Physical Optics Method Research Articles

ANALYSIS OF EFFECT OF KOREAN OFFSHORE WIND FARMS ON ACCURACY OF X-BAND TRACKING RADAR

Many government-led wind farms are being constructed in Korea as sources of clean and renewable energy. However, construction of these wind farms is continuously opposed by nearby military bases that operate X-band tracking radar because the amplitude and phase of the electromagnetic waves reflected from the wind turbines may interfere with tracking radars installed along the coastline of the Korean peninsula. This paper proposes a method to analyze the effect of a wind farm on tracking radar, and presents the results of using the radar cross section of the blade of a real turbine predicted by the method of physical optics. Simulation results using various flight scenarios demonstrate that the tracking accuracy may be considerably degraded; thus appropriate action is required to eliminate this effect.

BISTATIC RCS PREDICTION OF COMPOSITE SCATTERING FROM ELECTRICALLY VERY LARGE SHIP-SEA GEOMETRY WITH A HYBRID FACET-BASED KA AND SHADOW-CORRECTED GRECO SCHEME

This paper presents a hybrid scheme for fast calculation on the bistatic composite scattering from electrically very large ship-sea geometry at high frequencies. Based on the Kirchhofi approximation (KA), we try to break the large-scale sea surface into myriads of plane facets, then derive the Kirchhofi integration analytically on each individual discretized facet. The analytical expression obtained, so- called the \facet-based Kirchhofi approximation (FBKA), is suitable for a quick scattering calculation on the electrically very large sea surface, since it is beyond the intensively reflned meshes as the usual Monte Carlo implementation does. Meanwhile, combined with graphical electromagnetic computing method (GRECO) to extract the illuminated and shadow facets in accordance with the incident direction, the conventional physical optics method (PO) is improved by employing current marching technique (CMT) to calculate the currents in the shadow region. The shadow-corrected GRECO is presented in this hybrid model to solve the bistatic scattering from complex and very electrically large perfectly electric conducting (PEC) objects. The accuracy of the shadow-corrected GRECO is conflrmed well by exact numerical methods, especially at large scattering angles. The electromagnetic interactions between the ship and sea surface are estimated by the famous \four-path model, which has been proved to be valid for ship scattering at relatively calm sea state. Several numerical examples have been presented to demonstrate the e-ciency and accuracy of the proposed hybrid method.

Monte Carlo Simulation of the Echo Signals from Low-Flying Targets for Airborne Radar

A demonstrated hybrid method based on the combination of half-space physical optics method (PO), graphical-electromagnetic computing (GRECO), and Monte Carlo method on echo signals from low-flying targets based on actual environment for airborne radar is presented in this paper. The half-space physical optics method , combined with the graphical-electromagnetic computing (GRECO) method to eliminate the shadow regions quickly and rebuild the target automatically, is employed to calculate the radar cross section (RCS) of the conductive targets in half space fast and accurately. The direct echo is computed based on the radar equation. The reflected paths from sea or ground surface cause multipath effects. In order to accurately obtain the echo signals, the phase factors are modified for fluctuations in multipath, and the statistical average value of the echo signals is obtained using the Monte Carlo method. A typical simulation is performed, and the numerical results show the accuracy of the proposed method.

A NEW HYBRID MOM-GEC ASYMPTOTIC METHOD FOR ELECTROMAGNETIC SCATTERING COMPUTATION IN WAVEGUIDES

This paper presents a new hybridization between MoM-GEC and some asymptotic methods. In fact, a new hybrid current function based on Physical Optic (PO) and a modal method is developed. The approach consists in approximating the total current on an invariant metallic pattern on two parts; the inside of metal is governed by PO method; however, the edges are modeled by infinite cylinders and described by Hankel functions (modal method). The considered single test function is required then by MoM method to replace a lot of sinusoidal or triangular test functions, in order to get a rapid convergence and less computational time. For validation purposes, the new developed hybrid approach is applied to compute scattering in different structures. The obtained input impedances, currents and fields distributions are in agreement with those obtained by MoM method. Considerable gain in computational time and memory resources is achieved.

