching process, and why the effects apply to both single-look and multilook systems independently of look relocation. Two interpretations are given for the case when surface scatterers are stationary, but modulated in reflectivity (radar cross section) by a propagating wavefield. The first interpretation is what will be called a “degrade-and-shift” model. In it, a processor focusing adjustment degrades a point image. However, the overall image can be enhanced because an appropriate defocus results in a shifting of points in such a way that the image can most closely resemble the image of the time-invariant (or “frozen”) reflectivity. The second interpretation is a “defocus-and-refocus” model in which the image of a time-varying reflectivity is defocused and may be refocused to enhance the image. In justifying this “defocus-and-refocus” model, it is shown that the radar return from stationary scatterers of time-varying reflectivities is identical to that from physically moving scatterers of constant reflectivity. Thus, the two interpretations are not contradictory; they are, fundamentally, equivalent. The models support the use of a processor defocus corresponding to one half the wave phase velocity. Both qualitative and quantitative illustrations of the effects are given. Finally, it is shown that the same defocusing effect applies to image modulations brought about by the velocity bunching process     

Rough surface wavelength measurement through self mixing of Doppler microwave backscatter

A microwave backscatter technique is presented that has the ability to sense the dominant surface wavelength of a random rough surface. The purpose of this technique is to perform this measurement from an aircraft or spacecraft, wherein the horizontal velocity of the radar is an important parameter of the measurement system. Attention will be directed at water surface conditions for which a dominant wavelength can be defined, then the spatial variations of reflectivity will have a two dimensional spectrum that is sufficiently close to that of waves to be useful. The measurement concept is based on the relative motion between the water waves and a nadir looking radar, and the fact that while the instantaneous Doppler frequency at the receiver returned by any elementary group of scatterers on a water wave is monotonically changing, the difference in the Doppler frequency between any two scattering "patches" stays approximately constant as these waves travel parallel to the major axis of an elliptical antenna footprint. The results of a theoretical analysis and a laboratory experiment with a continuous wave (CW) radar that encompasses several of the largest waves in the illuminated area show how the structure in the Doppler spectrum of the backscattered signal is related to the surface spectrum and its parameters in an especially direct and simple way when an incoherent envelope detector is the receiver.     

Acousto-Plasmo-Magnonics: Coupling Spin Waves with Hybridized Phonon-Plasmon Waves in a 2D Artificial Magnonic Crystal Deposited on a Plasmonic Material

Coupling between spin waves (SWs) and other waves in nanostructured media has emerged as an important topic of research because of the rich physics and the potential for disruptive technologies. Herein, a new phenomenon is reported in this family involving coupling between SWs and hybridized phonon-plasmon waves in a 2D periodic array of magnetostrictive nanomagnets deposited on a silicon substrate with an intervening thin film of aluminium that acts as a source of surface plasmons. Hybridized phonon-plasmon waves naturally form in this composite material when exposed to ultrashort laser pulses and they non-linearly couple with SWs to produce a new breed of waves – acousto-plasmo-spin waves that can exhibit a “frequency comb” spanning more than one octave. This phenomenon, that we call acousto-plasmo-magnonics resulting from tripartite coupling of magnons, phonons and plasmons, is studied with time-resolved magneto-optical-Kerr-effect microscopy. The findings also reveal the presence of parametric amplification in this system; energy is transferred from the hybridized phonon-plasmon modes to the acousto-plasmo-spin wave modes to amplify the latter. This opens a path to design novel active metamaterials with tailored and enhanced response. It may enable high-efficiency magneto-mechanical-plasmonic frequency mixing in the GHz−THz frequency regime and provide a unique avenue to study non-linear coupling, parametric amplification, and frequency comb physics.     

Metamaterial covers over a small aperture

Recently, there has been an increased interest in the problem of wave transmission through sub-wavelength apertures, following successful experimental demonstration by several groups for enhancing optical power transmission through nano-scale holes in metallic screens due to properly designed periodic corrugation. Oliner, Jackson, and their co-workers explained and justified this phenomenon as the result of the excitation of the leaky waves supported by the corrugated screen. Here we discuss in detail the mechanism and analysis for another setup we have recently proposed, in which metamaterial layers with special parameters may be utilized as covers over a single sub-wavelength aperture in a perfectly electric conducting (PEC) flat screen in order to increase the wave transmission through this aperture, and we provide a detailed physical insights and analytical explanation for this aperture setup that may lead to similar, potentially even more pronounced effects when the proper metamaterial layers are used in the entrance and the exit face of the hole in the flat PEC screen with no corrugation. Some numerical results confirming this theory are presented and discussed. We also investigate the sensitivity of the transmission enhancement to the geometrical and electromagnetic parameters of this structure.     

Nonuniform azimuth image shift observed in the Radarsat images of ships in motion

This paper describes, for the first time to the authors' knowledge, the phenomenon of nonuniform azimuth image shift of a rigid body observed in the Radarsat synthetic aperture radar (SAR) images of cruising ships. The effect is caused by the different slant-range velocities of coherent scatterers across the hull associated with the ship motions. The slant-range velocity is estimated from the SAR image of an identified ship, and it is compared with that computed from a numerical model using the ship's specification and meteorological data. The result indicates that the dominant contribution to the nonuniform image shift is the pitching motion of the ship. Further comparison is made with the wave orbital velocity, and the results are shown to be in good agreement. Comparisons are also made between the SAR-derived slant-range velocities of two unknown ships and wave orbital velocities, and reasonable agreement is obtained. One of the ships' images exhibits not only nonuniform shift but also image skew. The latter skewing effect may be caused by rolling of the ship.     

