Analysis of radio wave scattering from rough sea surfaces based on high frequency approximation algorithm

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.

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.

In this paper, a study on the optimization of fuzzy classifier using radar cross section(RCS) values is presented to classify shell- shaped targets. Method of moments(MOM) is exploited to construct RCS database of generic shell-shaped targets in uniform angular intervals. Relative orientations are estimated from various flight scenarios of shell-shaped targets, and associated RCS values are inter-polated from the generated RCS database with uniform angular intervals. Initial membership functions are determined using the inter-polated RCS values, and particle swarm optimization(PSO) is utilized to optimize the membership functions of the fuzzy classifier in terms of probability of correct classification.Key words: Classification, Fuzzy, RCS, PSO]$ 2013^4LIG_1` $ C `78$ abc.d e fg-hij@fhg(Department of Electrical Engineering, Pohang University of Science and Technology) *LIG_1`(LIGNEX1) Manuscript received December 12, 2013 ; Revised January 13, 2014 ; Accepted February 10, 2014. (ID No. 20131212-127)Corresponding Author: Kyung-Tae Kim (e-mail: kkt@postech.ac.kr)

Based on the method of physical optics (PO) and the method of ray tracing (RT), we analyzed the near-field primary scattering of the panels, the near-field secondary scattering between the facets, the near-field composite scattering between facets and rough sea surface. Simultaneously, we calculated the near-field and far-field scattering versus incident angles by a dihedral made of two orthogonal plates and a ship model, respectively, and then discussed the characteristics of the near-field and far-field scattering. The near-field composite scattering between the ship model and sinusoidal composed surface were calculated and analyzed.

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 problem of electromagnetic scattering from a randomly rough surface is analyzed using the Kirchhoff approximation (stationary phase, scalar approximation), the small perturbation model and the two-scale model. The authors show that physical and geometrical optics approximations can be used for a rough sea surface. A small perturbation model gives a satisfactory solution. Unfortunately, it only applies to a smooth sea surface. Therefore, these methods have been shown to be limited in their regions of validity. Comparisons between the numerical calculations and the models are made for various surface rms heights and correlation lengths both normalized to the incident wave number (denoted by k/spl sigma/ and kL, respectively). The two-scale model proposed in this paper is the method which tries to conciliate the two previous aspects. One application is made in the ocean surface case. A set of experimental data, describing simultaneous bistatic radar cross section and forward radar cross section measurements of sea clutter taken with a moderately high resolution X-band scatterometer system operating near grazing incidence for both horizontal and vertical polarizations over a range of low grazing bistatic angles, is used but not presented in this paper to examine the applicability of bistatic scattering models.

In this paper, Physical Optics (PO) is used to calculate the electromagnetic scattering of electrically large complex target and background. Two dimensional surface is simulated by using spectral method based on PM spectrum. Then, the target and rough surface model are combined together to be a very accurate composite model, after which surface mesh is generated with the information of incidence frequency. Shading judgment and the radar cross section (RCS) of the composite model is calculated. The results of the validation model show good agreement with the FEKO (MoM) method. Finally, the bistatic RCS of the actual large rough sea surface with missile are given.

The parabolic equation is the main method to predict the propagation of radio waves in the marine environment. The rough sea surface model based on a two-scale roughness method is built in the paper. At the same time, the path losses of this rough sea surface under different propagation conditions are calculated using the three-dimensional parabolic equation (3DPE) method. The results show that the path loss using the 3DPE method changes under different wind directions. When the wind speed over the sea surface is high, the radio waves of propagation direction are greatly interfered by the surrounding electromagnetic fields. Therefore, the effect of sea breeze on radio wave propagation can be accurately reflected using this 3DPE method.

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.

A small-slope approximation (SSA) is used for numerical calculations of a radar backscattering cross section of the ocean surface for both K u - and C-bands for various wind speeds and incident angles. Both the lowest order of the SSA and the one that includes the next-order correction to it are considered. The calculations were made by assuming the surface-height spectrum of Elfouhaily et al for fully developed seas. Empirical scattering models CMOD2-I3 and SASS-II are used for comparison. Theoretical calculations are in good overall agreement with the experimental data represented by the empirical models, with the exception of HH-polarization in the upwind direction. It was assumed that steep breaking waves are responsible for this effect, and the probability density function of large slopes was calculated based on this assumption. The logarithm of this function in the upwind direction can be approximated by a linear combination of wind speed and the appropriate slope. The resulting backscattering cross section for upwind, downwind and cross-wind directions, for winds ranging between 5 and 15 m s−1, and for both polarizations in both wave bands corresponds to experimental results within 1–2 dB accuracy.

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.

The actual rough sea surface is described by Pierson-Moscowitz (PM) spectrum and Monte Carlo method; the composite electromagnetic scattering from the double rectangular crosssection conductor columns above the rough sea surface is investigated using the hybrid methodthe Method of Moment with the Kirchhoff approximation. Angular distribution curves of composite scattering coefficient are obtained; and this coefficient that varies with the parameter of sea surface, rectangular cross-section conductor columns and incident waves is calculated in detail. The characteristics of the composite scattering coefficient from the double rectangular cross-section conductor columns above the rough sea surface are also obtained. Results show that the hybrid methodthe Method of Moment with the Kirchhoff approximation, compared to the traditional Method of Moment, can obtain higher accuracy. Moreover, the hybrid method can effectively reduce the computation time and memory footprint, and the performance becomes more favorable with the increase of the sizes of rough surface.

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.

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.