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Showing papers in "Radio Science in 1991"


Journal ArticleDOI
TL;DR: A theoretical foundation for the use of the parabolic wave equation/Fourier split-step method for modeling electromagnetic tropospheric propagation is presented in this paper, where procedures are used to derive a scalar Helmholtz equation and to subsequently transform to a rectangular coordinate system without requiring approximations.
Abstract: A theoretical foundation for the use of the parabolic wave equation/Fourier split-step method for modeling electromagnetic tropospheric propagation is presented. New procedures are used to derive a scalar Helmholtz equation and to subsequently transform to a rectangular coordinate system without requiring approximations. The assumptions associated with reducing the resulting exact Helmholtz equation to the parabolic wave equation that is used for computations are then described. A similar discussion of the error sources associated with the Fourier split-step solution technique is provided as well. These discussions provide an important indication of the applicability of the parabolic equation/split-step method to electromagnetic tropospheric propagation problems. A rigorous method of incorporating an impedance boundary at the Earth's surface in the split-step algorithm is also presented for the first time. Finally, a few example calculations which demonstrate agreement with other propagation models are provided.

273 citations


Journal ArticleDOI
TL;DR: In this article, the authors proposed more stable and accurate recursing equations and a computing procedure to calculate scattering coefficients for a multilayered sphere, which can be used for large and small inhomogeneous spherical particles.
Abstract: We have proposed more stable and accurate recursing equations and a computing procedure to calculate scattering coefficients for a multilayered sphere. The procedure involves three logarithmic derivatives of Ricatti-Bessel functions ψn′(z)/ ψn(z), χn′(z)/ χn(z), and ξn ′(z)/ξn (z), as well as the ratio ψn(z)/χn(z). The asymptotic behavior, stability, and accuracy of the procedure and scattering coefficients an and bn are discussed for any complex refractive indices. This procedure can be used for large and small inhomogeneous spherical particles.

246 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the local time when the peak excursion of the ring current occurs (its decay from maximum negative values), affects the equatorial electric field and therefore the height of the F layer.
Abstract: For the equatorial ionosphere in years of high solar flux the F layer parameters are such that irregularities are produced almost every night in specific months The question that has emerged is why are there nights in these months without irregularities The hypothesis advanced is that for this period, one must look not for external seeding mechanisms but for inhibiting factors The ring current during magnetic storms appears to play a leading role directly or indirectly in establishing the conditions necessary for equatorial F layer irregularity generation and inhibition The hypothesis advanced is that for a particular ionospheric propagation intersection, the local time when the peak excursion of the ring current occurs (its decay from maximum negative values), affects the equatorial electric field and therefore the height of the F layer Using data from a number of magnetic storms with dates ranging from December 1971 to November 1981 primarily in years of high solar flux, the following categories were found If the maximum ring current energy as shown by Dst occurred during the midnight to postmidnight time period, irregularities were generated If the maximum Dst, the period before recovery set in, occurred in the early afternoon, irregularities were inhibited If the maximum occurred around sunset or shortly after sunset, then there was no effect on the generation of irregularities that night While the ideas on positive and negative correlation with magnetic indices have been shown in many early references, the importance of local time and its relationship with the maximum ring current energy have not been emphasized The use of widely spaced equatorial data from Manila and Huancayo allowed for contrasting reactions for the same storm and for validation of the hypothesis for the months and solar conditions stated For other periods, when occurrence of irregularities is lower, the effect on layer height may be only one factor in setting conditions for the generation of irregularities

160 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the fidelity of the reconstructed image by computing the response to an impulse-source function under various non-idealized but more realistic conditions, such as finite receiving aperture and circular geometry.
Abstract: The potential of imaging large ionospheric structures by applying tomographic techniques has only been proposed recently and has already received interest and increasing attention by workers in ionospheric research. In the combined case of an idealized plane geometry and complete and continuous data extending to {plus minus} infinity the inversion is exact and unique. This is no longer the case if any of the idealized conditions are removed. Specifically, the limitations are investigated that arise from limited angles, sampled data of finite receiving aperture and circular geometry. Under these various nonidealized but more realistic conditions, fidelity of the reconstructed image is investigated by computing the response to an impulse-source function. The deterioation of the reconstructed image, the sensitivity to the position in the image plane, and the possible existence of ghost images are all numerically investigated and illustrated. Possible methods of improvement are suggested. 23 refs.

