bluuebunny/crossref_metadata_2025 huggingface dataset:
Noniterative procedure for calculating ELF mode constants in the anisotropic earth‐ionosphere waveguide
Authors: Greifinger Carl , Greifinger Phyllis S.
Published: February 2008

A noniterative procedure is developed for calculating ELF mode constants and dipole excitation factors in the anisotropic earth‐ionosphere waveguide to an accuracy comparable with iterative full‐wave numerical techniques. The procedure enables one to calculate phase speeds, attenuation rates, excitation factors and polarization ratios for arbitrary ionospheric conductivity profiles and arbitrary ground conductivity. The accuracy of the method is demonstrated by comparison with full‐wave calculations at a frequency of 75 Hz for a normal nighttime and a disturbed daytime ionosphere, and two extremes of ground conductivity.
Pseudodecoupling ansatz for electromagnetic aperture coupling in three dimensions
Authors: Ziolkowski Richard W. , Johnson William A.
Published: February 2008

Hertzian potentials are commonly used to describe the scattering of electromagnetic waves in three dimensions. However, when apertures are present in the scattering object, mixed boundary conditions arise and the TE and TM modes of the scattered field become coupled. Numerous attempts to decouple these TE and TM modes have led to solutions of Maxwell's equations which fail to satisfy Meixner's edge conditions. It has been found that by introducing additional gauge potentials which are homogeneous solutions of the boundary condition equations, a pseudodecoupling of the TE and TM modes can be achieved while still satisfying Meixner's edge conditions.
A different approach to worst month rain statistics: Theory and experimental results
Authors: Dellagiacoma G. , Tarducci D.
Published: February 2008

A different approach to the estimate of “worst month” rain statistics is presented, which allows a rigorous evaluation of the q factor, defined as the ratio between the exceedance probabilities of a rain threshold R (or more generally any given quantity) during the worst month and an average year, which is required in order to estimate the terrestrial and satellite radio links performance, in the light of CCIR recommendations. Because of this formulation, it is possible to calculate several q factor statistical properties, such as moments (average value and standard deviation) and percentiles. The theoretical calculations are then compared with the experimental data, obtained from a very large rain gauge data base available in Italy (organized and worked out by CSELT), with very good results.
Surface impedance method for the analysis of phased arrays with a lossy ground plane
Authors: Stalzer Henry J. , Shmoys Jerry , Hessel Alexander
Published: February 2008

An approximate method for the analysis of phased arrays of waveguide apertures in a lossy surface is developed. The procedure uses a surface impedance boundary condition in place of modal expansions in the lossy region. The method is validated in the two‐dimensional case by comparing numerical results with those obtained by using modal expansions both in the feed waveguide and the lossy medium. The accuracy of the method improves as the magnitude of the lossy dielectric constant increases. The location of the blind spot when the lossy region is backed with a conductor is accurately predicted by the surface impedance method. Calculations of scan dependent matched reflection coefficient, element power gain and power loss to the surface are presented. Salient features of the effects of loss are discussed. The results of the paper permit an extrapolation of the surface impedance method to arrays of realistic waveguide phased array elements with lossy ground.
Vertical structure of ionization irregularities observed by HF vertical sounding in the lower E region in the presence of an acoustic wave
Authors: Blanc Elisabeth
Published: February 2008

An acoustic wave produced by a strong explosion at ground level induces in the lower E region partial reflections of the electromagnetic pulses transmitted by a vertical HF sounder at a few kilometers from the source point. Such reflections materialize the vertical path of the acoustic wave during about 40 km below the maximum of the E layer. They give continuous informations on the ionized medium with an altitude resolution determined by the speed of the acoustic wave and the repetition frequency of the sounding pulses of around 12 m. Their amplitude is of the order of 30 dB below the amplitude of the total reflected echoes but during short time range of the order of 0.2–1 s strong amplitude enhancements suggest an irregular small scale structure superimposed on the decreasing gradient of the lower E layer. The vertical scale of these irregularities varies from 150 to 500 m. Their structure and their relation with the studied acoustic wave are discussed.
Analysis of the rain outage performance of route diversity systems
Authors: Kanellopoulos J. D. , Koukoulas S. G.
Published: February 2008

Route diversity systems as available means to improve the rain outage performance are examined by comparison to tandem systems. A general and somewhat rigorous analysis for the evaluation of the joint probability on a pair of opposite hops has been performed, which is valid for any frequency. The total rain outage performance has been shown explicitly to be given approximately by the summation of joint probabilities on pair hops of the diversity system. Finally, a general and compact algorithm for the evaluation of the path length enlargement effect has been constructed which is valid for any frequency, above 10 GHz and single hop‐length of the corresponding tandem system.
Determination of the fluctuation level of ionospheric irregularities from radar backscatter measurements
Authors: Walker A. D. M. , Greenwald R. A. , Baker K. B.
Published: February 2008