HIGH-FREQUENCY METHOD FOR TERAHERTZ RADAR CROSS SECTION OF CONDUCTIVE TARGETS IN FREE SPACE

The high-frequency method for the prediction of the terahertz (THz) radar cross section (RCS) of conductive targets with extremely electrically large size in free space was presented. In order to consider the scattering flelds of the perfectly electric conducting (PEC) targets with extremely electrically large size in free space, the Green's function was introduced into the conventional physical optics (PO) method which was combined with the graphical electromagnetic computing (GRECO) method and improved using the partition display algorithm. The shadow regions were eliminated quickly by displaying lists of OpenGL to rebuild the targets, and the geometry information was attained by reading the color and depth of each pixel. The THz RCS of conductive targets can be exactly calculated in free space. The RCS comparison between the partition display GRECO prediction by the self-written Visual C++ 2010 program and the simulation of FEKO software with the large element PO method proves the validity and accuracy of the proposed method. The results provide an important basis and method for the potential applications of THz radar in many flelds such as military, astronomy and remote sensing.

MLFMA-PO Hybrid Technique for Efficient Analysis of Electrically Large Structures

A hybrid technique combining the multilevel fast multipole algorithm (MLFMA) and the physical optics (PO) is presented for analyzing scattering and radiation by electrically large structures. In this technique, MLFMA is used to characterize the complex parts such as antennas that cannot be analyzed by high-frequency asymptotic methods. The large smooth bodies are calculated using PO method because of its high computational efficiency. The interaction between method-of-moments (MoM) region and PO region is strictly considered and accelerated by MLFMA, which can guarantee the high efficiency without losing the calculation accuracy. Numerical results are given to demonstrate the reasonable accuracy and high capability of the proposed hybrid technique compared to the conventional MLFMA and MoM-PO method.

급전 함수를 고려한 수정곡면 캐서그레인 안테나 설계

에너지 보존 법칙과 스넬(Snell)의 법칙 및 경로 길이에 의한 동위상의 조건으로부터 수정곡면 캐서그레인 안테나 반사판의 단면을 설계하였다. 원형 개구면에서 균일 개구면 위상을 갖도록 개구면의 수직 및 수평 급전분포 함수에 따른 동위상 조건을 만족하는 3차원 반사판 곡면의 프로파일을 구하였다. 직경 55 <TEX>${\lambda}_0$</TEX>의 크기를 갖는 Ku 밴드용 수정 곡면 캐서그레인 안테나 반사판을 물리광학에 의한 시뮬레이션 결과, 36.4 dB, 33.9 dB의 부엽 레벨을 AZ 및 EL 방향에 대해서 각각 얻을 수 있었으며, 이전 안테나와 비교하여 10 %의 지향성 개선효과가 있었다. A shaped Cassegrain antenna is designed from the condition of the same path length at the equi-phase surface by using the conservation of energy and Snell's law. In order to improve the phase error efficiency of aperture surface, the surface profile of the main and sub-reflectors is found to satisfy the power distribution and the equi-phase condition at the aperture surface. The sidelobe levels of 36.4 dB and 33.9 dB are achieved at the AZ and EL planes, respectively from numerical calculation by physical optics method at Ku band and the directivity of designed antenna is 10 percent greater than that of conventional antenna.

Development of Acoustic Target Strength Analysis System for Submarine

【The acoustic target strength (TS) is one of the most important parameters for a submarine's stealth design. Because modem submarines are larger than their predecessors, TS must be managed at each design stage in order to reduce it. To predict the TS of a submarine, TASTRAN R1 was developed based on a Kirchhoff approximation in a high-frequency range. This program can present TS values that include multi-bounce effect in the exterior and interior of the structure by combining geometric optics (GO) and physical optics (PO) methods, anechoic coating effect by using the reflection coefficient, and response time pattern for a detected target. In this paper, TS calculations for a submarine model with the above effects are simulated by using this developed program, and the TS results are discussed.】

Fringe waves on an impedance cylinder

Surface diffraction of the plane waves from an impedance cylinder was investigated by the method of physical theory of diffraction (PTD). Physical optics (PO) method was used to gain the uniform scattered fields which are produced by the induced surface currents. Nonuniform currents and scattered fields were obtained by the method of PTD with the assistance of the exact solutions.

Physical optics methods for laser and nonlinear optics simulations

Abstract The development of optical systems today is strongly supported by computer simulations. Computer ray tracing tools are well established in the field of geometrical optics for designing objectives, light channels, etc. Ray tracing is conceptually quite simple, which makes the implementation as well as the interpretation of ray tracing results easy, although the design of high-quality lens systems remains a challenging task. In the field of wave optics, there are software tools as well, but they are still not as mature as the ray tracing tools. The main reason for this is that numerically solving partial differential equations is numerically more complex and not as intuitive as ray tracing. Besides simple wave propagation, the main field of use of the wave optics methods today is the development of lasers and laser resonator and nonlinear optics. The optical properties of the media, in general, depend on the temperature and mechanical stresses. The interaction of the electromagnetic fields with matter, thus, has to self-consistently include thermomechanical computations.