117 citations


Journal ArticleDOI
TL;DR: A number of iterative algorithms to solve integral equations arising in field problems are discussed and the essential features of the Neumann Series, overrelaxation methods, Krylov subspace methods, and the conjugate gradient technique are described.
Abstract: A number of iterative algorithms to solve integral equations arising in field problems are discussed. We describe the essential features of the Neumann Series, overrelaxation methods, Krylov subspace methods, and the conjugate gradient technique. Proofs of convergence of the conjugate gradient method are directly available when the underlying integral operator is self-adjoint, and in this case the method is equivalent to the Krylov method. However, for non-self-adjoint operators the conjugate gradient method requires an implicit symmetrization which results in poorer convergence than that obtained using the Krylov method. Some convergence results are also available for overrelaxation methods for both self-adjoint and non-self-adjoint operators. Relations between all of the methods will be described and numerical performance will be contrasted using a uniform square error criterion. All the methods are treated in the continuous operator form which is especially useful in using the physical setting to arrive at effective preconditioners.

100 citations


Journal ArticleDOI
TL;DR: In this paper, a new type of dominant mode power leakage from uniform lengths of printed-circuit waveguides, such as microstrip line, slot line, and coplanar waveguide, is reported.
Abstract: This study is concerned with a new type of dominant mode power leakage from uniform lengths of printed-circuit waveguides, such as microstrip line, slot line, and coplanar waveguide. These leakage effects, which are presently not recognized and can therefore be unexpected as well as undesired, occur at higher frequencies, and are therefore important for millimeter-wave integrated circuits and high-speed circuits. The cross talk and coupling that result can ruin the performance of a complex, high-density circuit unless the leakage effects are understood and controlled. The leakage effects reported here occur on uniform lengths of printed-circuit waveguide on which the dominant mode is purely bound at ordinary microwave frequencies but becomes leaky at higher frequencies; they appear in addition to the leakage produced by conversion into surface waves at various discontinuities. The nature of the leakage is described, and numerical results are presented for several structures as illustrations of the variety of ways this type of leakage can arise.

90 citations


Journal ArticleDOI
TL;DR: In this article, meteor wind observations with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (35°N, 136°E), utilizing an interferometer to determine the arrival angle of a meteor echo.
Abstract: We conducted meteor wind observations with the middle and upper atmosphere (MU) radar at Shigaraki, Japan (35°N, 136°E), utilizing an interferometer to determine the arrival angle of a meteor echo. We found that meteor echoes are widely distributed in zenith angles as large as 50° and that the narrow main lobe of a transmitting antenna cannot effectively detect meteor trails but that the sidelobes of the antenna illuminate most of meteor trails. Wind velocity profiles determined using meteor echoes are consistent with MU radar observations determined using turbulence echoes. The amplitudes and phases of both diurnal and semidiurnal tides were analyzed for a 96-hour continuous run in September 1989, which agreed reasonably well with the characteristics of tides determined from Kyoto meteor radar observations.

84 citations


Journal ArticleDOI
TL;DR: In this paper, the equivalent slab thickness of the ionosphere at 15 stations in middle and low latitudes was studied to determine its dependence on solar cycle and location, and the data were grouped by season.
Abstract: The equivalent slab thickness of the ionosphere at 15 stations in middle and low latitudes was studied to determine its dependence on solar cycle and location. The data were grouped by season. The following are the major conclusions. There appears to be little or no geographical, or geomagnetic, dependence. The slab thickness varies approximately linearly with the 12-month smoothed values of the 10.7-cm solar radio flux. In middle latitudes the winter midnight thickness is essentially independent of the flux, whereas in summer and equinox the midnight thickness increases with increase of solar flux. The noon thickness increases with increase of solar flux in all seasons. The zero-order Fourier coefficients for the diurnal curves at all 15 stations were expressed as linear functions of the 10.7-cm flux. The higher harmonic coefficients showed no appreciable dependence on solar flux. The pronounced predawn increase in slab thickness is caused by low values of the maximum electron density, not by increase of total electron content.