The expression for the radar cross section for backscatter from elongated, field‐aligned ionospheric irregularities is used to find a general formula for the backscattered power received by any radar for a given rms electron density fluctuation level. The formula involves an integral of the three‐dimensional irregularity power spectrum over a volume defined by the beam width of the antenna and the pulse length. The three cases considered are where: (1) the irregularities fill the beam but the beam width measured parallel to the earth's magnetic field is so large that a wide range of aspect angles falls within the beam, (2) the irregularities fill the beam and the beam width is so narrow that the aspect angle is approximately constant, and (3) the irregularities fill only a small portion of the beam, over which the aspect angle is constant. The three cases are typified by the Goose Bay HF radar, the Jicamarca radar, and STARE‐type radars, respectively. An empirical formula for the power spectrum is used to obtain explicit formulas for the three types of radar. The Goose Bay radar works at a low enough frequency so that ionospheric refraction can be used to achieve good aspect sensitivity. The effect of ionospheric refraction and focusing is therefore included in the formulas for backscatter. The results of numerical computations are presented in order to compare the sensitivity of the Goose Bay radar with other types of radar and to estimate the levels of fluctuation needed to produce backscatter. Curves of signal‐to‐noise ratio versus range are plotted for the radar.
TM scattering by a narrow conducting strip located at the interface between two semi‐infinite half‐spaces
Authors: Butler Chalmers M.
Published: February 2008

The far‐zone field scattered by a narrow conducting strip located at the interface between two semi‐infinite dielectric half‐spaces is determined analytically. The permeabilities of the two half‐spaces are equal, and the excitation is restricted to be transverse magnetic to, and invariant along, the axis of the strip. The solution of the approximate integral equation for the current on the strip is constructed as an infinite series of Chebyshev polynomials of the first kind weighted by a function exhibiting the proper edge behavior at the strip edges. Two methods, one depending upon integrals of the known excitation and the other upon its derivatives, are presented. The far‐zone scattered field is represented as series involving Bessel functions and coefficients determined from the excitation. In the special case of incident plane wave illumination, explicit expressions for the series coefficients are given.
Low‐order modes transformation in smooth waveguide couplings
Authors: Meyerova R. S. , Popov A. V. , Hoziosky S. A.
Published: February 2008

Propagation of low‐order modes in nonuniform waveguides is studied by means of the parabolic equation method. An analytic estimate of mode transformation coefficients is found.
Signal restoration from band‐pass filtered data
Authors: Guattari G. , Gori F. , De Mol C.
Published: February 2008

The restoration problem for a band‐pass linear system is examined in the case the input signal is weighted by a profile function. The singular system is evaluated analytically for three different forms of the profile function. An example of restoration process is presented.
Singular field behavior near the tip of a dielectric or dielectric‐metallic corner
Authors: De Smedt Ronald
Published: February 2008

The singular behavior of the electric field near the tip of a dielectric corner or a dielectric‐metallic corner is investigated by solving an appropriate eigenvalue problem on the surface of the unit sphere. As examples we study the dielectric corner, a dielectric corner on a metallic corner, and a metallic corner on a semi‐infinite dielectric medium. We also consider some particular configurations, in which the dielectric or the metallic has no influence on the singular behavior.
Application of propagating beam methods to electromagnetic and acoustic wave propagation problems: A review
Authors: Lagasse P. E. , Baets R.
Published: February 2008

Over the last years propagating beam methods have gained increasing acceptance for solving a broad range of wave propagation problems. The transformation of an extremely difficult boundary value problem into a much easier initial value problem, has allowed the solution of problems that were considered as intractable. The development of those propagating beam methods has occurred simultaneously, but almost independently, in the fields of acoustics and electromagnetics (primarily integrated and fiber optics). This paper reviews and assesses the main advantages and problems associated with the use of this method. Applications to acoustics and optics, to linear and nonlinear materials are considered and illustrated by examples.
Applications of field singularities at wedges and corners to time domain finite difference or finite element methods of field computations
Authors: Mei Kenneth K. , Cangellaris A. C.
Published: February 2008

Utilizing the knowledge of the singular behavior of the fields at a three‐dimensional corner, a simple algorithm is derived to account for the singularities in the numerical computations using the time domain finite difference or finite element methods. It is shown that the effects of the singularities have strong local effect near the corners or wedges for scattering problems.
The Moon as a calibration target of convenience for VHF‐UHF radar systems
Authors: Mathews J. D. , Breakall J. K. , Sulzer M. P.
Published: February 2008

