bluuebunny/crossref_metadata_2025 huggingface dataset:
Backscatter inversion in spherically asymmetric ionosphere
Authors: DuBroff R. E. , Rao N. Narayana , Yeh K. C.
Published: February 2008

It is well known that various diurnal and morphological features of the ionosphere reveal substantial departures from the normally simplified assumption of spherical symmetry at certain times of the day or at certain geographic locations. A radio ray passing through such an ionosphere must bear information about its horizontal gradients. The leading edge of a backscatter ionogram is formed by obliquely propagated radio rays of minimum time delay and hence is useful in deducing information about the ionospheric horizontal gradients. In this regard, the ionospheric electron density distribution is modeled by a locally quasi‐parabolic layer with six parameters. This six‐parameter space is known as the ‘ionostate.’ Our object is to seek the ‘best’ ionostate in the sense that the corresponding mean square error in group delay is a minimum. A computer program has been written to carry out the specified procedure. A number of sample calculations are presented and discussed.
A geometrical‐optics and an exact solution for internal fields in and energy absorption by a cylindrical model of man irradiated by an electromagnetic plane wave
Authors: Massoudi H. , Durney C. H. , Johnson C. C.
Published: February 2008

A geometrical‐optics approximation was used to calculate the mass‐normalized rate of RF energy absorption (the specific absorption rate or SAR) in cylindrical models of man and in experimental animals irradiated by an electromagnetic (EM) plane wave at high radio frequencies. Comparison of these results with corresponding geometrical‐optics calculations for prolate spheroidal models showed that the relative absorptive cross section of the prolate spheroidal and cylindrical models of man are essentially the same at frequencies above 20 GHz. The exact solution for the SAR in a lossy, infinitely long homogeneous circular cylinder exposed to an electromagnetic plane wave, for perpendicular incidence and with the incident E field both perpendicular and parallel to the axis, is also given. Although the formal solution is in the literature, data showing SARs for very lossy cylinders have apparently not been calculated. Curves showing SAR versus frequency for cylindrical models of man and animals in comparison with composite curves obtained from prolate spheroidal calculation are presented. It is shown that the exact solution for SARs in cylindrical models of animals and human beings appears to be a good approximation in the frequency range just below the geometrical‐optics limit, thus providing an important method for extending the calculation of SARs into a range of frequencies where calculations were not available previously.
A review of radar observations of the troposphere in clear air conditions
Authors: James P. K.
Published: February 2008

It is now 15 years since radars were first used intensively to probe the atmosphere in clear‐air conditions. The first experiments were concerned with the nature of the targets and accounting for the intensity of the measured echoes. It was found that inhomogeneities in the refractive index field were responsible for a large proportion of clear‐air returns. Such returns were detected in characteristic patterns in association with sheared statically stable layers, convective thermals, air mass boundaries, and a variety of wave structures. More sensitive Doppler radars have been developed in recent years which are able to obtain usable signals from all heights in the troposphere. Such radars, as well as providing additional information about the clear‐air patterns observed previously, enable continuous measurements of wind profiles to be made. This paper describes the phenomena which have been observed and the measurements which have been made with radars under clear‐air conditions. A summary is given of the types of targets observed, and the kinds of radars used in clear‐air studies are briefly described.
The input impedance of horizontal antennas above an imperfect plane earth
Authors: Nicol J. L.
Published: February 2008

A method of calculating the current distribution and input impedance of thin horizontal cylindrical antennas is presented and the impedance computed for cases in which the antennas are located at heights of 0.05 λ and 0.02 λ above a semi‐infinite plane earth. Results indicate that it would be practical to determine the electrical parameters εr and σ of the earth directly from measurements of the input impedance of a half‐wave dipole provided these parameters were essentially constant to a depth equal to the penetration depth of radiation at the operating frequency.
Generation of ionospheric irregularities associated with plasma lines induced by an HF wave
Authors: Muldrew D. B.
Published: February 2008

The magnetic field‐aligned ionization irregularities or ducts, in which the electrostatic waves exist that are responsible for the plasma lines induced by the HF heater wave, are small in diameter compared to the spatial separation of these ducts. There is reason to doubt that these ducts are formed by the thermal self‐focusing instability. A model is proposed for duct formation, in which a duct is analogous to a receiving antenna. Energy is extracted from the incident HF field over the ‘effective area’ of the duct, which is much greater than the cross‐sectional area of the duct itself. The separation of individual ducts is then determined by this effective area.
Investigation of a radial, parallel‐plate waveguide with an annular slot
Authors: Butler Chalmers M. , Keshavamurthy Talya L.
Published: February 2008