76 citations


Journal ArticleDOI
TL;DR: In this paper, numerical simulations of the scattering from time-dependent realizations of one-dimensional ocean surface waves are described, and the authors carry the simulations further by generating Doppler spectra from dynamically evolving surface realizations.
Abstract: In this paper, numerical simulations of the scattering from time-dependent realizations of one-dimensional ocean surface waves are described. A new technique is used that allows efficient generation of ocean surface realizations that preserve the dominant nonlinear hydrodynamic characteristics. Thus unique scattering effects of real ocean surface waves can be explored. Until very recently, numerical simulations of rough-surface scattering were used mainly to test and/or improve theoretical models that predict the average bistatic scatter cross section. We carry the simulations further by generating Doppler spectra from dynamically evolving surface realizations. Doppler spectra of signals scattered from the ocean surface are affected by both hydrodynamic nonlinearities and higher-order scatter terms. The simulated Doppler spectra from nonlinear surface realizations reproduce the measured characteristics of ocean and wave-tank data for low and high wind conditions. We also show that the results are essentially reproduced by the second-order Kirchhoff approximation.

70 citations


Journal ArticleDOI
TL;DR: In this article, a linear variation of the cross-spectral phase as a function of the radial velocity in the frequency domain for the radar interferometry (RI) cross spectra was analyzed for evidence of apparent beam direction that is off vertical even for a nominally vertically pointing beam direction.
Abstract: At VHF wavelengths, aspect sensitivity may result in an apparent beam direction that is off vertical even for a nominally vertically pointing beam direction if the refractivity surfaces responsible for the scatter are tilted with respect to the horizontal plane. Middle and upper atmosphere radar measurements obtained by using the system in a standard multireceiver configuration typical for radar interferometry (RI) and spaced antenna measurements have been analyzed for evidence of such effects. The analysis is based on the linear variation of the cross-spectral phase as a function of the radial velocity in the frequency domain for the RI cross spectra. True-vertical velocity estimates are obtained by using the fact that the phase difference between two antennas should be equal to zero when the echoes are being received from the vertical direction. The tilt angles of the refractivity surfaces were obtained from the phase of the cross-correlation function at zero lag, and the radial velocity in that direction was determined from the cross spectra. The results indicate that the vertical velocity derived from standard Doppler analyses is actually the velocity perpendicular to the refractivity surfaces and thus can be biased by the projection of the horizontal wind along the effective pointing direction.

67 citations


Journal ArticleDOI
TL;DR: This paper is dealing with the biomedical applications of active microwave imaging, and iterative space-domain formulations appear to provide a suitable base for achieving quantitative imaging thanks to a convenient use of the available a priori information.
Abstract: This paper is dealing with the biomedical applications of active microwave imaging. As a result of extensive preliminary evaluations conducted by means of a 2.45-GHz planar camera, it has been proven that active microwave imaging is able to provide a very sensitive means of investigation in such applications. While technological problems have been conveniently solved, the main problem still remains to determine the most efficient way to process the measured data in order to get the best quality of the reconstructed images. Until now, diffraction tomography algorithms based on a spectral approach have been used. The main limitations of this approach and some attempts to reduce them are analyzed. A more prospective discussion allows to identify the most promising reconstruction techniques. Among them, iterative space-domain formulations appear to provide a suitable base for achieving quantitative imaging thanks to a convenient use of the available a priori information.

