摘要:

We present a simple but effective technique for accelerating the convergence of iterative methods in the solution of electromagnetic scattering problems described by a second‐kind integral equation (SKIE). We call the technique “complexification and extrapolation,” or simply “complexification.” It is based on the mathematical principle of limiting absorption, and it alleviates the difficulties arising from the interior resonances of this SKIE, thus allowing the efficient solution of scattering from electrically large objects. The technique involves introducing an imaginary part to the real wavenumber and solving the problem, then repeating with a different imaginary part and extrapolating the solutions linearly back to the real axis. For higher‐order extrapolations we use additional complex wavenumbers. We have tested the method on a number of closed two‐dimensional conducting scatterers, using this SKIE discretized by Nyström's method and solved by the fast multipole method. We use a variant of the conjugate gradient (CG) method that we call the pseudoconjugate gradient (PCG) method. The PCG method as we employ it performs only 1.2 matrix vector products on average per iteration, as opposed to two for the standard CG. Complexification gives excellent results. Solutions are fast and accurate. The condition number of the discrete matrix is asymptotically bounded for a given problem as the number of points per wavelength increases. The empirical evidence we have gathered thus far also suggests that the condition number is essentially asymptotically bounded as the electrical size of the scatterer increases, holding the number of points per wavelength fixed. Thus the technique has great potential in the solution of scattering from electrically large objects. Note that the technique of complexification is not limited to the fast multipole method and should be of broad applicability in the numerical solution of scatt

ISSN:0048-6604    

DOI:10.1029/92RS01924

年代:1993

数据来源: WILEY

摘要:

In this article the statistical characteristics of the VHF radar returns, which are assumed to comprise components generated from the atmospheric isotropic turbulences plus anisotropic irregularities are theoretically studied. By employing a theory of the random variable, the analytic form of the amplitude and phase probability density functions of the VHF radar echoes are derived. After a somewhat tedious and complicated calculation it shows that the conventional Rayleigh distribution, the Rice distribution, and the Hoyt distribution are closely related to the derived generalized probability density function. The Nakagamimparameter corresponding to this generalized probability density function of the VHF radar signal amplitude is derived as well. It indicates that the magnitude of Nakagami m parameter is governed by the radar echo parameters, that is, 2S2, σ2, and μ, where 2S2is the power of the radar echo component scattered from the isotropic turbulences, μ and σ2are the mean and variance, respectively, of the radar signal component generated from the anisotropic irregularities. After examining the general behavior of the derived Nakagamimparameter in more detail, it is found that no matter what theSvalue is, the magnitude of Nakagamimparameter,m, is always in one of the following three categories, depending on the relative changes between μ and σ. Namely,m= 1 if σ2= (2½‐ 1)μ2; 0.5

ISSN:0048-6604    

DOI:10.1029/92RS01776

年代:1993

数据来源: WILEY

摘要:

A complete, plane wave spectral, vector wave function expansion of the electromagnetic, electric, and magnetic, dyadic Green's function for electric, as well as magnetic, point currents for a planar, multilayered, symmetric gyroelectric medium, where the gyroelectric anisotropy is confined in the plane parallel to planar interfaces is presented. It is given in terms ofz‐propagating, source‐free vector wave functions, whereis normal to the interfaces. It is shown that, because of the reflection symmetry of the electric permittivity tensor, where the modes are decoupled, an efficient representation of the field within each of the layers is possible. Some salient features of the Green's dyadic, along with a physical interpretation, are also descri

ISSN:0048-6604    

DOI:10.1029/92RS01926

年代:1993

数据来源: WILEY

摘要:

A simulation model of multipath propagation will be used to characterize the relationship between the cross‐polarization discrimination and the co‐polar attenuation in frequency‐reuse line‐of‐sight microwave links. The behavior of several links described in the literature will then be simulated, and the results will be compared with the corresponding measurements, to test the prediction capability of the model. A reasonable agreement between the corresponding results will be displayed. Further, it will also be shown that the cross‐polar phase patterns of the antennas could significantly affect the behavior of the cross‐polarization d

ISSN:0048-6604    

DOI:10.1029/91RS01974

年代:1993

数据来源: WILEY

摘要:

Calculation of the high‐latitude distribution of the vertical total electron content (TEC) is possible using a three‐dimensional, time‐dependent ionospheric model. Global and local comparisons may be made with observations of TEC. We compare the local diurnal variation of TEC calculated by the model with observations of TEC at Goose Bay, Labrador and Hamilton, Massachusetts. Data from the period of March 1–11, 1989, and monthly averaged data for solar maximum and solar minimum periods are examined. We extend the model to predict diurnal variations of TEC in the polar cap and compare these results with the observed TEC at Thule, Greenland, during an 8‐day campaign from January 28 through February 4,1984. We propose a possible explanation for the large variability observed. We show that the “equivalent vertical content” TEC is very sensitive to horizontalFlayer electron density gradients and that such “equivalent vertical” TECs may vary significantly from the true vertical TEC of the ionosphere. By incorporating these results, we calculate the vertical TEC distribution of the high‐latitude ionosphere for a wide range of solar activity, seasons, andKpvariation represented by a recently completed Utah State University time‐dependent ionospheric model data base. Finally, we discuss the possible uses of TEC as a diagnostic tool for testi

ISSN:0048-6604    

DOI:10.1029/92RS01928

年代:1993

数据来源: WILEY

摘要:

This study concerns the predicted performance of multifrequency ground‐based radiometers in estimating atmospheric moisture and the corresponding attenuation and wet path delay on an Earth‐space path. The analysis of the performance is based on a numerical simulation using possible combinations of radiometric channels at 10 microwave frequencies below 100 GHz. We first discuss the accuracy of retrievals of both integrated atmospheric vapor and cloud liquid from noisy radiometric measurements carried out at two frequencies and investigate the improvement attainable by using more than two radiometric channels. Then we focus on the problem of predicting attenuation and wet path delay at several frequencies in the millimeter wave range for a vertical Earth‐space path from radiometric data. Examples of possible combinations of two and three frequencies are presented, ranked according to their capability, first in retrieving vapor and liquid, then in predicting attenuation and wet path delay for two different climatol

ISSN:0048-6604    

DOI:10.1029/92RS02457

年代:1993

数据来源: WILEY

摘要:

An HF crossed arm sampling array was used to investigate the effect of the ionosphere on the direction‐of‐arrival (DOA) of 9.3‐MHz transionospheric transmissions from ISIS satellites. The experimental results were compared to theoretical results obtained from a transionospheric raytrace program. The difference in elevation between the transionospheric ray and the line‐of‐sight ray, referred to as the vertical DOA error, varies with elevation angle and maximizes at the iris of the ionosphere. The theoretical results show that if the transionospheric ray passes through a horizontal gradient or blob or trough in the electron density, a refractive error occurs. This error decreases if the ray passes through an electron density enhancement which is greater above the peak of theF2 layer than below it and increases if the ray passes through an enhancement which is greater below theF2 peak than above it. Experimental data exhibit this behavior. TheS4amplitude scintillation index is essentially independent of ray elevation and aspect angle regardless of the presence or absence of magnetic field‐aligned electron density irregularities (FAIs). For times when small‐scale FAIs are clearly present, the DOA of signals from a moving satellite fluctuates both most rapidly and most slowly when the orbit is polar and passes overhead. The very rapid fluctuations occur when the DOA of the transionospheric ray is near the magnetic zenith. This may be a diffractive effect. The fluctuations are slowest, probably indicating a refractive effect, when the satellite is approaching and receeding. This behavior is dependent upon the geometry of the situation but is absent when there are no FAIs. DOA fluctuations occurred at all the invariant latitudes observable (53°–65°), with the largest fluctuations occurring more frequently north of 60° and with increasing magnetic activity. The DOA fluctuations in this area were present in the summer and absent in the winter but were negligible when FAIs were absent. The seasonal variation is believed to be due to a seasonal variation in the chemistry of theF2 layer. When large blobs of FAIs appear to be located at the expected DOA, most connecting ray paths during the satellite transit appear to be refracted from the sides of adjacent blobs. However, one could also positively identify a small number of connecting ray paths per second which arrived at the expected DOA, indicating that a few rays appeared to have woven their way among the irregularit

ISSN:0048-6604    

DOI:10.1029/92RS01787

年代:1993

数据来源: WILEY

摘要:

The Disturbance Impact Assessment System (DIAS) is a computer software package designed to assess and predict the impact of solar flares on high‐latitude HF radio communications. The analysis spans the period between the flare onset (T0 hours) and the expected subsiding of its effects 5 days later (T120 hours). DIAS supports a HF communications system and when integrated into it, its operation will be entirely transparent to the user. A stand‐alone PC version was designed for tutorial purposes. Key features combine the capabilities of the algorithmicPROPHET HFsignal assessment system with expert system technology. The initial system provides qualitative advice and warnings. The rule sets in DIAS cover the following types of disturbances: (1) sudden ionospheric disturbances, (2) polar cap absorption, (3) ionospheric storm, (4) auroral zone absorption, and (5) auroral sporadicEand auror

ISSN:0048-6604    

DOI:10.1029/92RS01780

年代:1993

数据来源: WILEY