Knowledge of the absolute, versus relative, performance characteristics of VHF and UHF radars used in geophysical applications is often important. We suggest that the Moon may form a convenient, easily tracked calibration target for many such radars. The lunar absolute radar scattering crosssection is large, reasonably well known (∼7% of the visible disk) and is essentially wavelength independent over 6 m > λ > 1 cm. The Arecibo 430‐MHz radar system was calibrated using the Moon as the target. These measurements, which are discussed in detail, yielded cross sections of 4.0% and 4.6% for a special 6 W and the main 1.6‐MW transmitters, respectively. We develop and present the radar equation appropriate to all total power calibration procedures including the Arecibo “worst case” calibration. We also discuss error levels, the necessity for averaging to obtain valid cross sections, and other practical difficulties associated with the technique.
A note on auroral radar detection of electrostatic ion‐cyclotron waves
Authors: D'Angelo N. , Merlino R. L.
Published: February 2008

A brief discussion is presented of “forward” and “backward” electrostatic ion‐cyclotron waves in the high‐latitude ionosphere. The need for additional measurements by HF radars is emphasized.
Correction to “Magnetic elliptical polarization of Schumann resonances”
Authors: Sentman D. D.
Published: February 2008

In the paper “Magnetic elliptical polarization of Schumann resonances” (Radio Science, 22(4), 595–606, 1987), two typographical errors appear. The right‐hand side of equation (1) should be preceded by a plus sign instead of a minus sign, and the right‐hand side of equation (6) is missing a factor of 1/sin (θ). However, the entries in Table 1 are tabulated from the correct expressions, and subsequent conclusions are unaffected.

The author regrets the errors.
Focusing in the stationary phase approximation
Authors: Nickisch L. J.
Published: February 2008

A method for calculating focusing in the stationary phase approximation is presented. By considering variations of Hamilton's equations a set of equations is obtained which, by integration along a single ray, provides information on the convergence or divergence of infinitesimally separated rays. The method applies to three‐dimensional ionospheric structure and is shown to be much more accurate than conventional finite flux tube approaches.
Backscattering coefficients of a half‐space anisotropic random medium by the multiple‐scattering theory
Authors: Lee Jay K. , Mudaliar Saba
Published: February 2008

This paper is intended to study the electromagnetic wave scattering from a half‐space anisotropic random medium. The ladder‐approximated Bethe‐Salpeter equation in conjunction with the nonlinearly approximated Dyson equation is used to derive the modified radiative transfer (MRT) equations for wave propagation in the half‐space random medium. The MRT equations are solved under a first‐order approximation. Backscattering coefficients are calculated and are compared with those obtained using the Born approximation. The first important thing noticed is that the propagation constants in the Born results are changed to effective propagation constants. Secondly, there are some additional terms contributing to backscattering enhancement, which is an important direct result of the MRT theory. Several numerical results are illustrated to compare the MRT and the Born results.
Empirical modeling of nighttime easterly and westerly VLF propagation in the Earth‐ionosphere waveguide
Authors: Pappert Richard A. , Hitney Linda R.
Published: February 2008

It has been known for many years that easterly and westerly propagation of VLF in the Earth‐ionosphere waveguide is nonreciprocal with the mean attenuation rate to the west being greater than to the east for ground to ground transmissions. It is also true that nighttime propagation to the west is more difficult to accurately predict than is propagation to the east. For example, a simple exponential β = 0.5 km−1, h′ = 87 km (notation of Wait and Spies (1964)) profile does an excellent job of predicting nocturnal in‐flight measurements over the Hawaii‐San Diego path but does not accurately predict nighttime propagation over the Hawaii‐Wake path. This is partly because propagation to the west is less stable than propagation to the east. In this study, full wave solutions will be used to show that propagation to the west is more sensitive to upper levels of the ionosphere than is propagation to the east. It therefore seems reasonable that propagation to the west is more sensitive to variable regions of the ionosphere than is propagation to the east and that this, at least in part, is responsible for the difficulty of accurately predicting westerly propagation. Comparisons are made between calculations based on empirical electron density profiles and in‐flight measurements between Hawaii and San Diego, and Hawaii and Wake.
Polarized thermal microwave emission from oxygen in the mesosphere
Authors: Rosenkranz P. W. , Staelin D. H.
Published: February 2008

The thermally emitted microwave spectrum of the terrestrial atmosphere, viewed from above, is calculated for frequencies in the vicinity of a Zeeman‐split oxygen line. Previously omitted phase terms are included; their effect on brightness temperature can be as large as 1.5 K for linearly polarized measurements. For the purpose of using oxygen lines as a probe of the atmospheric temperature profile, circular polarization of the radiometer would yield better vertical resolution. A double‐sideband radiometer measuring two lines would include a polarizer that reverses the handedness of polarization between the upper and lower sidebands. With channels covering frequencies from −4 to +4 MHz from the center of the 7+ or 9+ lines a satellite radiometer could measure the temperature profile for altitudes from 43 to 70 km.
Effects of atmosphere on radio imaging for target detection
Authors: Chu T. S.
Published: February 2008