Journal ArticleDOI
TL;DR: The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E × B drift at the Earth's magnetic equator as mentioned in this paper.
Abstract: The latitudinal extent of the equatorial anomaly has been studied for equinoctial conditions using a theoretical model of the ionosphere which incorporates measured values of vertical E × B drift at the Earth's magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between ±20° magnetic latitude, is that part of the world where the highest values of electron density and total electron content (TEC) normally occur and hence is very important to high-frequency propagation and to transionospheric propagation effects. During the daytime, upward E × B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately ±15° dip latitude. The E × B drift mechanism is explained in detail by Hanson and Moffett (1966). The latitude range over which the anomaly makes a significant difference in values of ƒ0F2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be more highly variable and widespread in latitude and local time than is generally assumed.

Journal ArticleDOI
TL;DR: In this paper, the authors present a model for MMW scattering from snow using the vector radiative transfer theory and a Mie phase function, assuming snow to consist of randomly distributed spherical particles embedded in a mixture of air and water.
Abstract: Millimeter-wave (MMW) remote sensing of ground snow has attracted considerable interest in recent years. Because the size of the snow ice particle is comparable to the wavelength in the millimeter-wave region, we can no longer use a simple Rayleigh phase function or the small particle approximation usually used at microwave frequencies for calculating the extinction coefficient. In this paper we present a model for MMW scattering from snow using the vector radiative transfer theory and a Mie phase function. Assuming snow to consist of randomly distributed spherical particles embedded in a mixture of air and water, the vector radiative transfer theory is solved using the discrete ordinate method. The values of the extinction coefficient used in the calculations are based on a combination of experimental data and calculations using the quasi-crystalline approximation. The backscattering coefficient is calculated for different liquid water contents at 35, 95, and 140 GHz. We show that the backscattering coefficient is sensitive to liquid water content at all three frequencies, with 35 GHz being the most sensitive. Except for normal incidence, the effect of snow surface roughness is negligibly small for dry snow, and it is somewhat significant for wet snow at 35 GHz, but not at the higher frequencies.

Journal ArticleDOI
TL;DR: In this article, the geometrical theory of diffraction based equivalent current method is used to analyze the high-frequency electromagnetic scattering by open-ended waveguide cavities with a planar interior termination.
Abstract: Some approaches for analyzing the high-frequency electromagnetic scattering by open-ended waveguide cavities with a planar interior termination are discussed. The contribution to scattering arising from the open end is found using the geometrical theory of diffraction based equivalent current method, while the interior cavity scattering contribution can be treated by several different approaches, for example, hybrid modal, ray, or beam methods; other contributions are ignored for the reasons outlined in the paper. Numerical results based on these methods are presented.

Journal ArticleDOI
TL;DR: In this article, a database of equivalent slab thickness observations covering nearly two complete solar cycles has been obtained at a mid-latitude site (Hamilton, MA) for correlations between the various parameters as well as for dependencies on observable quantities.
Abstract: : A database of equivalent slab thickness observations covering nearly two complete solar cycles has been obtained at a mid-latitude site (Hamilton, MA). This database has been studied for correlations between the various parameters as well as for dependencies on observable quantities. The mean variations of tau are described both qualitatively, in terms of changes to profile shapes, and quantitatively, in terms of a simple numerical model. A preliminary investigation of the day-to-day variations of tau is also described. The similarity of the mean numerical tau model to one obtained using data from a site in Wales (U.K.) suggests that longitude variations of tau may well be small at mid-latitudes. This property has in turn been used to convert a network of ionosonde observations to a database of TEC, for comparisons with a number of currently available ionospheric models. These same ionospheric models were also used to predict values of tau over Hamilton, MA, and the comparisons provided additional model validation. Adjustments to the model profiles were suggested in a couple of cases. Keywords: Ionospheric models; Slab thickness; Total electron content; Day-to-day variations; Peak density.

