Browsing by Subject "Physical optics."

Now showing 1 - 4 of 4

Open Access
Application of characteristic basis function method for scattering from and propagation over terrain profiles
(2009) Yağbasan, Atacan
A computationally efficient hybrid method, that combines the characteristic basis function method and the physical optics as well as the forward backward method, is applied for the solution of integral equations used to investigate the electromagnetic scattering from and propagation over large scale rough terrain problems. The method utilizes high-level basis functions defined on macro-domains (named as blocks) namely characteristic basis functions that are constructed by aggregating low-level basis functions (i.e., conventional sub-domain basis functions). In the construction of the abovementioned characteristic basis functions, forward backward method as well as the physical optics approach (when applicable) are used. The conventional characteristic basis function method originally developed by Prakash et al. is slightly modified to handle large terrain problems, and is further embellished by accelerating it and by reducing its storage requirements via the use of an extrapolation procedure. Numerical results for the induced currents, total fields and path loss are presented and compared with either measured or previously published reference solutions to assess the efficiency and the accuracy of the method. Besides, certain parametric studies and convergence tests have been carried out.
Open Access
Comparison of two physical optics integration approaches for electromagnetic scattering
(2008) Öztürk, Ender
A computer program which uses two different Physical Optics (PO) approaches to calculate the Radar Cross Section (RCS) of perfectly conducting planar and spherical structures is developed. Comparison of these approaches is aimed in general by means of accuracy and efficiency. Given the certain geometry, it is first meshed using planar triangles. Then this imaginary surface is illuminated by a plane wave. After meshing, Physical Optics (PO) surface integral is numerically evaluated over the whole illuminated surface. Surface geometry and ratio between dimension of a facet and operating wavelength play a significant role in calculations. Simulations for planar and spherical structures modeled by planar triangles have been made in order to make a good comparison between the approaches. Method of Moments (MoM) solution is added in order to establish the accuracy. Backscattering and bistatic scattering scenarios are considered in simulations. The effect of polarization of incident wave is also investigated for some geometry. Main difference between approaches is in calculation of phase differences. By this study, a comprehensive idea about accuracy and usability due to computation cost is composed for different PO techniques through simulations under different circumstances such as different geometries (planar and curved), different initial polarizations, and different electromagnetic size of facets.
Open Access
Implementation of physical theory of diffraction for radar cross section calculations
(2002) Öztürk, Alper Kürşat
A computer program which uses the Physical Theory of Diffraction (PTD) method to calculate the Radar Cross Section (RCS) of perfectly conducting targets with arbitrary shape is developed. Given an arbitrary surface, it is first meshed using planar triangles. The area of each triangle is restricted to be smaller than 0.005λ 2 in order to obtain a good approximation to the actual surface. After meshing, Physical Optics (PO) surface integral is numerically evaluated over the whole surface. If the surface has edges or wedges, diffractions originating from these edges play a significant role in the overall scattered field. This part of the diffracted field is calculated using PTD-EEC method. Calculation of the edge currents is made possible by canonically modelling the arbitrary-shaped edge. If the surface of the scatterer has thin wires attached to it, then the thin wire scattering formulation in the literature is applied. Expressions for scattering mechanism on a straight wire are based on diffraction, attachment, reflection and launch. The results get sufficiently accurate especially for electrically large bodies.
Open Access
Memory-efficient multilevel physical optics algorithm for the solution of electromagnetic scattering problems
(2007) Manyas, Kaplan Alp
For the computation of electromagnetic scattering from electrically large targets, physical optics (PO) technique can provide approximate but very fast solutions. Moreover, higher order approximations, such as physical theory of diffraction (PTD) including the diffraction from the edges or sharp corners can also be added to the PO solution in order to enhance the accuracy of the PO. On the other hand, in real-life radar applications, where the computation of the scattering pattern over a range of frequencies and/or angles with sufficient number of samples is desired, further acceleration may be needed. Multilevel physical optics (MLPO) algorithm can be used for such applications, in which a remarkable speed-up can be achieved by evaluating the PO integral in a multilevel fashion. As the correction terms like PTD are evaluated independently just on the edges or sharp corners, whereas the PO integration is carried out on the entire target surface, PO integration is the dominant factor in the computational time of such higher order approximations. Therefore accelerating the PO integration will also reduce the computational time of such higher order approximations. In this thesis, we propose two different improvements on the MLPO algorithm.First improvement is the modification of the algorithm that enables the solution of the scattering problems involving nonuniform triangulations, thus decreasing the CPU time. Second improvement is the memory-efficient version, in which the O (N3 ) memory requirement is decreased to O (N2 log N). Efficiency of the two proposed improvements are demonstrated in numerical examples including a reallife scattering problem, with which the scattering pattern of a three-dimensional stealth target is evaluated as a function of elevation angle, azimuth angle, and frequency.