The predictions of atmospheric effects on millimeter wave radio imaging of launch vehicles can be obtained by theoretical extrapolation of measured data from propagation studies for millimeter wave satellite communication systems. Attenuation exceedance statistics versus percentage of time have been estimated for both relatively wet and dry temperate climates. Attenuation problems are minimal at 10 GHz, but increase with frequency. Excessive rain attenuation is expected at 30 GHz for a small fraction of time. Cloud attenuation becomes also significant at 100 GHz. Water vapor in clear air can often cause more than 10 dB attenuation at 300 GHz. By use of the reciprocity theorem the effects of angle‐of‐arrival fluctuations on beam spot resolution have been found to be negligible for a space‐based telescope detecting Earth‐launched vehicles. Beam coverage area averaging can reduce cloud‐induced signal scintillations to less than 1 dB. These statements are valid except at very low elevation angles approaching tangent to the Earth, where the dominant problem is excessive signal attenuation.
Electromagnetic scattering by rough conducting circular cylinders
Authors: Eftimiu Cornel
Published: February 2008

This paper examines the coherent and incoherent responses of a conducting circular cylinder which is rough in both angular and axial directions. The calculations are done to second order of perturbation theory, and the results are recast by using Padé approximants to extend the domain of validity of the original formulation.
A uniform asymptotic analysis of the diffraction by an edge in a curved screen
Authors: Chuang C. W. , Liang M. C.
Published: February 2008

An approximate asymptotic high‐frequency solution is obtained for the electromagnetic wave diffraction by an edge in a curved conducting screen. The solution is uniform across the various ray shadow boundaries and is expressed in the simple format of the geometrical theory of diffraction. The transition functions associated with this uniform solution involve previously published special functions and can be calculated efficiently. Since the effect of the whispering gallery modes associated with the concave side of the curved screen is not included in this study, the present asymptotic solution is valid when neither the source nor the field points are close to the concave side of the screen.
Seabed propagation of ULF/ELF electromagnetic fields from harmonic dipole sources located on the seafloor
Authors: Fraser‐Smith A. C. , Inan A. S. , Villard O. G. , Joiner R. G.
Published: February 2008

The amplitudes of the quasi‐static electromagnetic fields generated at points on the seafloor by harmonic dipole sources (vertically directed magnetic dipoles, horizontally directed magnetic dipoles, vertically directed electric dipoles, and horizontally directed electric dipoles) also located on the seafloor are computed using a numerical integration technique. The primary purpose of these computations is to obtain field amplitudes that can be used in undersea communication studies. An important secondary purpose is to examine the enhancements of the fields produced at moderate to large distances by the presence of the relatively less conducting seafloor, as compared with the fields produced at the same distances in a sea of infinite extent, for frequencies in the ULF/ELF bands (frequencies less than 3 kHz). These latter enhancements can be surprisingly large, with increases of 4 orders of magnitude or more being typical at distances of 20 seawater skin depths.
Filamentation of electromagnetic beams in an inhomogeneous ionosphere
Authors: Laxmi V. N. , Tripathi V. K.
Published: February 2008

The nonlinear evolution of filamentation instability has been investigated in an inhomogeneous ionosphere. It is seen that a plane wave of uniform intensity becomes unstable and gets filamented in the presence of transverse density fluctuations in the medium. The amplitude of the filaments increases with altitude and increasing incident wave power, behaving like an Airy's function. The horizontal scale size of the filaments is equal to the horizontal wave number of perturbation. The spatial growth length of instability is of the order of density scale length.
Propagation in a surface duct over a two‐dimensional sinusoidal rough Earth
Authors: Marcus Sherman W.
Published: February 2008

A solution for the problem of horizontally polarized radio waves propagating in a surface duct situated over a perfectly conducting, sinusoidally shaped ground with small amplitudes is obtained. The solution contains the assumption that the source is a horizontal line source perpendicular to the roughness variation. It is found that the specular contribution may be derived using an equivalent matrix with an effective reflection coefficient that accounts for scattering both out of and into the specular direction. The accuracy of a method for obtaining the rough Earth waveguide eigenvalues as a perturbation of those for the smooth Earth case is analyzed. Numerical field results are compared using the different approaches, demonstrating an attenuation effect due to the roughness.
Second‐order Wiener‐Hermite expansion in the electromagnetic scattering by conducting rough surfaces
Authors: Eftimiu Cornel
Published: February 2008

The surface current induced by a plane electromagnetic wave on a rough conducting surface and the scattered electromagnetic field are sought as expansions to second order in Wiener‐Hermite functionals of the profile function. The coherent and incoherent cross sections are calculated and compared to results obtained by means of perturbation theory, the Kirchhoff approximation, and Padé approximants.
Nonlinear wave fronts and ionospheric irregularities observed by HF sounding over a powerful acoustic source
Authors: Blanc Elisabeth , Rickel Dwight
Published: February 2008