Journal ArticleDOI
TL;DR: In this paper, the authors present a new theory to explain the cause of the atmospheric structures which are responsible for specular reflection of radar signals from the atmosphere, based on a form of highly damped wave which arises in a fluid as a result of the effects of viscosity and thermal conduction.
Abstract: We present a new theory to explain the cause of the atmospheric structures which are responsible for specular reflection of radar signals from the atmosphere. Evidence recorded with the middle and upper atmosphere (MU) VHF radar at Shigaraki in Japan, as well as previous reports of specular reflections from other radars and at other frequencies, is used to support our assertions. The theory postulates that the reflectors are produced by a form of highly damped wave which arises in a fluid as a result of the effects of viscosity and thermal conduction. They occur at altitudes where gravity waves are partially reflected and in the vicinity of gravity wave critical levels but are particularly prevalent when such reflections and critical level interactions occurs in a laminar region of the atmosphere. The typical scales of the waves, their reflection efficiency, and their likely location are all considered and found to be consistent with what is currently known about specular reflectors in the atmosphere. Some predictions which will allow further tests of the model are also presented.

Journal ArticleDOI
TL;DR: In this paper, the authors present experimental results on the angle of rotation and the axial ratio associated with the propagation of microwave waves in chiral composite samples (a phenomenon akin to optical rotation and optical dichroism).
Abstract: In this paper we present experimental results on the angle of rotation and the axial ratio (dichroism) associated with the propagation of microwave waves in chiral composite samples (a phenomenon akin to optical rotation and optical dichroism). In our experiments the chiral composite samples tested were made in the form of planar slabs and consisted of low loss dielectric matrix materials in which miniature copper springs of left only, right only, or an equal mixture of left and right handedness were randomly distributed and oriented. The normally incident wave was linearly polarized. In the chiral sample the linearly polarized wave decomposes into left and right circularly polarized waves which propagate with different speeds and different attenuations leading to an elliptically polarized transmitted wave whose orientation ( electromagnetic rotation ) and axial ratio (dichroism) are proportional to the concentration of springs. Rotation and dichroism are shown to be zero in the equichiral samples.

Journal ArticleDOI
TL;DR: In this article, the eigenmodes of a circular waveguide containing an isotropic chiral medium for monochromatic radiation of unspecified wavelength were investigated. And the dispersion equation for this case was derived analytically, solved numerically, and compared to the nonchiral case and to waveguides containing longitudinally magnetized ferrites.
Abstract: In this paper, Maxwell's equations and modified constitutive equations are used to investigate the eigenmodes in a circular waveguide containing an isotropic chiral medium for monochromatic radiation of unspecified wavelength. The dispersion equation for this case is derived analytically, solved numerically, and compared to the nonchiral case and to waveguides containing longitudinally magnetized ferrites. The field components are computed and plotted as a function of the radial coordinate as well as the transverse E-field lines in the circular cross section. Cutoff frequencies, evanescent modes, polarization characteristics of the fields, and the nondegeneracy of positive and negative mode numbers are discussed and contrasted with those of nonchiral, dielectric filled circular waveguides.

Journal ArticleDOI
TL;DR: In this article, a hybrid first-order numerical solution is proposed to evaluate the radar response to incidence angle, surface roughness, and liquid water content in a variety of snow conditions.
Abstract: Using a truck-mounted platform, backscatter measurements were made at 35, 95, and 140 GHz for a variety of snow conditions to evaluate the radar response to incidence angle, surface roughness, and liquid water content. Good agreement was obtained between the experimental observations and theoretical calculations based on the numerical solution of the radiative transfer equation presented in the preceding paper. A notable exception is when the snowpack is in the refreezing phase of the diurnal cycle, during which the snowpack is characterized by a dry surface boundary with wet layers underneath. To accommodate this type of condition, a hybrid first-order numerical solution is proposed. The hybrid approach provides excellent agreement between theory and experiment.