Passive inter-modulation scattering analysis of reflector considering contact nonlinearity

Passive inter-modulation (PIM) is a form of nonlinear distortion caused by the inherent nonlinearities of the passive devices and components in RF/microwave system. It will degenerate the performance of communication system with broad-band channel and high-sensitivity receiver. Therefore, it is necessary to construct a model to simulate this process in order to predict the level of PIM. This paper is aimed at constructing some plate models with one-dimensional and two-dimensional contact nonlinearity sections illuminated by two-tone waves, and calculating the scattered field at a fixed-point in space using time-domain physical optics method. By taking fast Fourier transform (FFT), we get the spectrum of the scattered field and then analyze the generated PIM products. At the end of this paper, some numerical examples are presented to show the influence rules of the relative factors on PIM. The results indicate the variation of the level of PIM with the number of the nonlinear regions, the nonlinear spacing, and the incident power levels.

A study on simulation method of starlight transmission in hypersonic conditions

By taking a hypersonic vehicle as the research object, the simulation method of starlight transmission in hypersonic conditions is established to qualitatively and quantitatively research the platform aero-optical effects on starlight navigation, and to provide a theoretical basis for the extended application of the starlight autonomous navigation. At first, based on Reynolds average, the k–ω/SST two-equation turbulence model is used to calculate the density distribution of external flow field when the star sensor is installed at different locations in the vehicle (head, middle and tail), and then the spatial refractive index distribution of the flow field is obtained. On this basis, we use the geometrical optics method and physical optics method to calculate the optical path difference, the point spread function, the image offset and such optical transmission effects caused by the laminar flow field, and then obtain a laminar flow diagram to lower clarity. At the mean time, we calculate wavefront distortion, density, standard deviation, variance and other optical phase-transfer effects of turbulent flow field by using statistical theory optical, and obtain a turbulent flow diagram to lower clarity.

MICRO-DOPPLER ANALYSIS OF KOREAN OFFSHORE WIND TURBINE ON THE L-BAND RADAR

To cope with the energy shortage and the rising cost of the fossil fuel, many wind farms are being constructed under the supervision of Korean government along the coasts of Korean peninsula to generate clean and renewable energy. However, construction of these wind farms may cause negative efiect on various L-band radars in operation. This paper presents the result of the micro-Doppler (MD) analysis of the in∞uence of the wind turbine on the L-band radar using the point scatterer model and the radar cross section of the real turbine predicted by the method of physical optics. The simulation results obtained at three observation angles show that the range of MD occupies a considerable portion of the helicopter MD range, and thus, the operations using helicopters need to be avoided in the wind farm region, and additional radars are required for the recognition of helicopter-like objects.

Time-Domain Physical Optics Method for the Analysis of Wide-Band EM Scattering from Two-Dimensional Conducting Rough Surface

Time-domain physical optics (TDPO) method is extended to investigate electromagnetic (EM) scattering from two-dimensional (2D) perfectly electrically conducting (PEC) rough surface in both time domain and frequency domain. The scheme requires relatively small amounts of computer memory and CPU time, and has advantage over the Kirchhoff Approximation (KA) method in obtaining transient response of rough surface by a program run. The 2D Gaussian randomly rough surface is generated by Monte Carlo method and then is partitioned into small triangle facets through the meshing preprocess. The accuracy of TDPO is validated by comparing the numerical results with those obtained by the KA method in both backward and specular directions. The transient response and its frequency distribution of radar cross section (RCS) from rough surface is shown, respectively. The scattering results from rough surface with different size in the specular direction are given. The influence of the root mean square height (σ) and correlation length (l) on electromagnetic scattering from PEC rough surface is discussed in detail. Finally, the comparisons of backscattering results at different incident angles are presented and analyzed.

Fast Far Field Computation of Single and Dual Reflector Antennas

The physical optics (PO) method has been widely used for the analysis of the electromagnetic behavior of single and dual reflector antennas. An extensive work has been done by the authors of this paper in order to increase the speed for obtaining far field patterns from single and dual geometries and also in order to increase the accuracy of the method. This paper reviews these contributions and improves the existing published work with the physical interpretation of the radiation from a single patch and the computer implications when using acceleration techniques such as OpenMP.

