Study on composite electromagnetic scattering from the double rectangular cross-section columns above rough sea surface using hybrid method

Compared with scattering from a rough surface only, composite scattering from a target above a rough surface has become so practical that it is a subject of great interest. At present, this problem has been solved by some numerical methods which will produce an enormous calculation amount. In order to overcome this shortcoming, the reciprocity theorem (RT) and the method of equivalent edge currents (MEC) are used in this paper. Due to the advantage of RT, the difficulty in computing the secondary scattered fields is reduced. Simultaneously, MEC, a high-frequency method with edge diffraction considered, is used to calculate the scattered field from the cone-cylinder target with a high accuracy and efficiency. The backscattered field and the polarization currents of the rough sea surface are evaluated by the Kirchhoff approximation (KA) method and physical optics (PO) method, respectively. The effects of the backscattering radar cross section (RCS) and the Doppler spectrum on the size of the target and the windspeed of the sea surface for different incident angles are analysed in detail.

In this paper, the electromagnetic (EM) scattering characteristics from the ship-induced Kelvin wake on the rough sea surface is investigated, which plays an important role in the ship detection and identification as well as the parameter inversions. The profile of ship-induced Kelvin wake is constructed according to the corresponding Kelvin model. And the rough sea surface is obtained through the double superposition method. The Integral Equation Model (IEM) is utilized to calculate the EM scattering from the ship-induced Kelvin wake on the rough sea surface. The bistatic scattering RCS from the ship-induced Kelvin wake on the rough sea surface is compared with that of simple sea surface. In addition, the effects of the ship speed on the scattering features are also discussed. In the end, the general mechanism of the ship-induced Kelvin wake on the rough sea surface is summarized.

Wide-band electromagnetic scattering from multiple objects shallowly buried beneath rough earth soil surfaces has been an important research topic in recent years because of its extensive applications in detecting the buried objects such as mines, pipes, and tunnels. Due to the advantages of finite-difference time-domain (FDTD) method in simulating wide-band electromagnetic scattering from rough surface in the presence of multiple objects, the FDTD method under Gaussian differential pulse wave incidence is utilized in the present study to analyze the frequency response of rough soil surfaces with shallowly buried objects, which serves as a basis for the detection and discrimination of objects buried below rough soil surfaces. The Topp equation model that can predict the dielectric constant of soil-water mixture is adopted in the present study to properly describe the dielectric property of earth soil with water. The actual rough land surface is modeled as the realization of a Gaussian random process with exponential spectrum by using Monte Carlo method. Simulation results show that the variation of composite scattering coefficient with frequency is oscillatory. It is also shown that the composite scattering coefficient versus frequency increases with the increase of root-mean-square of soil surface, water ratio of soil, the target section height, and the separation distance of target. However, simulation results indicate that the composite scattering coefficient versus frequency decreases with the increase of target section width. In summary, the variation of wide-band scattering coefficient is very complicated and is very sensitive to the incidence angle of electromagnetic wave. However, the wide-band scattering coefficient under Gaussian differential pulse wave incidence is less sensitive to the correlation length of rough soil surface, the depth of buried objects, and the dielectric constant of target. These qualitative results relating to the frequency response of rough soil surfaces in the presence of multiple objects are potentially valuable for detecting and discriminating the objects buried below rough soil surfaces by utilizing a wide-band ground penetrating radar system, although the present study is limited to one-dimensional rough soil surface due to the severe computational burden encountered in the large-scale Monte Carlo simulations. In addition, compared with frequency-domain numerical methods, the FDTD method has significant advantages in calculating wide-band composite scattering from rough surfaces in the presence of multiple objects, and thus has extensive applications in radar imaging simulation of multiple objects below or above rough surfaces, which goes beyond the scope of this paper.

Recent and ongoing efforts to characterize sea bed parameters from measured acoustic pulse decay have neglected the effects of sea surface roughness. In this paper, these effects are investigated using a rough surface version of RAMPE, RAMSURF, and random rough surface realizations, calculated from a 2D JONSWAP sea surface spectrum with directional spreading. Azimuthal dependence is investigated for sandy bottoms and found that the rate of pulse decay increases when the surface wave fronts are perpendicular to the path of acoustic propagation and higher significant wave height results in higher decay rates. Additionally, the effects from sea surface roughness are found to vary with different waveguide parameters including but not limited to sound speed profile, water depth, and seabed properties. Of particular interest are the combined effects of sea bed properties and rough sea surfaces. It is shown that when clay like sediments are present, higher-order modes are strongly attenuated and effects due to interaction with the rough sea surface are less pronounced. Finally, possible influences of sea-state and 3D out-of-plane propagation effects on the seabed characterization efforts will be discussed. [Work supported by ONR.]