Journal ArticleDOI
TL;DR: In this paper, the authors applied the theory of radar interferometry to a set of measurements obtained with the middle and upper atmosphere (MU) radar in order to demonstrate the technique and to test the theory.
Abstract: The theory of radar interferometry developed in the companion paper [ Van Baelen and Richmond, this issue] is applied to a set of measurements obtained with the middle and upper atmosphere (MU) radar in order to demonstrate the technique and to test the theory. The variations with respect to Doppler frequency of the coherencies and phases of the signals cross-correlated between receiver pairs depend on the amplitude and direction of the wind in the manner predicted by the theory. The horizontal wind derived by the interferometric technique agrees quite well with the so-called apparent velocity obtained by cross-correlating signals in the time domain from different receiver pairs, and under our experimental conditions the latter estimate has been determined to be a good representation of the true wind, in spite of slight tendencies to overestimate the wind amplitude. The vertical wind derived by the interferometric technique shows significant differences with the Doppler wind obtained from a vertically pointing antenna, but these differences disappear when the Doppler wind is corrected for the off-zenith distribution of the received power. An examination of the mean angle-of-arrival of the received signal reveals that there is a tendency for the angle to approach, though not attain, perpendicularity with the three-dimensional wind vector, and perhaps also to tend to be perpendicular to isentropes in the crosswind direction. A postset beam steering analysis of several examples of data illustrates the behavior of the angular distribution of received power in time and space. When the signal-to-noise ratio is large, there is a tendency for the power to be fairly isotropic with respect to zenith angle, but highly distorted patterns can be encountered when the signal is weaker. There appears to be a tendency for the patterns to vary relatively slowly over time, but strong variations in altitude are observed in passing from one 300-m range gate to the next. The direction of most intense received power does not necessarily correspond to the direction of mean angle-of-arrival of the signal as determined by interferometry.

Journal ArticleDOI
TL;DR: In this paper, the authors presented the first evidence of long-range detection of calibrated VLF signals resulting from the HF heating of the auroral electrojet, that is, signal detection at a point of direct "line of sight" of the heated patch of ionosphere.
Abstract: This paper presents the first evidence of long-range detection (>1000 km) of calibrated VLF signals resulting from the HF heating of the auroral electrojet, that is, signal detection at a point of direct “line of sight” of the heated patch of ionosphere. Other workers have presented calibrated data from shorter ranges (190–550 km) or claimed the detection of uncalibrated signals at ranges greater than 6000 km, but we believe that no such calibrated and clear signals, like those presented here from a range of greater than 2000 km, have previously been reported. Also, in contrast to earlier long-range detection experiments we record the “radial” as well as the “azimuthal” magnetic component of the signals and from their ratio obtain the waveguide mode polarization. Observed absolute magnetic field strengths and waveguide polarizations are found to be in line with the predictions of simple waveguide models.

Journal ArticleDOI
TL;DR: In this paper, a comprehensive analysis of the ionization rates of air by RF fields is presented, which relies on a time-dependent code which treats the electron energization with a Fokker-Planck type model and the inelastic energy losses with a multiple time scale technique.
Abstract: A comprehensive analysis of the ionization rates of air by RF fields is presented. The analysis relies on a time-dependent code which treats the electron energization with a Fokker-Planck type model and the inelastic energy losses with a multiple time scale technique. Derivation of ionization rates for parameters of interest To D region ionospheric by ground-based RF transmitters with frequency much higher than the electron neutral collision frequency is emphasized. The study provides a physical understanding of the ionization proces and its associated efficiency by combining the computational results with analytic theory. It is shown that for values of quiver energies « I, where I is the ionization potential, the electron production time corresponds to the electron energization time from energies below 2 eV to 20–25 eV. The analytic expressions derived are consistent with the computational results over 6 orders of magnitude in ionization rates and over 2 orders of magnitude in values of Power threshold definitions are clarified, and the pitfalls of using fluid descriptions or effective electric field notions are discussed. The paper concludes with an assessment the power requirements for ionization at 70-km ionospheric altitude with RF in the 100–900 MHz frequency range.