Different wave fronts affected by significant nonlinearities have been observed in the ionosphere by a pulsed HF sounding experiment at a distance of 38 km from the source point of a 4800‐kg ammonium nitrate and fuel oil (ANFO) explosion on the ground. These wave fronts are revealed by partial reflections of the radio sounding waves. A small‐scale irregular structure has been generated by a first wave front at the level of a sporadic E layer which characterized the ionosphere at the time of the experiment. The time scale of these fluctuations is about 1 to 2 s; its lifetime is about 2 min. Similar irregularities were also observed at the level of a second wave front in the F region. This structure appears also as diffusion on a continuous wave sounding at horizontal distances of the order of 200 km from the source. In contrast, a third front unaffected by irregularities may originate from the lowest layers of the ionosphere or from a supersonic wave front propagating at the base of the thermosphere. The origin of these structures is discussed.
Low‐frequency transverse electric atmospheric noise: Measurement and theory
Authors: Turtle John P. , Field Edward C. , Warber Chris R. , McGill Paul R.
Published: February 2008

This paper presents measurements of both transverse electric (TE) and transverse magnetic (TM) LF atmospheric noise made aboard a free‐floating balloon at altitudes up to 20 km. Data are given for rms noise level, TE/TM noise ratio, amplitude probability distribution, and impulsiveness parameter VD. The TE/TM noise ratio was greater than reported by other authors and implies major contributions from horizontally polarized sources. Calculations of the fields generated by lightning discharges as a function of altitude and orientation show that horizontal discharges are weak TE radiators at VLF/LF unless their altitude exceeds a few kilometers. Above several kilometers, however, horizontal discharges can radiate TE noise almost as efficiently as vertical ones radiate TM noise. Most horizontal lightning channels do, in fact, occur above 3 km and are more frequent than vertical channels. Moreover, the horizontal component of lightning is often two or three times greater in extent than the vertical component, even for cloud‐to‐ground discharges. It therefore appears that TE noise might well be caused primarily by direct radiation from horizontal lightning rather than by geomagnetically converted radiation from vertical lightning.
A wideband model for electromagnetic interference from corona on electric power lines
Authors: Olsen Robert G. , Wu Mingde
Published: February 2008

A new analytical model for predicting electromagnetic interference from corona on electric power lines has been developed. The corona is modeled as electric dipoles located on the surface of the power line conductor which is located above a homogeneous lossy Earth. The position and driving current of each dipole are described statistically. Expressions are found for the spectral density of the fields radiated by the corona and the currents induced on the power line. The main contribution of the new model is that it is valid in frequency ranges (i.e., 2–30 MHz) for which no model presently exists and for distances and directions from the power line which are essentially unrestricted. The model is shown to agree well with an empirical formula which was developed from the limited experimental data available.
A method of parameterizing VHF radar returns from the troposphere
Authors: Kuo Fu‐Shong , Lue Hsiou‐Yung
Published: February 2008

By numerical analysis of the stochastic model of the signal statistics of the VHR radar returns, we have isolated an echo parameter, called the modified Rice parameter γ, from the unknown stochastic parameters and have obtained a simple method of deducing γ from the returned signals. In contrast to γ, which is a low‐order moment, another echo parameter, the Nakagami m coefficient, which seems to involve fourth‐order statistics, is shown to be much more sensitive to stochastic effects and is an inappropriate parameter for characterizing the radar echo production mechanism for real observations of the atmosphere where stochastic effects abound. Examples of echo parameter analysis of Sounding System (SOUSY) and Chung‐Li VHF radar data are presented which demonstrate that the signal power is strongly related to the modified Rice parameter as expected.
Power deposition and focusing in a lossy cylinder by a concentric phased array
Authors: Lumori Mikaya L. D. , Wait James R. , Cetas Thomas C.
Published: February 2008

We present an analysis of phased array applicators for heating of torso, limbs, and neck. A homogeneous cylindrical model is adopted for the calculations. Four‐, eight‐, and 16‐horn apertures are considered. Focusing of power is demonstrated for an operation frequency of 915 MHz in a cylindrical phantom of radius 5.2 cm, with a conductivity of 1.28 S/m and a relative permittivity of 51. Experimental verification is shown for the case with four‐horn applicators. We thus demonstrate that by controlling the relative phases and amplitudes of the aperture sources, it is possible to focus electromagnetic power at desired locations such as tumors.
Loaded two‐element ring antenna for electron plasma waves
Authors: Morita Y. , Egashira S. , Honjoh M.
Published: February 2008

A short, coil‐loaded two‐element ring antenna including a transmission line, immersed in a warm, isotropic plasma, is investigated as a boundary value problem in the frequency range where electron plasma waves exist. The wave potential of electron plasma waves launched from a two‐element ring antenna has been enhanced by a factor of 4–6 in comparison with the case of an unloaded one‐element ring antenna, when a suitable inductive load is inserted at the feeding point of element 2, and the distance between element 1 and element 2 is chosen as an optimum element spacing. Theory is substantiated by the experiments. In addition, the unidirectional radiation patterns of the antenna for electron plasma waves are predicted by the present theory.
Removal of aliasing in pulse‐to‐pulse Doppler radar measurements
Authors: Sahr John D. , Farley Donald T. , Swartz Wesley E.
Published: February 2008