Shallow Applications of Geophysical Diffraction Tomography

Diffraction tomography is the generalization of conventional (X-ray) tomography to applications employing diffracting wave fields such as ultrasound, seismic, and low-frequency electromagnetic waves. One approach to diffraction tomography is the filtered back-back-propagation algorithm that has been suggested as a means for subsurface exploration in the oil industry. In this paper we investigate the influence of a number of factors on the quality of tomographic reconstructions obtained via the filtered backpropagation algorithm. These include the presence of strong scatterers, the approximate generation of plane waves, the attenuation of high-frequency components, the density of receivers, and the quality of the received signal. The objective of this study is the acquisition of information to serve as a basis for the design of field instrumentation and implementation of geophysical diffraction tomography. It is found that the density of receivers limits the size of the smallest features that can be imaged, while the loss of high-frequency components limits the image sharpness. The filtered backpropagation algorithm can yield adequate images with at least moderate noise added to the signal. Errors that can occur as a result of the approximate generation of plane waves can be overcome by an appropriate slant stack procedure. Isolated strong scatterers will be difficult to distinguish from weak scatterers in the reconstruction, and the presence of a strong scatterer could obscure nearby features.     

Physical wavelets and radar: a variational approach to remotesensing

Physical wavelets are acoustic or electromagnetic waves, resulting from the emission of a time signal by a localized acoustic or electromagnetic source moving along an arbitrary trajectory in space. Thus, they are localized solutions of the wave equation or Maxwell's equations. Under suitable conditions, such wavelets can be used as “basis” functions, to construct general acoustic or electromagnetic waves. This gives a local alternative to the construction of such waves in terms of (nonlocal) plane waves, via Fourier transforms. We give a brief, self-contained introduction to physical wavelets, and apply them to remote sensing. We define the ambiguity functional, generalization of the radar and sonar ambiguity functions, which applies not only to wideband signals, but also to targets and radar platforms executing arbitrary nonlinear motions     

Current-based hybrid analysis for surface-wave effects on largescatterers

Current-based hybrid analyses combine the method of moments (MM) with Ansatz currents derived from high-frequency methods such as physical optics, physical theory of diffraction (PTD), geometric theory of diffraction (GTD), and the Fock theory. The author introduces an analysis that incorporates a surface-wave basis set into the hybrid formulation. This approach substantially improves the modeling of nonspecular effects caused by surface waves. The discussion emphasizes the accurate representation of surface wave currents and the reduction of computational efforts in comparison with the conventional MM techniques. Scattering calculations for bodies of revolution (BORs) and two dimensional surfaces demonstrate the effectiveness of the analysis presented for large scatterers     

Numerical Solution of Steady-State Electromagnetic Scattering Problems Using the Time-Dependent Maxwell's Equations

A numerical method is described for the solution of the electromagnetic fields within an arbitrary dielectric scatterer of the order of one wavelength in diameter. The method treats the irradiation of the scatterer as an initial value problem. At t = 0, a plane-wave source of frequency f is assumed to be turned on. The diffraction of waves from this source is modeled by repeatedly solving a finite-difference analog of the time-dependent Maxwell's equations. Time stepping is continued until sinusoidual steady-state field values are observed at all points within the scatterer. The envelope of the standing wave is taken as the steady-state scattered field. As an example of this method, the computed results for a dielectric cylinder scatterer are presented. An error of less than /spl plusmn/10 percent in locating and evaluating the standing-wave peaks within the cylinder is achieved for a program execution time of 1 min. The extension of this method to the solution of the fields within three-dimensional dielectric scatterers is outlined.     

A Model for Currents and Voltages Induced Within Long Transmission Cables by an Electromagnetic Wave

A model is presented for determining the transient currents and voltages induced within a long coaxial cable by a uniform plane traveling wave whose variation with time may be specified. The cable may be located above or within a half-space lossy dielectric with the wave incident from the half-space lossless dielectric. This results in a model for either aerial or buried cables with the source of the wave located in the atmosphere. A method is also presented for generalizing the approach to more complex cables encountered in communication systems. A demonstration of the applicability of this model is presented by analyzing the response of a 0.375-in coaxial cable for both aerial and buried situations. An incident electromagnetic wave with a rectangular time domain waveform is used to demonstrate the transient response. This waveshape is adequate to clearly indicate the transient phenomena. It is found that voltage enhancement results in both aerial and buried cables when the incident wave approaches grazing incidence. This voltage enhancement can be significant especially for aerial cables. Cable shield current enhancement also results for an aerial but not for a buried cable.     

A three-wave FDTD approach to surface scattering with applicationsto remote sensing of geophysical surfaces

A major difficulty in physical interpretation of radio wave scattering from geophysical surfaces is the lack of detailed information on the signatures of geologically plausible discrete objects. Although the aggregate response will never be dominated by any single object, differences in the population of discrete objects on or near the surface (their sizes and shapes, for example) can change the character of a radio echo markedly. When the average surface is modelled as a flat, homogeneous half-space, the field that “drives” the scattering process is a composite consisting of the incident plane wave and the reflected and transmitted plane waves, all of which are known quantities; the total field can then be defined as the sum of the driving field and the scattered field. When a discrete object is near the surface, the total field can be calculated using finite-difference time-domain (FDTD) techniques, and the scattered near field can be calculated accordingly. The Green's functions for electric and magnetic currents above and below the surface, obtained by Sommerfeld theory and employed in conjunction with Huygens' principle, transform the local scattered fields to the far field. The FDTD implementation accommodates discrete lossy dielectric and magnetic scatterers in the vicinity of a dielectric surface; extension to a lossy half-space is straightforward. Two-dimensional results for scattering from perfectly conducting circular cylinders above and below a dielectric surface agree with moment method solutions within a few percent. Results for scattering from a dielectric wedge exhibit expected forward diffraction and internal reflection phenomena