A parallel‐plate, radial waveguide with an annular slot in its upper plate is excited by a known transverse electromagnetic wave. An integral equation is derived for the electric field in the annular aperture, and this equation is solved numerically. Attention is given to the construction of Green's functions for the waveguide interior as well as for the exterior half space above the guide. Efficient techniques are discussed for evaluating improper integrals and summing infinite series that arise in the numerical solution procedure. The electric field in both narrow and wide slots is determined as a solution of the integral equation, and when the interior and exterior dielectrics are different, the occurrence of surface waves in the slot region is observed. Values for scattering parameters and far‐zone radiated fields are presented.
Prediction of cross‐polarization discrimination statistics for propagation through spatially nonuniform rain
Authors: Kanellopoulos J. D. , Clarke R. H.
Published: February 2008

A method is presented for predicting the long‐term statistics for rain depolarization in the case of spatially nonuniform rain, based on previous calculations for the uniform case. These calculations apply to any type of linear polarization. Assuming a lognormal form for the point rain rate statistics and using the idea of space‐averaged rain rate, an approximate normal form is deduced for the probability density function of the cross‐polarization isolation (XPI) or discrimination (XPD) in decibels. The parameters of this normal density are shown to be dependent on the parameters of the radio link and the rain rate distributions. These include the probabilities of rainfall along that particular path, the median and standard deviation of the point rainfall rate, the spatial correlation coefficient of the rainfall rate, radio frequency, path length, and type of polarization.
Boundary formulation of propagation problems in guiding structures for integrated optics
Authors: Daniele V. , Montrosset I. , Zich R.
Published: February 2008

This paper presents a new technique for the determination of the dispersion diagrams of dielectric waveguides embedded in a stratified media. The method consists of a boundary formulation of the problem in terms of an integral equation on the waveguide surface. Numerical results are obtained by applying the moment method. As an example of application, the case of a guiding core of arbitrary cross section embedded in a homogeneous half‐space has been considered. The computer program requires small computer memory and time; in test cases the results agree with those obtained with other methods.
Calculation of dispersion for two optical fiber profiles by the propagating beam technique
Authors: Feit M. D. , Fleck J. A.
Published: February 2008

The propagating beam method provides an accurate configuration space description for the field in an optical fiber. From the Fourier transforms of certain correlation functions of the field it is possible to generate all mode properties of the field. These methods are applied to two fiber profiles in order to determine their dispersion properties.
Behaviour of transients in propagation constants series of evanescent wave theory
Authors: Jacobsen G. , Ramskov Hansen F.
Published: February 2008

Asymptotic series for the square of modal propagation constants of a general class of graded index refractive index profiles are investigated to including 0(k−8) using nonlinear transforms of order one, two and three. Here, the behaviour of spatial transients is considered. Description in terms of transforms of order one and two is locally compared to the third order description. The local sequence‐behaviour is reflected in the results presented.
Cross polarization during precipitation on terrestrial links: A review
Authors: Olsen R. L.
Published: February 2008

This paper reviews recent theoretical and experimental research on cross polarization that occurs during precipitation on terrestrial microwave links. Consideration is first given to the physical mechanisms, such as raindrop canting, that cause precipitation depolarization. Coherent rain depolarization models are then discussed and recent investigations of incoherent depolarization effects summarized. Particular attention is given to semi‐empirical models since they can be used most readily for statistical predictions in design applications. ‘Model oriented’ cross polarization measurements aimed at characterizing the physical properties of the depolarizing medium for use in model development are also given particular attention. Effective parameters of the raindrop canting angle distribution derived from these measurements are tabulated. Finally, some mention is made of the relative importance of rain depolarization and attenuation in causing outages on digital radio systems.
Partial coherence properties and spectral transfer functions of optical waves in dispersive waveguides
Authors: Miyazaki Y.
Published: February 2008

General transmission theory of transient coherence of optical fields in dispersive fibers is shown. Temporal and spatial coherence properties of optical waves in random gradient fibers excited by partially coherent sources are studied by stochastic process theory for irregularities along fibers. Generalized coupling equations concerned with transient mutual correlation functions of random fields are derived. Using transient mutual correlation functions, exact spectral transfer functions for base band power responses are discussed with dispersive effects. Based on this theory, optical pulse signals in gradient fibers with radially inhomogeneous refractive index and lossy cladding that gives excellent filtering properties are studied. Scattering, pulse broadening, and base band power response are discussed.
Gaussian approximation of the emission from a semiconductor laser
Authors: Tanaka M. , Takenaka T. , Fukumitsu O.
Published: February 2008

AGaussian approximation of the emission from a semiconductor laseris investigated by using the plane wave spectrum representation of the electric field. A Gaussian spectrum, which fits either the spectrum of the laser emission or its homogeneous component at the laser‐air interface, is determined. The spread of a Gaussian function closely approximating the amplitude of the far field is obtained. The results show good agreement between the spread calculated for the Gaussian spectrum and that for the spectrum of the laser emission. The formulations and calculations are mainly carried out for a symmetric structure. The effect of the reflection at the laser‐air interface and that of the asymmetry of the structure are also discussed.
On the completeness of spectral expansion of the electromagnetic field in the set of dielectric circular rod waveguide eigen waves
Authors: Shevchenko V. V.
Published: February 2008