Frequency aliasing is a problem often encountered in pulsed radar studies of some deep targets, such as the radar aurora. It is possible to resolve the aliasing ambiguities by transmitting closely spaced pulse pairs, rather than single pulses. Instead of the usual power spectrum, we compute a cross spectrum of the received signals. The amplitude of this cross spectrum is just the normal, possibly aliased, power spectrum, and the phase determines the aliasing, if any, at each frequency bin. The technique works as long as the spectrum is not seriously wrapped around upon itself, that is, as long as each frequency bin is dominated by signals with a single Doppler shift, no matter what that Doppler shift may be. The price paid for this extra information is the addition of radar clutter, uncorrelated signals from unwanted ranges, but this price may be acceptable.
Quantum mechanical uncertainty limitations on deep space navigation by Doppler tracking and very long baseline interferometry
Authors: Cannon W. H.
Published: February 2008

The ultimate precision with which very long baseline interferometry (VLBI) can determine the angular position of a spacecraft is determined by the quantum mechanical limitations on the performance of the interferometer receivers and the quantum mechanical uncertainty relation ΔNΔΦ ≥ 1. It is shown that for the navigation of a typical deep space mission using present‐day techniques, fundamental physics imposes the following limits on the precision of spacecraft navigation: (1) Minimum noise on determination of phase of spacecraft navigation tone, ΔΦmin ≈ 1.9 × 10−5 radians per AU, (2) minimum noise on determination of phase of VLBI navigation fringes, ΔΘmin ≈ 2.6 × 10−5 radians per AU, (3) minimum noise on determination of VLBI navigation fringe frequency, Δfmin ≈ 2.9 × 10−9 Hz per AU, (4) minimum noise on determination of VLBI group delay, Δτgmin ≈ 0.5 ps per AU, and (5) minimum noise on determination of spacecraft angular position, ΔΨmin ≈ 2.9 × 10−11 radians per AU. The above limitations which are a consequence of quantum mechanical uncertainty on the determinations of the phase of a spacecraft tracking signal may be circumvented in principle by the application of squeezed quantum states.
Numerical modeling of passive microwave O2 observations over precipitation
Authors: Gasiewski A. J. , Staelin D. H.
Published: February 2008

The microwave scattering and emission properties of precipitation cells are investigated by comparing 118‐GHz radiometric observations with a planar‐stratified numerical radiative transfer model. Liquid and frozen hydrometeors are modeled as spherical Marshall‐Palmer and Sekhon‐Srivastava distributed Mie scattering polydispersions, respectively, with Henyey‐Greenstein phase functions. Comparisons are made between computed brightness temperatures based on weather radar observations of a convective precipitation cell couplet during the Cooperative Huntsville Meteorological Experiment (COHMEX), 1986, and brightnesses observed coincidentally by an airborne imaging spectrometer. Agreement between observed and computed brightness perturbations is within 10% over the radiometrically opaque, mature regions of the cell, although the model brightnesses over the partially transparent anvil region are highly sensitive to the assumed mean ice particle size. The sensitivity of the 118‐GHz channels to temperature at various levels in the troposphere is exhibited, in the presence of precipitation, through the perturbed temperature weighting functions. Calculations using the perturbed and nonscattering weighting functions suggest 118‐GHz transparent‐channel cell top reflectivities of up to 50% in the convective core region and 6% in the anvil region. A rain cell model parameterized by cell top altitude and total (liquid and ice) water density is used to analyze the information content of 118‐GHz precipitation cell spectra. The model suggests that at 118 GHz, cells with uniform water densities greater than 0.5 g m−3 are opaque, and cells with uniform densities less than 0.1 g m−3 are transparent. The dominant 118‐GHz spectral modes are found to contain useful information on the cell top altitude and can be used to detect transparent anvil regions. The physical retrieval of cell water density is shown to be facilitated by coincident observations using similar weighting‐function channels at 53 and 118 GHz, although the retrieval is sensitive to the assumed mean ice particle size.
Thermospheric dynamics: A system theory approach
Authors: Codrescu M. , Forbes J. M. , Roble R. G.
Published: February 2008

A system theory approach to thermospheric modeling is developed, based upon a linearization method which is capable of preserving nonlinear features of a dynamical system. The method is tested using a large, nonlinear, time‐varying system, namely the thermospheric general circulation model (TGCM) of the National Center for Atmospheric Research. We compare TGCM simulation results with results from our approximation procedure at Millstone Hill and Arecibo, two locations where there may be interest for using this approach. In the linearized version an equivalent system, defined for one of the desired TGCM output variables, is characterized by a set of response functions that is constructed from corresponding quasi‐steady state and unit sample response functions. The number of functions in the set depends on the degree of nonlinearity of the original system and the desired accuracy of the approximation. Using the set of response functions, we are able to approximate time‐dependent TGCM simulation results with reasonable accuracy. The linearized version of the system runs on a personal computer and produces an approximation of the desired TGCM output field height profile at a given geographic location. The approximation is obtained by performing a specialized convolution between the cross polar cap potential drop as input and the set of response functions. This enables an efficient first‐order comparison between experimental data and theoretical simulations and may provide an operationally efficient means of more realistically predicting density variations for satellite ephemeris or reentry trajectory determinations during geomagnetic quiet periods as well as during magnetic storms.
The use of mode structure diagrams in the prediction of high‐latitude HF propagation
Authors: Davé Nikhil
Published: February 2008