Journal ArticleDOI
TL;DR: In this paper, the sensitivity of five evaporation duct height algorithms to errors in shipboard meteorological data is examined, and the algorithms are intercompared, showing that the sensitivity to errors caused by sensor inaccuracies ranges from 10% to 50% relative uncertainty in the calculated duct height.
Abstract: The sensitivity of five evaporation duct height algorithms to errors in shipboard meteorological data is examined, and the algorithms are intercompared. The data set used was generated parametrically so a large variation of environmental conditions could be considered. The errors in the meteorological data consisted of two types: uncorrelated random errors associated with sensor inaccuracies and both random and systematic errors due to the influence of ship-induced distortions. Before considering any errors, however, algorithm-to-algorithm differences of 10%–70% in the computed duct height are demonstrated and related to the assumptions and simplifications used during algorithm development. The sensitivity study shows that although the evaporation duct height algorithms have different genealogies, they have similar sensitivities. The sensitivities to errors caused by sensor inaccuracies range from 10% to 50% relative uncertainty in the calculated duct height, except for extremely low duct heights where the uncertainties are greater. During the daytime the relative uncertainties due to errors caused by ship-induced distortions are approximately 10%–20% higher than those due to the sensor errors, and they are about 50%–75% smaller at night because of a lack of solar heating of the ship. These conclusions represent a best-case scenario for Navy operational applications because of the omission of some sources of error, the optimistic ship error characteristics used, and the assumption of horizontal homogeneity in the near-surface refractivity field.

Journal ArticleDOI
TL;DR: In this article, a uniform geometrical theory of diffraction solution is developed for the two-dimensional problem of high-frequency plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes.
Abstract: A uniform geometrical theory of diffraction solution is developed for the two-dimensional problem of high-frequency plane wave diffraction by a planar junction of two thin dielectric/ferrite half planes. Each material half plane in this two-part configuration is assumed to be electrically thin so that it can be replaced in the analysis by a generalized resistive boundary condition of 0(t), where t is the corresponding material slab thickness. The solution obtained is based on the Wiener-Hopf technique, and it is shown that the present boundary value problem can be completely solved by imposing a junction condition at the junction of the two dielectric half planes in addition to the boundary and radiation conditions as well as the usual edge condition. This junction condition can be obtained if the field near the junction is modeled by a quasi-static solution. The solution developed here can be further specialized to limiting cases for which either of the material half planes can become free space, perfectly conducting, or purely resistive, respectively. Several numerical examples are presented, and it is shown that the present solution reduces to known results and agrees very well with a moment method solution.

Journal ArticleDOI
TL;DR: In this paper, a general spectral domain formulation to the problem of radiation of arbitrary distribution of sources embedded in a horizontally stratified arbitrarily magnetized linear plasma is presented, where the fields are obtained in terms of electric and magnetic type dyadic Green's functions.
Abstract: A general spectral domain formulation to the problem of radiation of arbitrary distribution of sources embedded in a horizontally stratified arbitrarily magnetized linear plasma is presented. The fields are obtained in terms of electric and magnetic type dyadic Green's functions. The formulation is considerably simplified by using the kDB system of coordinates in conjunction with the Fourier transform. The distributional singular behavior of the various dyadic Green's functions in the source region is investigated and taken into account by extracting the delta function singularities. Finally, the fields in any arbitrary layer are obtained in terms of appropriately defined global upward and downward reflection and transmission matrices.

Journal ArticleDOI
TL;DR: In this article, an error analysis for the image reconstruction algorithm currently employed by the electronically steered thinned array radiometer aircraft instrument is presented, which is shown to produce artifacts in the image which appear as “streaks” running along the instrument flight line.
Abstract: An error analysis is presented for the image reconstruction algorithm currently employed by the electronically steered thinned array radiometer aircraft instrument. This instrument samples components of the Fourier transform of the brightness temperature distribution at 1.4 GHz. Sources of error are identified, including systematic offsets and stochastic noise in the measurements, errors in the interference pattern antenna calibration, multipath scattering from the antenna support structure, mutual coupling between nearby array elements, and variations in the antenna patterns of the array elements. These errors are shown to produce artifacts in the image which appear as “streaks” running along the instrument flight line. Possible hardware corrections are discussed. These include a reduction in the profile of the antenna support structure and a decoupling of adjacent and other nearby array elements using corrugated “skirts.” Also discussed are several image enhancement techniques which detect and correct for some of the streaking. These techniques include a renormalization of the interference patterns based on the measured element patterns, a perturbation based interpretation of image contrast, and a covariance analysis of noise present in the image.

