ISSN:0920-5071    

DOI:10.1163/156939392X01994

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A Monte Carlo microwave emission model for calculating the brightness temperature from a vegetative medium is presented in this paper. The vegetation is modelled as a two layer random medium: the top layer contains discrete dielectric scatterers while the bottom layer is a half-space with a rough interface representing the ground. The leaves, stalks and trunks in the vegetation are approximated as disks, small needles and vertical cylinders with suitable complex dielectric constants. The ground is modelled as a Kirchhoff rough surface based on the stationary-phase approximation. Effects of various physical and geometrical parameters such as dielectric constants, volume fractions, sizes and orientation distributions of the scatterers are studied. It is found that (i) at low frequency, the surface emission is dominant, (ii) as frequency increases, the effect of the layer of scatterers determine the trends of the brightness temperature, and (iii) higher order multiple scattering effects should be included, especially in the high frequency range (C- and K-band).

ISSN:0920-5071    

DOI:10.1163/156939392X01453

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

A Monte Carlo microwave emission model for calculating the brightness temperature from a vegetative medium is presented in this paper. The vegetation is modelled as a three layer random medium: the top layer contains a mixture of randomly orientated discrete dielectric scatterers representing the leaves and the stems, while the second layer contains vertical dielectric cylinders representing the stalks, and the bottom layer a rough half-space representing the ground. The leaves, stems, and stalks in the vegetation are approximated as disks or needles (as the case may be), small cylinders and vertical cylinders, respectively, with suitable complex dielectric constants. The phase matrices of these disks and needles are formulated using the generalized Rayleigh-Gans approximation whereas those of the cylinders are obtained from the infinite-cylinder approximation. The ground is modelled as a Kirchhoff's rough surface based on the stationary-phase approximation. The relative contributions to the brightness temperature from various components of the plant canopy are studied. There appears to be a scarcity of experimental measurements with sufficient ground truth data which we can use to compare with our model. Our calculations yield good fits to the measured data from some fields of corn and soybean.

ISSN:0920-5071    

DOI:10.1163/156939392X01462

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

One possible realization of an optoelectronic 90° -hybrid consists of an optical 3 x 3 fiber coupler, three optical detectors and an electrical signal processing unit. This paper describes the design and optimization of such a configuration. Based on measured power coupling coefficients, the complex 3 x 3-coupler matrix is determined first. In a second step the optimum parameters of the signal processing unit are calculated. It is shown that with a given real 3 x 3 fiber coupler under shot noise limit conditions, an optoelectronic 90° -hybrid can approach the ideal 90° -hybrid within 0.1 dB.

ISSN:0920-5071    

DOI:10.1163/156939392X01471

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Basic characteristics of a cutoff waveguide aperture antenna used for large scale ICRF (ion cyclotron range of frequency) plasma heating are investigated. Main interests are focused on the effects of the Faraday shield covering the aperture surface. The analysis is performed for the antennas radiating into semi-infinite vacuum space rather than a magnetoplasma to understand the basic characteristics of the antenna where the loading effect due to the presence of the plasma is excluded. Theoretical results show that the input admittance of the aperture antenna with the Faraday shield is similar to that without the shield qualitatively. On the other hand the aperture electric field distribution is quite different for these two cases. The validity of the anisotropic boundary condition which has been often used to incorporate the effects of the Faraday shield is also discussed. In order to confirm the validity of the analysis, experimental results of the input admittance and aperture field are presented.

ISSN:0920-5071    

DOI:10.1163/156939392X01480

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

In this paper, we propose an adaptive polarimetric system which allows the minimization of the contribution of stationary clutter by adapting the polarization state of the receiving antenna to the polarimetric signature of the environment by using the minimum power criterion. From the knowledge of the polarimetric signature of the environment and by applying the least square criterion, we can detect the presence of targets and estimate their polarimetric parameters. Some results of simulation are presented, in order to show the feasibility of the system and the robustness against the spread of clutter variation.

ISSN:0920-5071    

DOI:10.1163/156939392X01499

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Unintentional slot apertures formed at the joints in the outer shell of a system are typically terminated by bolts or rivets. When the slot is narrow (length and wavelength much larger than the slot cross section) the question arises: How can these loads be incorporated into the antenna model for the slot? Impedance loads for the antenna model are derived by considering the difference between the Hallén filamentary feed representation and the true bolt geometry. Simple formulas are derived for the inductance of such loads. The leading order formulas are derived by means of conformal mapping considering only the interior of the slot. A correction resulting from the exterior of the slot is also given. These formulas show (in the case where the bolt or rivet makes contact with the conductors on each side of the width of the slot) that the bolt or rivet may be treated approximately as a short circuit. This follows because the condition that the slot be narrow, required to treat it by means of an antenna model, is also required for the bolt load to approach a short circuit. However, in the case where an annular gap exists around the bolt or rivet in the slot conducting wall, and the length of this annular region (typically the thickness of the metal wall) is much larger than the slot width, the resulting inductance can dominate the bolt load effect and in certain cases make the bolt or rivet act more as an open circuit; bolt or rivet contact impedance can lead to similar results.

ISSN:0920-5071    

DOI:10.1163/156939392X01507

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

The use of Levin's u-transform is shown to accelerate the summation of slowly convergent series. The transform works effectively on alternating as well as monotonic series. The usefulness of the transform is illustrated by its application to the very slowly converging free-space periodic Green's function series involving single and double infinite summation. Numerical results are presented for the cases in which the series have the slowest convergence.

ISSN:0920-5071    

DOI:10.1163/156939392X01516

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

摘要:

Polarimetric radar calibration algorithms using a combination of point targets and reciprocal distributed targets are developed. From distributed target reciprocity, a matrix equation is derived which can be converted into an equivalent point target response, and can also be utilized to make a polarimetric radar reciprocal. The equivalent point target corresponds physically to a 90-degree polarization rotator which is nonreciprocal and rotation-invariant. Due to this equivalent-point-target interpretation, the cases of polarimetric calibration using two point targets and one distributed target then reduce to those using three point targets, which have been solved in previous research. Regarding the calibration using one point target and one reciprocal distributed target, two cases are analyzed with the point target being a sphere (trihedral reflector) or a polarimetric active radar calibrator (PARC). For both cases, the general solutions of system distortion matrices are written as a product of a particular solution and a matrix with one free parameter, and then an additional assumption about the distributed target is made to determine the free parameter. For the trihedral-reflector case, when the particular solution is applied for calibration, the measured polarimetric data can be calibrated to the level that only rotation errors remain, and once azimuthal symmetry is assumed for the distributed target, an iterative scheme is devised to solve the rotation parameter. For the PARC case, the residual error is like the channel imbalance after the data are calibrated by the particular solution, and the free parameter can be determined by knowing one ratio of two covariance matrix elements of the distributed target. Numerical results were simulated to demonstrate the validity of the algorithms developed in this paper.

ISSN:0920-5071    

DOI:10.1163/156939392X01525

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor

ISSN:0920-5071    

DOI:10.1163/156939392X01534

出版商:Taylor & Francis Group    

年代:1992

数据来源: Taylor