The rigorous evaluation of the NRCS (Normalized Radar Cross Section) of an object above a one-dimensional sea surface (2D case) needs to numerically solve a set of discretized integral equations involving a large number of unknowns. Thus, the direct solution of the impedance matrix equation via LU decomposition becomes the most expensive step in the MoM (Method of Moments) procedure. So, in order to minimize the computation cost, the iterative domain decomposition method called EPILE (Extended Propagation-Inside-Layer Expansion) was used and then was combined with the FBSA (Forward-Backward with Spectral Acceleration) to calculate the local interactions on the rough sea surface. The resulting fast method is called EPILE+FBSA. In this paper, we take advantage of the rank-deflcient nature of the coupling matrices, corresponding to the object-surface interactions, to further reduce the complexity of the method by using the ACA (Adaptive Cross Approximation). Thus, the coupling matrices are strongly compressed without a loss of accuracy and the memory requirement is then strongly reduced. For a cylinder above a rough sea surface, the results show the e-ciency of the accelerated EPILE+FBSA+ACA method.

This paper presents the characteristics of radio wave scattering from rough sea surfaces in Ka-band. The physical optics method, which is one of the high frequency approximation algorithm, is employed to calculate radar cross section of electromagnetic simulation models of rough sea surfaces with Ship-EDF software. Rough sea surfaces are divided into six levels according to the wind speed over them. The simulation results show the effection that wind speed, the grazinng angle and frequency of radio wave have on the monostatic radar cross section of rough sea surfaces under the conditions of both horizontal and vertical polarization. With the increase of the roughness of sea surfaces and grazing angle of radio wave, the radar cross section value of rough sea surfaces increase. The frequencies of Ka-band have little contribution to the radar cross section value.

A fast hybrid method based on physical optics (PO) and the reciprocity theorem is proposed to analyse the Doppler spectrum of a two-dimensional, moving plasma-coated object above a one-dimensional time-evolving lossy dielectric rough sea surface. This hybrid method eliminates the need for numerical solution of the polarization currents on the object and the time-evolving sea surface, thereby greatly improving the computing speed. The method is verified by conventional numerical methods. The characteristics of the Doppler spectrum of a plasma-coated object above a time-evolving sea surface are analysed in detail, including the effects of several parameters such as the incident angle and the velocity of the moving object. The effects of some key parameters on the stealth performance are also examined.

The problem of very high frequency (VHF) radiowave propagation over the sea surface at low grazing angles is of a great interest for a lot of applications including radar backscattering at different polarizations. Existing theoretical approaches predict much bigger backscattering cross section for vertical polarized (TM) waves as compared to horizontal polarized (TE) waves, and are unable to give satisfactory explanation of experimental results showing that these two values occasionally can be of the same order. In this paper two main features are taken into account in a theoretical investigation of the processes of VHF radiowave backscattering by the rough sea surface at low grazing angles: a) distortion of the incident plane wave by diffraction effects at the sea wave crests prior to scattering by smallscale roughness; b) the concave form of the most part of the visible undulating sea surface. The incident wave diffraction caused by sea wave crests is considered using the simplest model of the crest shape, i.e. a perfectly conducting wedge. It is shown that the diffraction in forward direction at low grazing angles increases the strength of the horizontal polarized field as compared to the vertical polarized one just near the boundary between shadowed and illuminated parts of the surface. Back scattering of electromagnetic waves from cylindrical impedance concave statistically rough surface is studied analytically by the small perturbation method.

The electromagnetic wave transmission of the rough surface of Gaussian layered medium is studied with a hybrid algorithm which is based on MoM (method of moment) and combines with KA (Kirchhoff approximation). In the hybrid algorithm, the upper rough surface and the lower rough surface are placed in the MoM area and the KA area respectively, and the curve of transmission coefficient to change with the scattering angle of transmission wave is solved through numerical calculation. Finally, the relationships of transmission coefficient with the root-mean-square height, the dielectric constant and thickness of the middle-layer medium, and the frequency of incident wave of rough surface are discussed, and then a complete electromagnetic wave transmission characteristic is obtained.

The emissivity model for rough sea surface [Remote Sensing Environ. 24, 313-329 (1988)] is inspected in light of the measured surface emissivity. In the presence of moderate wind (5 m/s or less), the emissivity model is found to be adequate for small to moderate view angles. For large view angles, the discrepancy between the computed and the measured emissivity is large, but one can reduce this considerably by incorporating the reflected sea surface emission into the emissivity model. In addition, examination of the spectral variation of the observed and computed emissivity suggests the need for refined measurements of the complex refractive index. An improved model is constructed to calculate the rough sea surface emissivity that can be used to provide accurate estimates of sea surface skin temperatures from remotely sensed radiometric measurements. An important feature of the improved model is that the computed sea surface emissivity is only weakly dependent on wind speed for most view angles used in practice.