Implementation of Reflection on Curved Surfaces and Physical Optics in Ray Tracing for Tunnel Propagation

For the modeling of multipath propagation in every wireless systems, the ray tracing method has been widely studied. However, large errors may result due to the approximation of geometrical optics in curved surfaces. This paper therefore focused on the curved surfaces and edges, which are difficult to handle in ray tracing. Examples of curved surfaces can be found in arched cross-section tunnels which are common in highway networks of mountainous areas. The traditional ray tracing method of dividing the curved surface into smaller flat plates is not so accurate as the size of smaller plates may not satisfy the geometrical optics assumption, and the reflection point which satisfies Fermat's principle may not exist. In this work, a new ray tracing method is proposed with 2 contributions. The first one is the implementation of the reflection coefficient for curved surfaces in ray tracing. The second is applying the physical optics method on the caustics region. To evaluate these methods, path gain simulation results for an arched cross-section model are compared with measurements made inside an arched tunnel. To further improve the simulation results, the effect of rough surface is introduced, and the results are again compared with measurement.

FRINGE WAVES IN AN IMPEDANCE HALF-PLANE

The uniform expressions of scalar fringe waves which are based on the physical theory of difiraction (PTD) were obtained for the impedance half plane in terms of the Fresnel integrals. Asymptotic and uniform forms of the fringe flelds were compared. The radiated flelds of the fringe expressions were analyzed numerically. Physical optics (PO) is based on the integration of surface currents which are induced by the incident flelds. This concept is valid and accurate for metallic scatterer surfaces and gives exact geometric optics (GO) waves in the high frequencies. There are lots of studies in the literature about the application of the PO method (1{3). The edge difiracted flelds which are found from the contribution of the PO surface integral are not correct. In addition PO currents just consider the illuminated part of the scatterer and ignore the shadow regions. The physical theory of difiraction (PTD) method was proposed by Uflmtsev to overcome the flrst defeat (4). The PTD consists of two main parts. The flrst part is the uniform part which is obtained from the PO and the second part is the contribution of the non-uniform or fringe part. The fringe part is the result of the fringe currents. It is pointed out that the fringe currents are obtained by dividing the induced surface currents into two parts. The flelds, radiated by the fringe currents, can be found by subtracting of uniform part from the exact solution. The method is widely used in the literature for investigation of the scattering problems (5{8). Using the uniform fringe flelds instead of the non-uniform version is more reliable. Non- uniform fringe flelds can be obtained by the subtraction of asymptotic

Numerical study of extinction of visible and infrared radiation transformed by preferentially oriented plate crystals

Extinction properties of an ensemble of semi-transparent plate crystals in preferred orientation are studied at wavelengths of visible and infrared ranges using the physical optics method. The work illustrates some calculations of the differential scattering coefficient in the near-forward and the exact forward directions. The features of energy and polarization characteristics of directly transmitted radiation through a set of particles are discussed, as depended on parameters of medium and the incident radiation. It is shown that the effect of polarization on extinction by large plates at certain combination of microphysical parameters of medium and the wavelength can be pronounced even for clean ice. By increasing the real part of complex refractive index, the polarized effect can be increased a few times. It is illustrated as an influence of flutter of plates on the considered characteristics.

GPU-Based Combination of GO and PO for Electromagnetic Scattering of Satellite

In the radar cross section (RCS) prediction of complex target, the intensive computational burden occurs while calculating the multiple scattering effect. In order to overcome the large computing, we present the program executing on graphics processing units (GPUs). In this paper, we analyze the scattering properties of the satellite, on which the antennas are described as cubes and columns, by employing the GPU-based combinational method of geometrical optics (GO) and physical optics (PO) together with the kd-tree technique. Furthermore, due to this distinctive treatment, the improved method yields a superior performance at high frequency. Some examples will be displayed in the following text. The agreement of the results yielded in this paper with the experimental and other exact results demonstrates the accuracy and efficiency of this useful technique.

RCS of Complex Targets: Original Representation Validated by Measurements—Application to ISAR Imagery

In this paper, we propose and investigate a model based on asymptotic methods, designed to compute the radar cross section (RCS) of complex targets. A combined method of physical optics, geometrical optics, and the equivalent current method is applied to establish an effective backscattering analysis procedure. The scattering model includes shadowing effects, multiple bounce, and diffraction by edges. This model is used for RCS estimation and to generate inverse synthetic aperture radar (ISAR) raw data for imaging applications. We review the theoretical aspects and describe the proposed model and experimental setup in detail. Numerical results are provided and compared with the experimental results obtained in the anechoic chamber of ENSTA Bretagne to validate the approach through the computation of the RCS for canonical objects and a generic boat. Finally, a parallelepiped representation of the RCS is presented to be used for ISAR imagery, and the first results concerning the ISAR imaging of a generic boat are provided.