Journal ArticleDOI
TL;DR: In this article, the authors evaluated the performance of six publicly available ionospheric models as predictors of total electron content (TEC) data from a range of mid-latitudes and low latitudes and longitudes for a wide range of solar activity.
Abstract: We have gathered total electron content (TEC) data from a range of mid-latitudes and low latitudes and longitudes for a wide range of solar activity. This data was used to evaluate the performance of six publicly available ionospheric models as predictors of total electron content. TEC is important for correcting modern DoD space systems, which propagate radio waves from the earth to satellites, for time delay effects of the ionosphere. The TEC data were obtained from polarimeter receivers located in North America, the Pacific, and the East coast of Asia. The ionospheric models evaluated are (1) the International Reference Ionosphere, (2) the Bent model, (3) the Ionospheric Conductivity and Electron Density model, (4) the Penn State model, (5) the Fully Analytic Ionospheric Model, and (6) a hybrid model consisting of the Union Radio Scientifique Internationale 88 (URSI-88) coefficients coupled with the Damen-Hartranft profile model. We will present extensive comparisons between monthly median TEC and model TEC obtained by integrating electron density profiles produced by the six models. These comparisons demonstrate that although most of the models do very well at representing ƒ0F2, none of them do very well with TEC, probably because of inaccurate representation of the topside profile. We suggest that one approach to obtaining better representations of TEC is the use of ƒ0F2 from the CCIR or URSI-88 coefficients coupled with a good climatological slab thickness model.

Journal ArticleDOI
TL;DR: In this article, the scattering problem of obliquely incident waves on a multilayered elliptical cylinder is considered, where the cylinder is assumed homogeneous and isotropic but lossy.
Abstract: The scattering problem of obliquely incident waves on a multilayered elliptical cylinder is considered. The cylinder is assumed homogeneous and isotropic but lossy. Both polarization of the incident wave, the E wave, and the H wave are considered. In the theoretical analysis the number of layers are unlimited, but in the numerical analysis we limit the number of layers so we can avoid excessively long computer running time. The numerical results include the bistatic radar cross sections, the specular differential cross sections, and the efficiency factors for absorption, scattering, and extinction. The computed results show the dependence of the scattering on the size, shape, material, and the angle of the incident.

Journal ArticleDOI
TL;DR: In this article, the authors show that spectral analysis, rather than fringe visibility measurements, can be used to determine correlation properties of the field and a principle is also formulated, according to which, in many interferometric measurements, a trade-off can be made between the length of the baseline and the frequency at which measurements are made.
Abstract: By making use of some new results in optical coherence theory we show that spectral analysis, rather than fringe visibility measurements, can be used to determine correlation properties of the field. A principle is also formulated, according to which, in many interferometric measurements a trade-off can be made between the length of the baseline and the frequency at which measurements are made. This principle is a rigorous generalization of the so-called space-frequency equivalence theorem for two antenna detection systems, and it applies to radiation from a broader class of sources than previously considered.

Journal ArticleDOI
TL;DR: In this paper, a combination of fourth-order Runge-Kutta and Adams methods, with error control and extension to variable step size, has been applied to an investigation of high-frequency near vertical incidence sky wave (NVIS) effects.
Abstract: An earlier, three-dimensional ray-tracing program has been upgraded to include magnetic field effects, variable step sizes, improved propagation loss computation, and efficient homing and focusing computation algorithms. A combination of fourth-order Runge-Kutta and Adams methods, with error control and extension to variable step size, has been included. The program is applied to an investigation of high-frequency near vertical incidence sky wave (NVIS) effects. Model simulations of oblique ionograms are found to be consistent with experiment. Implications for NVIS geolocation are discussed.