The Kirchhoff approximation (KA) for elastic wave scattering from two-dimensional (2D) and three-dimensional (3D) rough surfaces is critically examined using finite-element (FE) simulations capable of extracting highly accurate data while retaining a fine-scale rough surface. The FE approach efficiently couples a time domain FE solver with a boundary integration method to compute the scattered signals from specific realizations of rough surfaces. Multiple random rough surfaces whose profiles have Gaussian statistics are studied by both Kirchhoff and FE models and the results are compared; Monte Carlo simulations are used to assess the comparison statistically. The comparison focuses on the averaged peak amplitude of the scattered signals, as it is an important characteristic measured in experiments. Comparisons, in both two dimensions and three dimensions, determine the accuracy of Kirchhoff theory in terms of an empirically estimated parameterσ2/λ0(σis the RMS value, and λ0is the correlation length, of the roughness), being considered accurate when this is less than some upper boundc, (σ2/λ0<c). The incidence and scattering angles also play important roles in the validity of the Kirchhoff theory and it is found that for modest incidence angles of less than 30°, the accuracy of the KA is improved even whenσ2/λ0>c. In addition, the evaluation results are compared using 3D isotropic rough surfaces and 2D surfaces with the same surface parameters.

The scattering of an acoustic signal incident from below at low angles on a rough sea surface is treated by the integral equation method in the parabolic approximation. Equations are obtained allowing the mean scattered field to be calculated even when the surface causes a large phase modulation in the incident wave. Solutions are found using the method of Laplace transforms and some results are presented for a specific type of rough surface.

At wind speeds higher than a few meters per second, when breaking waves are present, the sea surface roughness is accompanied by an assemblage of microbubbles forming different structures or clouds, varying in range and in time. For simplicity, in terms of an average bubble population, the time and space varying assemblage of microbubbles is often assumed to be uniform and referred to as the subsurface bubble layer. Of fundamental importance is the role that this subsurface bubble layer may play in connection with scattering from a rough air/sea interface. The purpose of this paper is to determine the effect that the rough surface in the presence of this subsurface bubble layer has on the total field at low frequencies. Numerical calculations indicate that at low frequencies the bubble layer shifts the incident angle on the surface to steeper angles. In addition it was found that the enhancement of the total field is a consequence of scattering at the rough surface in the presence of the upper refracting bubble layer. An enhancement of approximately 40 dB with respect to the bubble-free medium was obtained very near the surface for a frequency of 400 Hz, with a nominal grazing angle of 20 deg, and a void fraction at the surface of 3.2×10−5. The enhancement decreases to about 10–15 dB at 10 m below the surface, but is still significant at depth exceeding the bubbly region. For a void fraction at the surface of 3.1×10−6 the enhancement is approximately 5 dB.

Summarizes the scattering properties of a rough surface as a function of observation parameters, physic and geometric characteristics of the surface. This study is applied to the oceanic surface. Results are presented in the form of polarimetric signature as a function of the transmitter location and receiver location for a bistatic radio-link over the rough sea surface. The like- and cross-polarized received power are directly calculated using the scattering coefficients, without calculating the Mueller matrix. The Kirchhoff model and the small perturbation method are applied to determine the scattering coefficients. The polarization states, obtained for the minima and maxima received power in order to increase the performance of a bistatic radio-link, or to improve the discrimination power of a radar target are investigated. It is observed that for a fixed transmitter location and receiver location it is possible to choose a polarization state minimizing the scattered power from a rough sea surface, independently of the sea state. An electromagnetic wave scattered from a target is a vectorial quantity containing polarimetric information. This information is a function of the target, and also depends on the observation parameters. The purpose of this paper is to present, the polarimetric signature of the rough sea surface in order to increase the performance of a bistatic radio link (decreasing fading effect, for example), or to improve the discrimination power of a radar target or to identify a target.

A variable depth step implementation of the range-dependent acoustic model (RAM) is applied to the modeling of forward scattering from a rough sea surface. The sea surface is treated within RAM simply as an internal interface between a water layer and an air upper halfspace. A comparison with a numerically exact integral equation is undertaken for the scattering of single frequencies from Pierson-Moskowitz sea surfaces. The method is extended to model the variability of linear frequency modulated pulses from a series of frozen sea surfaces in a shallow water waveguide. The subsequent effect of rough boundary scattering on the replica correlation process is investigated.