Improvement in high‐frequency (HF) communication and frequency management, especially at high latitudes, depends on knowledge of ground ranges accessible to propagating modes and frequencies not only at the maximum usable frequency (MUF) but also at frequencies below the MUF, these lower frequencies being often used in communication to assure operation below about 90% of the population of observed MUF values. To this end, calculations from an advanced HF ray‐tracing program, “AMBCOM,” of receivable frequencies for basic HF ionospheric modes at the high latitude station Andoya, Norway are plotted in the frequency/ground‐range plane, for a chosen set of geotemporal conditions and sunspot number. The plots are made for transmitters located on northern, eastern, southern, and western bearings relative to Andoya, and for ground ranges of up to 6000 km from this assumed receive site. This method of representation of HF ray trace calculations results in the delineation of the regions of occurrence in the frequency/ground‐range plane of the main HF propagation modes for chosen conditions. Such plots are appropriately termed mode structure diagrams. The HF mode structure diagrams provide valuable information, discussed herein, pertaining to (1) the determination of what modes and frequencies are best suited for specific paths and ground ranges at high latitudes, (2) identification of factors affecting the performance of HF communication at frequencies below the MUF, and (3) the conception and performance of further studies to resolve certain outstanding HF communication problems. Approximate formulae for the frequency range available in 1‐hop F2 modes at high latitudes, motivated by mode structure diagrams, are developed and discussed.
Further investigation of the interference minimums in the low‐frequency electromagnetic fields produced by a submerged vertical magnetic dipole
Authors: Inan A. S. , Fraser‐Smith A. C.
Published: February 2008

The quasi‐static electromagnetic fields generated along the sea surface by a submerged vertical magnetic dipole are evaluated numerically using exact expressions and the results are plotted in a parametric form for source depths varying from 2 to 14 seawater skin depths δ. The curves show that there is a minimum in the amplitude of the vertical component of the magnetic field for horizontal distances from the source in the range 9–14δ and for dipole depths ranging from 2 to 8δ, with the deepest minimum occurring at a horizontal distance of pmin ≃ 11.07δ when the dipole is at a critical depth of dc ≃ 4.22δ. There also exists a similar minimum point in the variation along the surface of the amplitude of the total electric field for horizontal distances from the source in the range 10–20δ and dipole depths ranging from 4 to 23δ, with the deepest minimum occurring at a horizontal distance of pmin ≃ 12.95δ when the dipole is at a depth of dc ≃ 9.38δ. Both minimums are due to the strong destructive interference between the direct and the lateral wave components of the fields. No such minimum point exists for the variation of the amplitude of the horizontal component of the magnetic field.
Average reflected power from a one‐dimensional slab of discrete scatterers
Authors: Saatchi Sasan S. , Lang Roger H.
Published: February 2008

Reflection from a one‐dimensional random medium of discrete scatterers is considered. The discrete scattering medium is modeled by a Poisson impulse process with concentration λ. By employing the Markov property of the Poisson impulse process, an exact functional integrodifferential equation of the Kolmogorov‐Feller type is found for the average reflected power. Approximate solutions to this equation are obtained by regular perturbation and two variable expansion techniques in the limit of small λ. The regular perturbation results is valid for small slab thicknesses, while the two‐variable result is uniformly valid for any thickness. The two‐variable result shows that as the slab size becomes infinite all of the incident power is reflected on the average.
Quasi‐static analysis of a two‐wire transmission line located at an interface
Authors: Hill David A.
Published: February 2008

Simple quasi‐static expressions have been derived for the propagation constant, the characteristic impedance, and the field distribution of a two‐wire transmission line located at the air‐Earth interface. Both the complex permittivity and the complex permeability of the Earth are allowed to differ from the free‐space values, and a numerical solution of the mode equation shows that quasi‐static approximation is valid when the wire separation is much less than a free‐space wavelength. The quasi‐static approximation can be used to determine both the complex permittivity and the complex permeability of the Earth from measurements of the propagation constant and the characteristic impedance of the transmission line.
Transverse electric wave scattering by two‐dimensional surfaces of arbitrary shape in the presence of a wedge
Authors: Zheng Dalian , Michalski Krzysztof A. , Crowell Kelly J. , Nevels Robert D.
Published: February 2008

Two forms of the so‐called mixed‐potential electric field integral equation (MPIE) are developed for two‐dimensional, perfectly conducting (PC) surfaces of arbitrary shape in the presence of an infinite, PC wedge, subject to transverse electric excitation. One of the MPIEs is based on the Coulomb gauge while the other employs the Lorentz gauge. In either case the effect of the wedge is incorporated in the integral equation by means of the appropriate Green's functions, leaving the current distribution on the arbitrary surface as the only unknown. The Green's functions are derived by the eigenfunction expansion technique. A well‐established moment method procedure is adapted to numerically solve both forms of the MPIE. Computed results are presented for several cases of interest, and the relative merits of the Coulomb and Lorentz gauge MPIEs are discussed.
Uncertainties in estimates of the mean vertical velocity from MST radar observations
Authors: Nastrom G. D. , Gage K. S. , Ecklund W. L.
Published: February 2008

Mean values of the vertical velocity based on time series of clear‐air Doppler radar observations are subject to uncertainties due to several factors. This paper examines the statistical uncertainties due to estimating the population means using only finite periods of record and the uncertainties that are introduced by systematic gaps in the series of observations. The statistical uncertainty, the standard error of the mean, depends on the variance of the data and on the effective sample size. The largest change in the variance of vertical velocity is about a factor of 3 and is seen between clear and cloudy sky conditions. Very little change in the effective sample size is seen under any conditions. The averaging time required to estimate the mean with a standard error of the mean of 2 cm/s is found to be about 6 hours under typical conditions at Flatland. The uncertainty of the mean due to systematic gaps in the data increases with the length of the gaps, even though the total observation period remains constant. The root‐mean‐square value of this uncertainty is about 2 cm/s; about the size of the standard error of the mean and about half as large as typical expected subsynoptic scale motions. These results suggest that future sampling programs should be designed to eliminate systematic gaps in the data.
Range dependence of volume scattering from a fractal medium: Simulation results
Authors: Rastogi Prabhat K. , Scheucher Karl F.
Published: February 2008

Range variation of scattered signal power in atmospheric radar experiments seems to be well understood for a point target and for volume scattering from a homogeneous random medium. In these cases the signal power decays, respectively, as inverse fourth power and inverse square of range. Intermediate cases of scattering from patchy random media with partial space‐filling characteristics are numerically examined for a simple fractal model. Planar targets, oriented along the beam, are successively divided into cells of smaller size, eventually reduced to isotropic “point” targets. At each stage of division the cells are activated with a probability p. This probability controls the degree to which the medium fills the scattering region, and it is related to its fractal dimension D. For each realization of the medium the signal power is obtained by coherently adding the complex received voltage due to each point target. It is found that volume scattering is a good approximation even for targets that only fill up a plane, i.e., have fractal dimension D∼2.
Microwave imaging by three‐dimensional Born linearization of electromagnetic scattering
Authors: Caorsi S. , Gragnani G. L. , Pastorino M.
Published: February 2008

An approach to microwave imaging is proposed that uses a three‐dimensional vectorial form of the Born approximation to linearize the equation of electromagnetic scattering. The inverse scattering problem is numerically solved for three‐dimensional geometries by means of the moment method. A pseudoinversion algorithm is adopted to overcome ill conditioning. Results show that the method is well suited for qualitative imaging purposes, while its capability for exactly reconstructing the complex dielectric permittivity is affected by the limitations inherent in the Born approximation and in ill conditioning.
Simulations of electromagnetic wave plasma interactions
Authors: Kruer William L.
Published: February 2008

The interaction of electromagnetic waves with plasmas is a topic rich in nonlinear effects with applications ranging from laser fusion to space plasmas. Computer simulations are a powerful tool to help understand the growing number of experiments in this area. Here we discuss some simulations of the Raman and two‐plasmon decay instabilities and the nonlinear evolution of ponderomotively driven ion acoustic waves. Some salient features are compared with experiments.
Strain, stress and uplift
Authors: Toriumi Mitsuhiro
Published: February 2008

Strain distribution across the high‐pressure‐type Sambagawa regional metamorphic belt was measured by studies of shape change of initially spherical radiolaria. Strains change from 5 to 2000% with increasing metamorphic temperatures. Aburpt strain increase apparently occurs in the intermediate metamorphic grades of the terrane. Deformation stresses during metamorphism were inferred using the grain‐sized quartz piezometer method, which is applicable to dynamically recrystallized quartzose rocks. Stress distribution across the terrane was composed of two levels; the stress level in the lower‐grade zones was about 130 MPa but in the higher‐grade zones was about 50 MPa. Combining stress distribution, strain distribution, and temperatures of the studied metamorphic terrane, the activation energy of the deformation of these quartzose rocks is estimated to have been about 80–90 kJ/mol. The huge amount of deformation of the deep‐seated Sambagawa metamorphic terrane, which was formed probably at the lower crustal to uppermost mantle levels over the subducting lithosphere, was sufficient to bring about the uplift of parts of the terrane to the earth's surface.