In this paper some results on the vector theory of the cross‐section open waveguide boundary problem (for dielectric circular rod waveguides) are given as a generalization from the same scalar theory (for plane‐stratified waveguides).
Intensity scintillation parameters for characterizing transionospheric radio signals
Authors: Rino C. L. , Liu C. H.
Published: February 2008

The structure of intensity scintillation is characterized by the fourth‐order complex signal moment Γ4. The differential equation that governs the behavior of Γ4 is written in terms of dimensionless variables to isolate parameters that can be inferred from intensity scintillation data without a priori knowledge of scale sizes, layer thickness, propagation angles, etc. It is shown, for example, that when the outer scale is large compared to the Fresnel radius, the scintillation index measured under weak scatter conditions can be used to derive a universal scintillation index curve. Multifrequency scintillation data are presented to demonstrate the concept.
Simulation of a lightning channel by a prolate spheroid
Authors: Volland Hans
Published: February 2008

The channel of a lightning stroke is modeled by an extremely elongated prolate spheroid of moderate electric conductivity. The model simulates return strokes if the center of the spheroid has electrical contact to an electric wall (the earth's surface). Otherwise, it simulates K strokes. Discharging of an electric charge within such spheroid occurs in an impulsive way due to the wave guide characteristics of the channel. The eigenvalues, eigenfrequencies, and eigenfunctions of electromagnetic fields and currents inside and outside the channel are calculated. They are compared with corresponding values of a cylindrical model. The agreement is reasonable as long as ∣ αl/c∣ ≦ 3, with α being the eigenfrequency, 2l being the channel length, and c being the speed of light. Most observed wave forms of return stroke currents and of sferics from return strokes or K strokes meet that condition.
Incoherent scatter radar observations of the plasma line with a chirped pulse system
Authors: Hagfors T.
Published: February 2008

It is shown that the scattering from plasma density fluctuations near the Langmuir frequency can be detected with greatly improved signal to noise ratio when the incident radiation is chirped so as to match the electron density gradient. The factor of improvement in going from a nonchirped to a chirped modulation scheme is derived. A method of implementing the search for the optimum chirp rate is described, and several uses of the method for the measurement of ionospheric parameters are suggested.
HF direction finding by wave front testing in a fading signal environment
Authors: Rice D. W.
Published: February 2008

Calculations are reported of the probability of observation of various degrees of phase front nonlinearity, for two and three signals incident from different directions upon a phase‐measuring array. It is assumed that the signals fade independently and that their amplitude probability densities are described by a Rayleigh law. The measure of phase front nonlinearity employed is the rms deviation from the best straight‐line fit to the measured phases along a linear array. The worst‐case condition is that of equal average powers in the incident signals. When there are two such signals present, it is shown that the probability is 0.5 that a single observation of the rms phase deviation will yield a result of 25° or less. When there are three equal‐power signals present, the corresponding probability is 0.2. Experimental measurements over a 911‐km mid‐latitude path and a 2100‐km auroral‐zone path confirm the general behavior predicted by the theory. The experimental program included concurrent measurements using FMCW, CW, and single‐sideband signals from a controlled transmitter. The range resolution capability of the FMCW signals was utilized to provide direction‐of‐arrival statistics on a mode‐separated basis. The standard deviation of the direction of arrival varied from 0.3° for E mode signals to 1.4° for F2 high‐angle signals. The CW and single‐sideband signals were processed by selecting only those phase fronts with rms phase deviations of 10° or less. For these, direction‐of‐arrival standard deviations were in the range 0.6°–0.9° with slightly smaller deviations being observed for the CW signals.
A review of volume scatter theories for modeling applications
Authors: Fung A. K.
Published: February 2008

Volume scatter theories commonly applied to develop scatter models for vegetation, snow, and sea ice are reviewed. These include (1) the first‐order theories based on the Born approximation, the distorted Born approximation, and the renormalization method and (2) the multiple scatter theories based on the radiative transfer method and the matrix doubling method. Applications of the scatter models developed from a first‐order theory and a multiple scatter theory are illustrated. Additional applications available in the literature are cited.
Depolarization of the scattered radiation fields by conducting objects of irregular shape above rough land and sea: Full wave solutions
Authors: Bahar Ezekiel
Published: February 2008

The full wave approach to problems of scattering by rough surfaces has been applied to the problem of depolarization of the scattered radiation fields by objects of finite conductivity and irregular shape. In the analysis, complete expansions are employed, exact boundary conditions are imposed, and a variable coordinate system that conforms with the local features of the irregular surface is used. The full wave solutions are expressed in forms that can be readily compared with earlier solutions, and they can be used to reconcile the differences and bridge the wide gap between these solutions. Thus the full wave solutions for the backscatter cross section are shown to reduce to the physical optics solutions when the high‐frequency, stationary phase approximations are used. Similarly, for slightly rough surfaces the full wave expressions reduce to the perturbational solutions for the backscatter cross section. The full wave solutions are shown to be consistent with the duality reciprocity and realizability relationships in electromagnetic theory. These solutions are invariant to coordinate transformations. Since upward and downward scattering are considered in the analysis, multiple scattering and shadowing effects are taken into account in a self‐consistent manner. Thus the total scattered field varies continuously as the observer moves across a shadow boundary, and there is no need to introduce transition terms derived from other theories. The full wave approach can be applied to deterministic, periodic, and random rough surfaces. It can also be used to determine the scattering by finite scatterers in the presence of rough land or sea.
The image theory electromagnetic fields of a horizontal electric dipole in the presence of a conducting half space
Authors: Bannister Peter R.
Published: February 2008

The problem considered is a horizontal electric dipole that is located above or below the plane surface of a conducting half space. Expressions for the field components are obtained by employing finitely conducting earth‐image theory techniques. The formulas developed are of simple form and are valid from the quasi‐static to the far‐field ranges as long as the measurement distance is greater than 3 times the burial depth of the source and/or receiver.
Measurements of atmospheric water vapor with microwave radiometry
Authors: Elgered G. , Rönnäng B. O. , Askne J. I. H.
Published: February 2008

A dual‐channel, ground‐based microwave radiometer, working at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer, and radiosondes have been used for comparative measurements of the integrated amount of precipitable water vapor in the atmosphere over a period with zenith water vapor contents varying between 6 and 26 mm. The microwave radiometer was found to give comparable or better formal accuracy than the radiosondes, the absolute accuracy of which is believed to be about 1 mm. The rms difference of the integrated amount of water vapor in the zenith direction measured with the microwave radiometer and with radiosondes was 1.2 mm for all data, and 0.8 mm for a selected group of good weather data. These are lower formal errors than previously reported. It is shown that the simplified relation between the radiometer antenna termperatures and the integrated amount of water vapor in this case contributes with a formal error of about 0.3 mm. It is suggested that mean ground weather data can be used to adapt this relation to other sites and seasons.
Results of 11.7‐GHz CTS rain attenuation measurements at Waltham, Massachusetts
Authors: Nackoney O. G. , Davidson D.
Published: February 2008

Satellite‐to‐earth rain attenuation at 11.7 GHz was measured at Waltham, Massachusetts (42.4°N, 71.3°W; elevation angle 24°), utilizing the Communications Technology Satellite (CTS) beacon signal. This paper summarizes 29 months of measurements by presenting attenuation and rain rate distributions, fade duration distributions, fading rate distributions, and statistical relationships between attenuation and rain rate. For a two‐year statistical base, rain attenuation exceeded 9.7 dB for 0.01% of the time and 2.0 dB for 0.1% of the time.
Initial results from the VPI&SU SIRIO diversity experiment
Authors: Towner G. C. , Marshall R. E. , Stutzman W. L. , Bostian C. W. , Pratt T. , Manus E. A. , Wiley P. H.
Published: February 2008

The first year of observations of precipitation effects on the 11.6‐GHz beacon signal from the SIRIO satellite are reported. This experiment is unique in that it uses dual‐polarized receivers in a diversity configuration (two sites separated by 7.3 km) and a low path elevation angle (10.7°). Rain rate, attenuation, and isolation statistics are presented for both sites. In addition, attenuation diversity gain is plotted and compared to current models. Isolation diversity gain is also discussed. It was found that little improvement in dual‐channel isolation is obtained with site diversity at this low elevation angle. The influence of ice clouds was noted in the months of October and November.
An attenuation function for multiple knife‐edge diffraction
Authors: Vogler Lewis E.
Published: February 2008

A recently derived attenuation function for multiple knife‐edge diffraction is described, and some example calculations are presented. The function, in the form of a multiple integral, is transformed into a series representation which is amenable to computer implementation. A program has been written to compute the attenuation over propagation paths containing up to a maximum of 10 knife‐edges. Attenuation curves for three example paths are presented showing the effects of multiple knife‐edges on the received signal. A means of estimating the statistical distribution of attenuation for a given path from statistical knowledge of the atmospheric surface refractivity is suggested.
A method of estimating the electron density profile of the D layer from a knowledge of the VLF reflection coefficients
Authors: Mambo Masayoshi , Nagano Isamu , Nakamura Kiyomi , Fukami Tetsuo
Published: February 2008

A technique for calculating an estimate of the lower ionospheric electron density profile from a knowledge of the VLF radio wave reflection coefficients is studied. The conditions of success of this technique are discussed in detail. Numerical simulation of this method has been carried out on the basis of a full wave calculation. It was confirmed that the deduced profile converged well to the specified profile after only a few iterations. Some numerical results are presented.
The scintillations for weak atmospheric turbulence using a partially coherent source
Authors: Baykal Y. , Plonus M. A. , Wang S. J.
Published: February 2008

The scintillation index for weak atmospheric turbulence is formulated for a partially coherent source (in space and time). We obtain results for two cases when the detector has a slow and a fast response time. Our solutions correctly reduce to the known scintillation index in the limit of coherent and incoherent sources. Transmitter aperture averaging and frequency averaging effects are shown as special cases of our results. The limitations of the source model used and the solution obtained for a fast detector are discussed.
Loop antennas for directive transmission into a material half space
Authors: Smith Glenn S. , An Lam N.
Published: February 2008

The horizontal circular loop and the coaxial array of loops above a material half space are studied as antennas for directive transmission into the half space. In a practical situation the loops might be located in air with the directive transmission into the earth. In determining the optimum geometry for the single loop and the array, the far‐zone field patterns and directivities of these antennas when placed over lossless dielectrics are considered first. The directive properties for the lossless dielectric are found to be indicative of those for the same antenna over a medium with low loss when proper account is taken of the exponential attenuation experienced in the lossy medium. Parametric studies are used to obtain the maximum directivities for these antennas. For the single loop of resonant size, the optimum height over the interface is determined, and for the two‐element array consisting of a driven loop of resonant size with a single parasite, the optimum size and spacing of the parasitic reflector are found. Measured electric field patterns and gains of model antennas above an interface between air and fresh water are in good agreement with the theoretical results.
Ionospheric irregularities and their potential impact on synthetic aperture radars
Authors: Szuszczewicz E. P. , Rodriguez P. , Singh M. , Mango S.
Published: February 2008

Accumulating data are making it increasingly evident that major plasma irregularities populate substantial portions of the ionosphere. In contrast with these findings, satellite‐borne synthetic aperture radar (SAR) systems tacitly assume that the ionosphere is uniformly layered and unchanging under the orbiting SAR. Analysis of plasma irregularity structures measured directly on the S3‐4 satellite shows that this assumption is readily violated near the nighttime equator during the occurrence of spread F and at high‐latitudes on a nearly 24‐hour basis. The irregularities can be very intense, covering scale sizes from meters to hundreds of kilometers. Associated along‐track phase path calculations point to a potentially serious problem in SAR imaging integrity in restricted ionospheric space‐time domains.
Recent studies of the structure and morphology of auroral zone F region irregularities
Authors: Rino C. L. , Livingston R. C. , Tsunoda R. T. , Robinson R. M. , Vickrey J. F. , Senior C. , Cousins M. D. , Owen J. , Klobuchar J. A.
Published: February 2008

Recent analyses of auroral‐zone spaced‐receiver measurements have shown that the regions where sheetlike irregularities occur are confined to the equatorward portion of the nighttime scintillation zone where the westward and eastward electrojets flow. Poleward of this region, the irregularities are rodlike. For satellites in highly eccentric orbits, the spaced‐receiver technique can be used to measure ionospheric drifts. Simultaneous incoherent‐scatter radar measurements have revealed two types of F region ionization enhancements that are believed to be the source regions of persistent scintillation features on polar satellite transmissions. One type is found at the equatorward edge of the diffuse aurora and can persist for more than 10 hours. More dynamic structures often occur in pairs, which suggests an association with ‘inverted‐V’ precipitation events. Radar data have also revealed large‐scale east‐west structure in the poleward enhancements.
Scattering of electromagnetic waves from a half space of densely distributed dielectric scatterers
Authors: Tsang L. , Kong J. A.
Published: February 2008

The scattering of a plane wave obliquely incident on a half space of densely distributed spherical dielectric scatterers is studied. The quasi‐crystalline approximation is applied to truncate the hierarchy of multiple scattering equations, and the Perçus‐Yevick and the Verlet‐Weis results are used to represent the pair distribution function. The coherent reflected wave is studied with these approximations. The incoherent scattered wave is calculated with the distorted Born approximation. In the low‐frequency limit, closed‐form expressions are obtained for the effective propagation constants, the coherent reflected wave, and the bistatic scattering coefficients. Results at higher frequencies are calculated numerically. The advantage of the present approach is that in the low‐frequency limit, it reproduces the effects of specular reflection, Fresnel reflection coefficient, Brewster angle, and Clausius‐Mosotti relation. In addition to the classical results, the bistatic scattering coefficients are also calculated. The theory is also applied to match backscattering data from dry snow at microwave frequencies.
The D region winter anomaly at high and middle latitudes induced by planetary waves
Authors: Kawahira Kohji
Published: February 2008

An observational study of the D region winter anomaly at high and middle latitudes has been made during the period of a sudden stratospheric warming of the 1967/1968 winter. From isopleth analysis of the absorption index, ƒmin, systematic large‐scale distributions of the absorption are found at even higher latitudes than 60° geomagnetic latitude as well as at middle latitudes. On the basis of evidence that geostrophic winds induced by well‐developed planetary waves extend to the upper D region, it is found that when southward winds are dominant, the absorptions become weaker at high latitude but stronger at middle latitude, indicating that abundant NO could be carried away from high‐latitude source regions to middle‐latitude sink regions. Thus large amplitude planetary waves in the D region could induce the winter anomaly through NO transport not only at middle but also at high latitudes.
Antenna‐induced range smearing in ST and MST radars
Authors: Watkins B. J. , Johnston P. E.
Published: February 2008

For ST and MST radars, finite antenna beam width causes range‐smearing that depends on observation range and antenna pointing angle. This effect is examined by first developing a range‐smearing function and then applying this function to model atmospheric turbulent structures for two typical antenna systems. The effect is most severe at mesospheric heights (80–100 km) for off‐vertical antenna directions and for broader‐beam antennas. This has implications for radar design and the scientific interpretation of the data when high resolution is a goal. The results emphasize the need for the narrowest antenna beam that the design and budget will permit. For data interpretations, a highly discontinuous turbulent structure (in the vertical direction) may be recorded, and interpreted, as a continuous height distribution. This will affect the interpretation of measured parameters such as the occurrence of turbulence, Doppler velocity, spectra widths, and calculations of turbulent diffusion rates. For situations when there are very strong returned signals from some heights (i.e., signals ≥26 dB above the minimum detectable level), the antenna side lobes extend the smearing and it may be difficult or impossible to discriminate against these false signals.
On the nature of atmospheric waves and turbulence
Authors: Dewan Edmond M.
Published: February 2008

The basic physical properties of waves and turbulence are reviewed and, in particular, the differences between buoyancy range turbulence (BRT), inertial range turbulence (IRT), and waves are explained. As a result, the “buoyancy length” concept is extended to give the transitional scale between BRT and waves. In addition, the physical nature of BRT is described. Certain practical methods to distinguish between waves and turbulence are considered, and the waves versus two‐dimensional turbulence interpretation controversy is viewed in the light of the above considerations.
Some findings on correlation between the stratospheric echo power and the wind shear observed by the Arecibo UHF radar
Authors: Tsuda T. , Hirose K. , Kato S. , Sulzer M. P.
Published: February 2008

This paper is concerned with the correlation between the echo power and the wind shear observed on June 10 and 11, 1983, by the Arecibo UHF radar at the altitude of 13–22 km. Correlation values of 0.76 and 0.82 were obtained on each day between the range‐corrected echo power and the square of the wind shear for the horizontal wind fields due to both the mean wind and waves with a relatively large vertical scale. The maximum echo intensity occurred at approximately 500 m higher altitude than that of the wind shear. A ratio between the range‐corrected echo power and the square of the wind shear averaged in the whole altitude region was similar for observations done on 2 successive days.
New techniques in ground‐based ionospheric sounding and studies
Authors: Reinisch Bodo W.
Published: February 2008

Rapid progress in the integrated circuit market has led to new advanced techniques in the remote probing of the ionosphere with HF radio waves. The classical ionosonde which measured virtual height as a function of frequency expanded into a geophysical research tool by measuring all the observables contained in the electromagnetic signals reflected from the ionosphere: amplitude, phase, Doppler, incidence angle, and polarization. A receiving antenna array and high speed digital processing provide the desired spatial and temporal resolution. The current emphasis is on both the on‐line and off‐line postprocessing of the multiparameter ionogram data to extract the geophysically important ionospheric characteristics: the vertical electron density profiles, horizontal gradients (tilts and waves), plasma drift, the mid‐latitude F region trough, and auroral and equatorial spread F. Digital ionosonds deployed in the polar cap and the auroral zone have helped to obtain a better understanding of some of the high‐latitude features, and measurements of the equatorial spread F have shown the development and motion of the F region bubbles. HF coherent radar techniques for studying ionospheric irregularity structures measure the velocity of irregularities with scale sizes of one half the radio wavelength. They have mainly been used in the northern auroral zone.
Nighttime VHF scintillations at 23°N magnetic latitude and their association with equatorial F region irregularities
Authors: Dabas R. S. , Reddy B. M.
Published: February 2008

A study of postsunset VHF scintillations observed during recent solar maximum period (1980) at 23°N magnetic latitude, which is substantially beyond the daytime crest of the equatorial anomaly along 84.4°E geographic meridian, is carried out in order to examine their association with the irregularities of the equatorial F region. This is an extension of the study reported earlier from a chain of stations along the same meridian but covering a magnetic latitude belt of 3°‐21°N (Somayajulu et al, 1984). The characteristics and the occurrence pattern of scintillations observed at 23°N agree with the results reported upto 21°N. Also, the occurrence of scintillations at 23°N is found to be conditional to their prior occurrence at lower latitudes. This further confirms that the occurrence of nighttime scintillations up to 23°N magnetic latitude in the Indian zone are essentially controlled by the generation of equatorial F region irregularities. The study also provides evidence that the equatorial F region irregularities extend to altitudes in excess of 1300 km at least during equinoctial periods of high sunspot years and consequently cause scintillations in a wider latitudinal belt. The occurrence frequency of nighttime scientillations at 84°E, when compared with that of 93°E and 122°E longitudes for the same period of time, indicates strong longitudinal dependence, particularly in the Indian sector.
Multiple scattering by conducting particles with random rough surfaces at infrared and optical frequencies
Authors: Bahar Ezekiel , Fitzwater Mary Ann
Published: February 2008

The incoherent specific intensities for the waves scattered by a random distribution of particles with rough surfaces are derived. Since large roughness scales are considered, the diffuse scattering contributions to the like and cross polarized scattering cross sections are given by the full wave solutions. The scattering matrix in the expression for the equation of transfer is given by a weighted sum of the scattering matrix for the smooth particle and the diffuse contribution due to the rough surface of the particle. Illustrative examples are presented for the propagation of a circularly polarized wave normally incident upon a parallel layer of particles. Particles with different surface height spectral density functions, roughness scales, complex permittivities and sizes are considered. Both first order (single scatter) and multiple scatter solutions are provided and the results for particles with smooth and rough surfaces are compared.
MU radar observations of the aspect sensitivity of backscattered VHF echo power in the troposphere and lower stratosphere
Authors: Tsuda Toshitaka , Sato Toru , Hirose Kazuo , Fukao Shoichiro , Kato Susumu
Published: February 2008

By using the fast beam steerability of the MU radar, we have studied the characteristics of the aspect sensitivity of the VHF backscattered echo power in the troposphere and lower stratosphere. The tropopause clearly divides the altitude range into regions with and without large aspect sensitivity. In the stratosphere, large aspect sensitivity occurs in the entire altitude region and is clearly correlated with the echo power received in the vertical direction. From investigation of the Doppler spectra, it is found that the aspect sensitivity of the echo power is correlated with the zenith angle dependence of the spectral width. Fairly large aspect sensitive echoes are also received from intense scattering layers in the troposphere, although their time‐height variation is large. Because of the large aspect sensitivity of the stratospheric echo power, the effective zenith angle of the antenna beam is smaller than the bore sight angle, so that the horizontal wind velocity can not be estimated correctly from the radial wind velocity measurements done at small zenith angles.
Techniques for calculating the electric field near a perturbed reflection layer in the ionosphere
Authors: Hansen J. D.
Published: February 2008

Two methods (Langer's method and a Green's function technique) for calculating the electric field near the perturbed reflection layer of an electromagnetic wave propagating along the magnetic field in a polar ionosphere (k ∥ B0 ∥ ∇n0) are compared. Perturbations on the zero order density profile are modelled by localized Gaussians whose depth and extent along the geomagnetic field can be varied. The Green's function technique is found to be useful when short scale length perturbations are present, but is limited by its domain of convergence and numerical efficiency. Comparison to the convergent Green's function technique shows inaccuracies in the Langer method for perturbations whose normalized spatial extent is small in relation to its normalized depth. Results are presented of phase shifts due to short scale density cavities, as may arise from ponderomotive forces due to electrostatic waves. These results delineate the sensitivity required by experiments that use phase shift measurements as a diagnostic tool. The Langer method is used to calculate the electric field for long‐scale cavities typical of ohmic heating modifications.
Modeling the interactions between the ocean and the environment for microwave radar sensing
Authors: Weissman David E.
Published: February 2008

An extensive set of X‐band microwave backscatter measurements has been analyzed to determine its dependence on winds near the surface, atmospheric stability, and long‐wave slopes. These radar measurements, made from a tower in the Gulf of Mexico, were performed in conjunction with an extensive set of simultaneous environmental measurements. The CW microwave system operated at an incidence angle of 45°, with antennas directed into winds and waves. Model functions for the radar cross section (RCS) and the modulation transfer function (MTF) are developed that depend on the geophysical variables, including wind stress. Statistical analysis shows that these models functions yield a significantly smaller error when fit to the RCS data than a simple wind speed function displays. A Taylor series expansion of the returned power from a small area provides a unified function that demonstrates the complementary roles of wave slope and atmospheric fluctuations on both the RCS, the MTF and the coherence. The issue of linearity of the MTF is addressed with this data set, yielding evidence that this is a valid assumption. These results have direct application to the remote sensing of the mean and fluctuating winds, and the wave spectrum with coherent and incoherent active radars.
Harmonic and transient scattering from weakly nonlinear objects
Authors: Censor Dan
Published: February 2008

A mathematical model for scattering of electromagnetic waves from weakly nonlinear objects is developed. The constitutive relations are based on Volterra series, but additional, physically plausible, heuristic assumptions have to be introduced in order to solve the scattering problem. The general theory is discussed in connection with scattering from circular cylinders. These canonical problems demonstrate the new phenomena involved. It is shown that the first order effects of the nonlinear scattering problem involve modification of the linear scattering coefficients and production of new multipole terms at the fundamental frequency. In addition, part of the energy is transformed into harmonics. The corresponding problem of transient scattering is considered. The new effects of pole migration and pole creation are discussed. The present study contributes to understanding the theoretical aspects of nonlinear scattering, and may also provide a method for remote sensing of nonlinear targets.
The frequency management system of the Jindalee over‐the‐horizon backscatter HF radar
Authors: Earl G. F. , Ward B. D.
Published: February 2008

Real‐time frequency management is a vital element in the effective operation of an over‐the‐horizon backscatter (OTH‐B) HF radar. In the case of the Australian project Jindalee OTH‐B HF radar, a comprehensive frequency management system has been developed for this purpose. The frequency management system includes subsystems for ionospheric backscatter, vertical and oblique incidence sounding, HF spectral surveillance, and incorporates a low powered backscatter radar. The system is capable of providing real‐time frequency management advice to the main radar, and of acquiring synoptic data for off‐line analysis. In this paper we describe the hardware architecture and signal processing schemes, together with sample data, for each of the subsystems. We then describe the manipulation of the environmental data in order to provide frequency management advice to the main radar.
Use of maximum entropy spectral analysis to improve ship detection by over‐the‐horizon radar
Authors: Bourdillon A. , Gauthier F. , Parent J.
Published: February 2008

The over‐the‐horizon radar is able to detect ships over very large oceanic areas. Detection performance is sometimes reduced because of smearing of the sea echo due to variations of the Doppler shift imposed by the ionosphere. In this paper we propose a decontamination procedure, based upon maximum entropy spectral analysis of sea scatter Bragg lines, to correct the phase of the signal before performing classical fast Fourier transform processing. The improvement in quality of the signal after phase correction results in a sharpening of spectral peaks. In the example presented the signal was chosen to contain spectral lines of a transponder installed on board a ship, to be used as reference lines. The decrease in the spectral width is significant for the sea scatter Bragg lines and for the transponder lines. As the Bragg lines sharpen, the ship echo is more easily detected in the spectrum. The proposed decontamination procedure involves more computation load than the simpler frequency shift method used for sea state monitoring, but, on the other hand, it can handle Doppler shift variations during coherent integration time.
A meteor echo detection and collection system for use on VHF radars
Authors: Wang S. T. , Tetenbaum D. , Balsley B. B. , Obert R. L. , Avery S. K. , Avery J. P.
Published: February 2008

We have developed a meteor echo detection and collection system (MEDAC) that can be attached to many ST or MST radars operating in the VHF range. The system can be operated in parallel with the normal data collection of turbulent echoes. The use of the meteor echoes extends MST mesosphere observations into the night and extends ST capabilities into the mesosphere and lower thermosphere. This paper describes the system and presents preliminary wind measurements using the Platteville, Colorado, ST radar.
Analysis of two‐dimensional electromagnetic scattering from a periodic grating of cylinders using a hybrid current model
Authors: Boag Amir , Leviatan Yehuda , Boag Alona
Published: February 2008

A method of moments solution is presented for the problem of two‐dimensional electromagnetic scattering from a periodic grating composed of an infinite set of penetrable cylinders illuminated by a TM (transverse magnetic relative to the cylinder axis) plane wave. The reduction of the general problem to a consideration of the fields over a suitably selected period, referred to as the unit cell, is facilitated by the Floquet theorem. The solution then uses fictitious spatially periodic and properly modulated electric current strips to simulate the scattered field in the exterior region within the unit cell and fictitious filamentary currents to simulate the field penetrated into the cylinder enclosed within this cell. The fields radiated by the current strips are represented in terms of Floquet modes. Finally, the simulated fields are forced to obey the continuity conditions for the tangential components of the electric and magnetic fields at a selected set of points on the cylinder boundary within the unit cell. The procedure is simple to implement, rapidly converging, and is applicable to cylinders of arbitrary smooth cross section. Perfectly conducting cylinders are treated as reduced cases of the general procedure for penetrable cylinders. Results are given and compared with available data. The efficiency of the suggested method is demonstrated.
Radio acoustic sounding of the atmosphere using a FM CW radar
Authors: Peters G. , Hasselmann D. , Pang S.
Published: February 2008

The use of a FM CW radar for the measurement of atmospheric sound velocity profiles is investigated. In contrast to particulate scatterers or clear air turbulence the sound waves represent a coherent scattering structure, and the consequences are examined. We derive the theoretical shape of the signal spectrum and present examples of velocity profiles of vertically propagating sound waves. As the sound and clear air echoes are separable by frequency, simultaneous measurements of atmospheric wind and temperature profiles should be feasible. Because of the excellent spatial and temporal resolution of the FM CW technique, these measurements perhaps offer a possibility of future remote heat flux measurements.