摘要:

In this paper we present an overview of the two and three dimensional absorbing boundary conditions (ABCs) that can be imposed at an artificial boundary in solving open region electromagnetic scattering problems, using finite difference or finite element techniques. A listing of the different types of ABCs available in the literature is given together with a brief description of the problems being solved by imposing these boundary conditions.

ISSN:0920-5071    

DOI:10.1163/156939391X00347

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A pulsed Laser Radar "LADAR" system is evaluated for detecting and tracking cooperative retro-reflector carrying targets against noise, clutter, back-scatter, and other propagation effects. A set of equations, developed for the propagation and tracking of conventional Microwave Radar systems, was reconfigured and adapted to the optical case. As a result, an efficient statistical method is developed for computing the pulsed LADAR system effective range as a function of false alarm and detection probabilities. An optimization process enables significant increase in operation range with conventional equipment. New methods for improving the LADAR systems range and performance are discussed. The presented analysis and computation method has proved to be useful in the design stages of operational LADAR detection and tracking systems.

ISSN:0920-5071    

DOI:10.1163/156939391X00356

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Random errors on a reflector surface will result in antenna gain loss, beam efficiency reduction, and sidelobe level degradation. In this study, an improved mathematical model is presented for handling correlated random surface errors. The average power patterns of arbitrarily distorted antennas are determined by performing a complete numerical integration. Furthermore, in the case of a parabolic reflector antenna with random surface errors that have relatively short correlation length, the effective aperture distribution parameters, Be and pe, can be defined and analytically derived. Numerical data for a parabolic reflector antenna with dimensions recommended by a recent NASA study are generated to investigate the effects of uniform and linear random surface errors on power pattern, boresight gain loss, and beam efficiency. The results properly characterize the pattern degradation and the beam efficiency reduction as a function of random surface errors. For high beam efficiency and low sidelobe specifications, the acceptable tolerance on the surface errors becomes more stringent. Several charts on the beam efficiency are presented as general guidelines to be used by the designers of large reflector antennas.

ISSN:0920-5071    

DOI:10.1163/156939391X00365

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Rain measurements are being conducted by microwave passive techniques from air as well as from space. Microwave radiometry is also applied to ocean surface measurements. They are usually examined separately, and the rain and ocean surface roughness due to wind are regarded as a perturbing factor to each other's objectives. From a standpoint of rain measurements, however, ocean surface condition is important, since it determines the lower boundary condition for microwave radiative transfer. In this paper, an influence of ocean surface roughness on microwave passive measurements of rain from an airborne platform is evaluated by a numerical computation. A two-scale rough surface model is adopted to characterize the microwave scattering signature from a rough ocean surface. The results of computation show that the nadir-looking configuration minimizes the influence of ocean surface roughness on rain measurements. The influence can be partly reduced by using excess brightness temperatures for rain rate estimation. A comparison between the theoretical results and the measurements is also made.

ISSN:0920-5071    

DOI:10.1163/156939391X00374

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The WKB method and its generalization are extended to stratified chiral media for waves with oblique incidence. Wave propagation in chiral media inhomogeneous in one space coordinate is considered through normalized wave fields, a certain combination of the electric and magnetic fields, which are slightly coupled in slightly inhomogeneous media. Omitting the coupling leads to the WKB solution for waves propagating without reflection. The reflection effect is obtained as a first order correction to the WKB solution. For sufficiently slowly varying media, the reflection dyadic due to the stratification can be expressed in closed integral form.

ISSN:0920-5071    

DOI:10.1163/156939391X00383

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A problem originating in radar polarimetry is considered for which the radar target is to be characterized by its 4 x 4 Mueller matrix [M] properties in terms of a partially polarized wave treatment for the monostatic reciprocal and non-reciprocal cases. In order to compare our results with previous coherent treatments of optimizing the corresponding 2 x 2 Sinclair matrix [S], first the vector formalism for the coherent case is introduced. The coherent formulation is then extended via the coherency matrix (vector) approach to the partially polarized case with the aid of the Kronecker expansion matrix and the Stokes vector formalism. Distinction of the Stokes Reflection Matrices is made between the one-antenna and two-antenna cases. For the one-antenna case, the reception of the optimal energy densities and powers is accomplished separately for the co-polarized and cross-polarized channels, which is a natural approach in radar polarimetry. For the two-antenna case, the reception of the optimal power is accomplished by adjusting the polarization state of the receiving antenna to match or mismatch the polarization state of the scattered wave. The "degenerate Mueller matrix case" for purely coherent wave reception is analyzed first to facilitate comparison with previous methods of optimizing the corresponding Sinclair matrix [8]. Here, the Lagrangian multiplier method is used for determining the characteristic optimal polarization states subject to the constraint that the incident wave is purely polarized. It is then shown that for the more general partially polarized scattered wave case there exists, next to the total energy density, three specific types of energy densities: (i) the adjustable purely polarized intensity, (ii) the noise-like unpolarized intensity, and (iii) the receivable polarized intensity, for which separate optimization methods are developed. For the first category, a closed solution may be found, whereas for the second two criteria numerical solutions must be used. The paper concludes with a comparison of the various results obtained here with those obtained by other methods.

ISSN:0920-5071    

DOI:10.1163/156939391X00392

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Near-field measurements can be used as diagnostic tools to reconstruct aperture fields of array antennas and to locate their defective elements. In this paper, a computer simulation is performed to evaluate the effect of various near-field parameters such as the measurement distance and the sample spacing on the diagnostics of faulty elements in array antennas. The near-field of an N x N dipole array containing defective elements is simulated on a planar surface a distance d from the antenna with a given sampling spacing of S. Next, additive Gaussian noise is added to the near-field and the result is back-propagated to reconstruct the source. The measurement distance, the sample spacing, and the noise level are variables in this simulation study.

ISSN:0920-5071    

DOI:10.1163/156939391X00400

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939391X00419

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939391X00428

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939391X00437

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor