bluuebunny/crossref_metadata_2025 huggingface dataset:
The distribution of radio refractive index structure parameter in boundary layers undergoing spatial or temporal transition
Authors: Gossard E. E.
Published: February 2008

The height distribution of Cϕ (the structure parameter of potential refractive index) in a steady, horizontally uniform boundary layer are calculated assuming power laws for meteorological quantities . The results are then compared with those of more rigorous models examined by Wyngaard et al. It is shown that the results are in very satisfactory agreement when the power is properly chosen. Finally, power‐law profiles are used to calculate horizontal and temporal variations in vertical profiles of Cϕ and to calculate radio‐duct thickness for several boundary transitional conditions.
Observation of winds in the clear air using an FM‐CW Doppler radar
Authors: Gossard E. E. , Chadwick R. B. , Moran K. P. , Strauch R. G. , Morrison G. E. , Campbell W. C.
Published: February 2008

The Wave Propagation Laboratory of NOAA has developed a frequency‐modulated, continuous‐wave, clear‐air radar with a Doppler capability for measuring both the mean wind and turbulent intensity. In this paper, observations of turbulence are shown and related to the background wind and temperature structure. The radar's capability for measuring the structure parameters and dissipation rates of both velocity and refractive index in the inertial subrange is discussed.
Inference of static and dynamic vertical structure of the lower troposphere from ground‐based infrared radiometry
Authors: Basili Patrizia , Solimini Domenico
Published: February 2008

Measurement of atmospheric brightness at different wavelengths in the infrared can give information about several parameters which characterize the vertical structure of the lower troposphere. In this paper some of the results obtained from a six‐month experiment carried out in 1975 are reported. The temperature profile has been retrieved from measurements of the average brightness at different elevation angles. In addition, measurements of both the variance and the low‐frequency spectrum of the radiometer output signal have been performed to obtain information on the intensity, scale length and spectra of temperature fluctuations. Particularly under stable tropospheric conditions, sharp periodicity of the radiometer signal has occasionally been observed, suggesting the occurrence of a periodic variation of the atmospheric parameters such as that associated with atmospheric waves. The data which have been collected appear to confirm the capability of ground‐based radiometry for the accurate and timely monitoring of the static and dynamic state of the lower troposphere.
Remote sensing of directional gravity wave spectra and surface currents using a microwave dual‐frequency radar
Authors: Schuler Dale L.
Published: February 2008

The modulation of small‐scale centimeter (and later decimeter) water waves induced by larger‐scale 2 to 18 m gravity waves has been studied by using a coherent, dual‐frequency radar technique. Experiments using a prototype CW X‐band radar operating at 9.3 GHz have been performed. Experience with the X‐band system led to the subsequent development of a pulsed dual‐frequency L‐band radar operating at 1.2 GHz. The gravity wave modulation manifests itself as a narrow, Doppler‐shifted, resonance peak in the product power spectrum of the backscattered returns. The dispersion relation (for both deep and shallow water) of the modulation pattern matches that of gravity waves. Modulation amplitude spectra have been experimentally obtained which, after sufficient averaging, closely resemble directional gravity wave spectra simultaneously obtained from capacitance wave probe and Sea Photo Analysis measurements. Temporal stationarity of the large‐scale gravity wave structure may only be assumed for finite data acquisition times of, perhaps, the order of one hour, or less. Experiments with the X‐band system, however, have shown that high resolution, sufficiently averaged, densely sampled spectra require measurement periods much longer than one hour. To reduce the overall data acquisition time, multiplexing of the individual spectral samples has been employed. The amount of spectral averaging required was substantially reduced by developing and using a dual‐frequency L‐band radar (Bragg resonant with 12 cm short‐gravity waves) which proved to be much less sensitive to wind‐induced fluctuations in the small‐scale wave return. A method is also presented for enhancing the amplitude of the resonance peak through selective Doppler filtering. This technique was subsequently used to facilitate detection of small tidal currents flowing in the Chesapeake Bay.
Wave propagation experiments at medium frequencies between two ionospheric satellites, 1, General results
Authors: James H. G.
Published: February 2008

Occasionally the ISIS I and ISIS II spacecraft rendezvous in parts of the world where simultaneous telemetry coverage is feasible. Electromagnetic propagation between ISIS I and ISIS II has been studied using their topside sounders at medium and high frequencies. Above the gyrofrequency, the O and upper‐frequency X modes have been identified often during separations of up to a few thousand kilometers. In a few instances, the lower‐frequency X, or Z, mode has been observed with separations less than 1000 km. This mode is continuous below the gyrofrequency where the whistler mode is also seen. Intersatellite ionograms confirm that the O‐, X‐, and Z‐mode pulses propagate largely as expected on the basis of magnetoionic theory applied to a smoothly varying medium, with little distortion except near cutoff. On the other hand, whistler‐mode pulses are significantly elongated by the medium. Their interpretation is the subject of a companion paper, part 2.
Refraction theories of radio wave propagation through the troposphere—A review
Authors: Arora R. K. , Wait J. R.
Published: February 2008

The paper presents an overall review of the literature on the subject of refraction theories of electromagnetic wave propagation through the earth's troposphere, with particular emphasis on the more recent work. Following a historical introduction to the subject, a qualitative discussion is given of the phenomenon of tropospheric refraction. The physical mechanisms of propagation associated with different types of refractive index profiles are discussed. The paper then proceeds to review the main features of the rather extensive theoretical techniques that have been employed in the past to study the phenomenon.
Evaporation duct influences on beyond‐the‐horizon high altitude signals
Authors: Hitney H. V. , Pappert R. A. , Hattan C. P. , Goodhart C. L.
Published: February 2008

A case study of beyond‐the‐horizon propagation previously examined in terms of an earth‐detached duct produced by an elevated layer at 2000 ft is reexamined in terms of ducting produced by evaporation ducts (duct heights ≤100 ft). The case study applies to a frequency of 3087.7 MHz, a transmitter altitude of 68 ft and a receiver altitude of 3000 ft. It is concluded that relatively shallow surface ducts of the type examined can significantly affect signal level beyond the horizon at elevations which are an order of magnitude or more greater than the duct height and could conceivably account in large measure for the disparities previously reported.
Guidance of whistler‐mode waves through the plasmasphere
Authors: Gupta G. P. , Singh R. N.
Published: February 2008

Using appropriate plasmaspheric models, whistler‐mode wave propagation has been studied. It is shown that the ray curvature of propagating whistler‐mode waves is quite different from the geomagnetic field curvature. The presence of field‐aligned irregularities superposed on various models of ambient plasmaspheric electron density introduces the compatibility of ray and field curvatures. The presence of field‐aligned irregularities and modified refractive index has been used to study the guidance of whistler‐mode waves along the field line. The critical angle of elevation, track width, and electron density gradient needed for guidance of whistler waves have been computed. Using modified refractive index, the propagation time of whistlers has been computed. It is argued that the analyses of whistler sonograms accounting for some of these features would result in a useful diagnostic.
Vertical profiles of refractivity turbulence structure constant: Comparison of observations by the Sunset Radar with a new theoretical model
Authors: VanZandt T. E. , Green J. L. , Gage K. S. , Clark W. L.
Published: February 2008

Vertical profiles of refractivity turbulence structure constant C2n (which is proportional to the radar volume reflectivity) from about 5 to 15 km are measured by the Sunset Radar every 50 seconds. The method of determining such profiles from the radar Doppler spectra is described. The C2n profiles for about an hour are averaged to form (C2n(radar)). The profiles of (C2n(radar)) are quite variable: on the average they decrease by about two orders of magnitude from about 5 to 15 km, and they often change by one order of magnitude from day to day at a given height. A theoretical model is developed that enables the calculation of C2n from routine rawinsonde profiles of wind, temperature, and humidity. This model is based on the assumption that the fluctuations of refractivity that scatter the radio waves are in equilibrium with homogeneous, isotropic, steady‐state turbulence in the inertial subrange. An essential and new feature of this model is an estimate of the mean fraction of the radar‐observed volume that is turbulent. The resulting profiles of (C2n(model)) agree well with the measured profiles of (C2n(radar)) in general shape, in changes from day to day, and in many details from kilometer to kilometer. This agreement implies that: (1) The vertical profile of C2n can be measured by Doppler radar. (2) The vertical profile of C2n can also be estimated by calculation from routine rawinsonde profiles, using our theoretical model.
Doppler frequency shift of a wave propagating in a time‐varying, inhomogeneous, anisotropic medium
Authors: Noda Akira
Published: February 2008

A general formula for the Doppler shift of a wave propagating in a time‐varying, inhomogeneous, anisotropic medium is derived by using the eikonal method for both relativistic and nonrelativistic cases.
ULF/ELF electromagnetic fields generated in a sea of finite depth by a submerged vertically‐directed harmonic magnetic dipole
Authors: Bubenik D. M. , Fraser‐Smith A. C.
Published: February 2008

In this paper we compute the undersea quasi‐static electromagnetic fields of a submerged vertically‐directed harmonic magnetic dipole (VMD), taking into account the presence of a sea floor, for the purpose of evaluating the relative importance of the modes of propagation of the fields at frequencies in the ULF/lower‐ELF bands (frequencies less than 300 Hz). Both the source and the receiver are located at points equidistant between the surface and floor of a sea one skin depth (δ) deep, and the horizontal separation (r) between them is varied over the range 0.1 δ and 100 δ. When the horizontal separation between the receiver and the VMD source is less than about 6 δ, we find that each field component behaves as though the source were in a sea of wholly infinite extent. At larger separations, the ratio of actual field amplitude to the “infinite extent,” or direct, field amplitude increases rapidly and reaches a value exceeding 1036 at a separation of 100 δ, indicating that the fields at such distances have not propagated directly through the sea water. Analysis of the propagation modes reveals that the chief contribution at large separations (r/δ > 20) is the surface mode, and the field amplitudes in this asymptotic regime are independent of sea floor conductivity. At closer ranges (r/δ < 10), the surface and sea‐floor modes are of equal importance. Between these limiting cases (10 < r/δ < 20) is a transitional regime where the first‐order sea floor contribution is sometimes the dominant mode. The changeover between the transitional and the asymptotic regimes moves outward from the source as the sea floor conductivity decreases, and the field components in the asymptotic regime are characteristic of a quadrupole or higher‐order source.
A comparison of the beat method of determining low electron densities from topside ionograms with the plasma‐frequency resonance method
Authors: Whitteker J. H.
Published: February 2008

A comparison is made between electron densities derived from the frequency of the plasma resonance and densities derived from the frequency of the beat between the electron gyrofrequency (ƒH) and the upper hybrid frequency (ƒT), over the density range 150 to 300 cm−3. Determinations were made from 77 ISIS 2 high‐latitude ionograms on which both methods could be used. Densities derived from the ƒT−ƒH beat frequency were found to be lower than those from the plasma‐frequency resonance by an average of 10%, with a dispersion of 10%. Although this result does not apply directly to electron densities of < 100 cm−3 for which the beat method is normally used, it indicates that the method is more accurate than might be supposed from theory applied to higher densities. The result also puts experimental constraints on the frequencies at which the ƒH and ƒT resonances occur.
Mutual admittance between circular apertures on a large conducting sphere
Authors: Hessel A. , Liu Y.‐L. , Shmoys J.
Published: February 2008

Explicit expressions for the self admittance and the mutual admittance Yij between two arbitrarily polarized circular apertures are presented. The aperture field distribution is that of TE11 circular waveguide mode. The analysis makes use of a spherical mode expansion and of the addition theorem for tesseral harmonics. Numerical results show Yij as a function of angular separation θ. The E‐plane coupling exhibits asymptotic (large θ) behavior consistent with the lowest “hard” surface ray order results. The slope of the H‐plane coupling for large values of θ does not correspond to the lowest‐order “soft” surface ray attenuation, but rather to that of the lowest‐order “hard” ray. This behavior is explained.
New formulas for tropospheric scatter path loss
Authors: Parl Steen A.
Published: February 2008

The predicted performance of tropospheric scatter transmission is investigated in terms of spectrum slope, refractive index variance, and scale of turbulence. The common volume integral is evaluated to yield a useful expression for path loss in terms of these parameters. Formulas for the coupling loss in the asymptotic case of very wide and very narrow antenna beams are derived. Approximate formulas for intermediate size antenna beams are deduced and compared with more exact computer calculations. Our results suggest that previous path loss prediction methods do not place enough emphasis on turbulent scatter and that a model with spectrum slope 11/3 should be used. The resulting model provides an initial step toward the development of a simpler and more realistic alternative to the NBS model, and also refines a related approach by Gjessing.
Approximate solutions for a slab geometry problem based on Bremmer series concepts
Authors: Sluijter Frans W. , Rompa Hubertus W.
Published: February 2008

In this article a generalization is given of a method due to Mirovitskii and Budagyan, and it is shown that their method amounts to calculating two Bremmer series with a different normalization.
National time and frequency standards of Finland
Authors: Kalliomäki K. , Wallin P. , Vuorinen J.
Published: February 2008

National time and frequency standards in Finland are based on Loran‐C phase‐locked clocks. The Loran‐C clock comprises a high‐stability crystal oscillator (HP 105 or equivalent), a Loran‐C receiver, a digital clock, and a specially constructed digital phase controller. There are 10 clocks in continuous use in Finland, one in Sweden, and one in Norway. In Southern Finland the clocks utilize a transmission of 7970 W (Sylt) and in Northern Finland that of 7970 X (Bø). The time constant of the digital phase controller is about 1 hour. Thus short‐term fluctuations of the receiver phase are damped. Comparisons with rubidium and cesium clocks show that the standard deviation of the phase of the crystal oscillator is from 0.1 to 0.3 μs. The short‐term (1 hour) frequency stability is better than 2×10−11. In addition, Loran‐C is regularly compared with other time signal transmissions. Several clocks utilizing the time code of DCF 77 (West Germany) have been constructed. The accuracy of these clocks varies from 0.1 to 1 ms depending on their construction. The reception of Soviet TV (Tallin) synchronization pulse was started in January 1978. Initial results indicate the standard deviation of UTC(Helsinki)–UTC(Tallin) to be less than 1 μs. This project is being carried out in cooperation with Soviet scientists.
Surface currents and charges on a cross formed by an electrically thick cylinder and a flat plate in a normally incident plane‐wave field
Authors: King R. W. P. , Blejer D. J. , Burton R. W.
Published: February 2008

With the aid of graphs a detailed description is given of the experimentally determined distributions of the currents and charges induced by a normally incident plane electromagnetic wave on all of the surfaces of a metal cross formed by a vertical tubular cylinder and a horizontal flat plate. The cylinder is a wavelength in circumference and 1.75λ high over a large ground plane. The horizontal plate consists of two equal arms that extend 0.75λ from the surface of the cylinder; it is 0.2125λ wide and 0.0255λ thick. It is centered at a height of 1.25λ from the ground plane. The distributions are compared with the corresponding ones on crossed tubular cylinders and on isolated cylinders.
On natural slot antennas
Authors: Wait James R.
Published: February 2008

The concept of a natural slot antenna for very low frequency is discussed. It is suggested that a viable system needs to have an insulated cavity backing the slot aperture.
Pulse spreading and wandering in random media
Authors: Liu C. H. , Yeh K. C.
Published: February 2008

Two possible mechanisms are responsible for the temporal broadening of pulses propagating in a turbulent medium. The ensemble averaged pulse may be broadened owing to the fact that each realization of the ensemble is spread in time as a consequence of scattering. It may also come about because of the wandering of the pulse. In this paper these two mechanisms are studied. First, the two‐frequency and the four‐frequency mutual coherence functions for the field are obtained for both the Rytov approximation and the asymptotic saturated condition. They are then used to compute the spreading and wandering of the pulse. It is found that under the condition of weak scattering, pulse wandering dominates the contribution to the broadening of the pulse. In the saturated region, however, pulse spreading becomes the dominant factor. Explicit expressions for the spreading and wandering are derived for some examples.
Radiation into the air above a horizontal dipole in the earth
Authors: King R. W. P. , Shen L. C.
Published: February 2008

The electromagnetic field in the air directly above a horizontal dipole embedded in the earth is evaluated from the exact integrals. Also discussed are Baños' approximate formulas and improvements on these. Graphs are shown when the earth's surface is sea water, lake water, and dry earth. In all cases the Poynting vector points upward into the air along the vertical axis through the dipole.
Absorption of higher‐frequency RF energy by biological models: Calculations based on geometrical optics
Authors: Rowlandson G. I. , Barber P. W.
Published: February 2008

Theoretical calculations of absorption of radio‐frequency (RF) energy by appropriate models of man have heretofore failed to yield numerical results much beyond the first absorptive peak of whole‐body resonance. Current efforts are directed toward a better definition of whole‐body absorption at supraresonant frequencies. A method based on geometrical optics has been developed recently, and it was used by us to compute absorptive characteristics of a prolate spheroidal model of man to a high‐frequency limit. The technique approximates the surface of the prolate spheroid by small planar subareas. The energy being transmitted into each subarea is determined and is assumed to be completely absorbed due to the small depth of penetration of electromagnetic waves into lossy biological bodies in the supraresonant region. The total energy absorbed is found by summing over all subareas. Validity testing with Mie theory, in conjunction with consideration of the localization principle of geometrical optics, indicates that this technique is applicable to the model of man at frequencies beyond 6 GHz. Computer‐generated results for a 70‐kg prolate spheroidal model of man indicate that (1) the dependence of energy absorption on the incident wave's polarization and angle of incidence is markedly different from dependencies observed at lower frequencies; (2) the rate of energy absorption increases with frequency within the asymptotic limit; and (3) the use of simple planar models is inadequate to determine energy‐absorption characteristics of biological bodies at high RF frequencies.
Diagnosis of pulmonary edema by a surgically noninvasive microwave technique
Authors: Iskander M. F. , Durney C. H. , Shoff D. J. , Bragg D. G.
Published: February 2008

A method for measuring content of water in the lung is described. The method utilizes changes in the amplitude and phase of the microwave signal transmitted through the thorax. The phase of the transmission coefficient is particularly sensitive and provides a means of detecting and monitoring of pulmonary edema. Theoretical predictions of sensitivity were confirmed experimentally in agar models of the thorax. The optimal frequency lies between 740 MHz and 1.5 GHz. A 50‐ohm open‐transmission line was designed and used to couple 915‐MHz energy into tissues with minimal external radiation. Other advantages of this strip‐line concept include its low mass, small size, and flat configuration, which render it utile for clinical purposes. The feasibility of the method was also evaluated in experiments on dogs. In a typical dog experiment, the phase of the microwave‐transmitted signal changed by an average value of 2° for each 10 milliliters of infused blood. These results provide strong encouragement that incipient pulmonary edema can be detected before clinical emergencies arise, thus providing the physician a safe and sensitive tool for diagnosis of pulmonary function.
Destruction of solid tumors by heating with radio‐frequency energy
Authors: Dickson John A.
Published: February 2008

Tumors of various types and size in the limbs of rats and rabbits have been destroyed by elevations of temperature to levels near 50 °C Destruction was achieved by diathermy heating at 13.56 MHz; the condenser‐field technique was used, and direct‐contact electrodes were applied at the site of a tumor. Special features of the heating system provide fo efficient coupling of RF currents to tissues, and for optimizing heating by the currents. Temperatur are continuously monitored via sensors at multiple sites within and without the tumor, which enables a lethal dosage of thermal energy to be delivered to the tumor and to be quantitated in terms of temperature × time. The approach enables a favorable ratio of deep‐to‐surface heating, which facilitates tumor destruction without damage to normal tissues such as skin and muscle. Initial work indicates that the technique has potential for the selective heating of limb tumors in man.
Excitation of LHR electrostatic waves in a warm magnetoplasma by current pulses: Application to the VLF echoes excited by the Isis 2 HF transmitter
Authors: Hamelin M.
Published: February 2008

The potential created by a current pulse
is derived using a warm‐plasma description in the vicinity of the lower hybrid resonance (LHR). The results are somewhat different from the previous ones obtained with the cold‐plasma theory; the amplitude of the excited wave has an asymptotic decay with time as t−1 rather than t−½. The spatial distribution of the potential shows a propagation effect that gives a reasonable explanation of the duration of the VLF echoes excited by the Isis 2 HF transmitter; it might also provide a technique for the measurement of the ionic thermal velocity and the drift velocity component perpendicular to the magnetic field.
Precision of tropospheric/stratospheric winds measured by the Chatanika radar
Authors: Chang Norman J. F.
Published: February 2008

A method for reducing tropospheric/stratospheric radar backscatter measurements to wind components and for relating uncertainties in these components to uncertainties in Doppler backscatter moments estimation is described. The centroid of the Doppler spectrum is used to estimate the wind component along the radar line of sight, while the variance of this estimator is used to judge the precision that can be attached to the measurement. Error estimates are given in geographical coordinates so that errors in resolved velocities can be related directly to radar measurements. To assess the validity of the procedure, wind profiles derived from Chatanika radar measurements are compared with wind profiles obtained from a series of 12 windsondes. These windsondes were launched nearby (at approximately 45‐min intervals) on April 6 and 7, 1978. Radar measurements include selected horizontal wind profiles (from about 4‐ to 14‐km altitude) and winds at 9.05 km extending over 15 hours. Windsonde and radar‐derived velocities generally agree within the precision of the measurements, but in regions of strong wind shear, differences exceeding the calculated error bars are found. These discrepancies are shown to be caused by several independent scattering layers that often exist within the scattering volume. When these layers have sufficiently different velocities, causing multiple peaks to appear in the Doppler spectrum, and when the relative strengths of these spectral peaks change with azimuth, the centroid no longer accurately reflects the mean motion of the scatterers. Significant errors in resolved velocities can thus occur. Because regions of high reflectivity apparently exist in thin layers separated by regions of low reflectivity, radar backscatter will provide an accurate sampling of the wind field, only if the scattering layers can be accurately located. To overcome these limitations in accuracy, radars that are used to measure tropospheric/stratospheric winds should have range resolutions comparable with the thickness of the scattering layers.
Ionospheric wave measurements with satellite‐borne cross‐power spectrum analyzers
Authors: Harker K. J. , Crawford F. W.
Published: February 2008

The wave spectrum of the fluctuations in a plasma can be determined by first measuring the cross‐power density spectrum and then performing a spatial Fourier transform. The feasibility of using this method to measure the fluctuations in the ionosphere by placing a cross‐power spectrum analyzer on an orbiting satellite is assessed by determining the integration time required to make the measurement. The theory considers in particular spherical probes biased into the electron saturation region. The method consists in assuming an equilibrium plasma wave spectrum, from which the power spectrum directed to each spherical probe and the available power at the amplifier input are determined. Using statistical theory, this information is used to determine the integration time needed to measure the cross‐power spectral density at the output terminals of the cross‐power spectrum analyzer to a specified accuracy. For typical experimental conditions the integration times are found to increase from 0.1 to 1000 s as the probe spacing increases from 2 to 10 Debye wavelengths for ion wave frequencies. For electron plasma wave frequencies the corresponding increase is from 10 to 104 s.
Diffraction of an H‐polarized electromagnetic wave by a semicircular strip
Authors: Jain D. L. , Lal Bansi
Published: February 2008

We present here solution of the two‐dimensional problem of diffraction of an H‐polarized electromagnetic plane wave by a perfectly conducting semicircular infinite strip. Approximate expressions are derived for the far‐field amplitude of the magnetic intensity associated with the scattered wave and the scattering cross section when the wavelength is much larger than the radius of the semicircular strip. Numerical calculations of the quantities of physical interest are presented graphically.
The Rayleigh hypothesis in the theory of diffraction by a perturbation in a plane surface
Authors: van den Berg P. M. , Fokkema J. T.
Published: February 2008

In this paper the Rayleigh hypothesis in the theory of reflection by a cylindrical perturbation in a plane surface is investigated analytically. The hypothesis asserts that above the surface the scattered field may be expanded in terms of outward‐going wave functions. As such, it is analogous to the assumption made by Lord Rayleigh in his treatment of diffraction by a reflection grating. We show that the validity of the Rayleigh hypothesis is governed by the distribution of singularities in the analytic continuation of the exterior scattered field. Conditions are derived under which the Rayleigh hypothesis is rigorously valid. A procedure is presented that enables the validity of the Rayleigh hypothesis to be checked for a surface whose profile can be described by an analytic function. Numerical results are presented.
Current induced on a pair of wires above earth by a vertical electric dipole for grazing angles of incidence
Authors: Olsen Robert G. , Aburwein Abdulmagid
Published: February 2008

Exact expressions for the current induced on a pair of horizontal wires above the earth by a vertical electric dipole have been available for some time. It is well known that the wire currents can be expanded as a continuous spectrum current plus a sum of discrete modal currents. Each current consists of a monofilar and a bifilar component. In this paper, conditions for which the currents can be approximated either by the discrete modal currents or a simple approximation to the continuous spectrum current are found. For simplicity it is assumed that the earth is perfectly conducting and that the wires are thin and can be characterized by a surface impedance boundary condition. Using steepest descent integration, the case for grazing incidence (distance from source to observation point R greater than the perpendicular distance from the source to the center of the wire array r) is examined. It is found that either the monofilar or the bifilar component of current can be written as a single term which is proportional to 1/R if R is of the order of r or to 1/R5/2 if R is ‘very large.’ In the intermediate range the discrete modal currents dominate the solution. Transition solutions valid between these ranges are given.
Propagation of ion‐acoustic solitons in a lossy plasma with density gradients
Authors: Yagi Tsuneo , Noguchi Akira
Published: February 2008

It is shown that a certain type of nonuniform media with a relaxation effect can still support exact multisoliton. Through the original Gardner‐Morikawa transformation the propagation of a solitary ion‐acoustic wave in a plasma with both ion‐neutral collisions and density gradients is shown in the Korteweg‐de Vries equation with loss and nommiformity terms. If there exists a certain balancing between them, the corresponding solutions are shown to express solitons damping their amplitude in time, without emitting any oscillatory tails.
Effect of the shape of plasma density power spectra on ionospheric scintillations
Authors: Serafim Philip E.
Published: February 2008

We present a systematic way to investigate the effect of the shape of an arbitrary power spectrum for the ionospheric plasma density on amplitude and phase scintillations through the study of appropriate wave number moments. These moments were found by following the random scattering of the wave inside the ionosphere and its diffraction into free space. We show that the inner and outer scales of the density spectrum are not sufficient for the description of several scintillation effects. We propose the necessary additional scales. For example, the distance at which amplitude scintillations become strong is not related to the inner scale of the plasma turbulence, as it had been assumed previously. In other words, it is not related to the variance 〈k2〉. Instead, we find it to be related to the inner scale of the structure at the boundary between the ionosphere and free space. Equivalently, we prove it to be related to the moments 〈k〉 and 〈k−1〉. The differences may be very significant. As another example, we show that the near‐zone scintillation index depends on the moment 〈k3〉. The theory so developed is applied to the case of a very general von Karman power spectrum. Some conclusions are drawn related to current research problems (Wideband satellite experiment, SHF scintillations).
Satellite measurements of the HF radio environment in the topside ionosphere
Authors: Rush C. M. , Snyder A. L. , Ziemba E. , Patterson V. , Tascione T. , Nelson D.
Published: February 2008

Measurements made on board the orbiting Air Force meteorological satellite equipped with a high‐frequency (HF) receiver have been studied in order to describe the HF radio environment at the satellite height. The satellite is in a sun‐synchronous dawn‐dusk orbit at a height of nearly 500 nmi (860 km) above the surface on the earth. The receiver sweeps repeatedly through the band 1.2–13.9 MHz in 100‐kHz steps once every 32 s, providing observations at each frequency approximately every 200 km along the satellite orbit. Measurements from a 3‐month period early in the satellite lifetime—September, October, and November, 1977—have been grouped and analyzed by frequency to yield results that are consistent with the hypothesis that the primary source of the observations is emissions from terrestrial transmitters. The relative differences in the received signals vary with frequency in a manner that agrees with the allocation of the HF spectrum.
HF interference measurements at sea
Authors: Hansen P. M. , Brown G. J. , Blitch J. D.
Published: February 2008

Interference data were collected in the HF band (2–30 MHz) during a Pacific transit. Measurements were made during four time periods corresponding to midday, midnight, and the sunrise and sunset transition periods. Man‐made signals (other users) were the predominant interference source between the maximum observed frequency (MOF) and the lowest observed frequency (LOF). The occupancy statistics and power levels were found to exhibit characteristic curves for each time period, with the maximum percentage occupancy and power level falling between the expected MOF and LOF. Typical other user occupancy between the MOF and LOF was about 50%; however, occupancies as high as 80% were observed near Japan at certain times.
On the transport theory of mutual intensity function propagation in a multiple scattering medium
Authors: Moran Steven E.
Published: February 2008

The statement that the mutual intensity function is equal to the two‐dimensional spatial Fourier transform of the traditional radiance is analyzed within the framework of the theory of partial coherence and the generalized radiometry. It is found that this relationship, which plays a central role in the transport theory of mutual intensity function propagation in a multiple scattering medium, is valid (1) in the source plane if and only if the source is quasi‐monochromatic, wide sense spatially incoherent, and Lambertian and (2) in the radiation field if and only if the region of interest lies in the Fresnel diffraction zone of a quasi‐monochromatic, wide sense spatially incoherent Lambertian source. It is also shown in the Fresnel approximation that the aforementioned relationship is universally correct for the generalized radiance and that the traditional and generalized radiance functions are identical under conditions 1 and 2 above. The concept of mutual radiance is introduced, and a generalized radiance invariance principle is developed. An example that illustrates these results is worked out, and conditions for the validity of the small angle transport equation are developed.
Effects of a satellite power system on ground‐based radio and radar astronomy
Authors: Thompson A. R.
Published: February 2008

Intensive studies of the feasibility of a satellite power system (SPS) are currently being carried out under a program initiated by the U.S. Department of Energy and the National Aeronautics and Space Administration. In such a system, satellites in the geostationary orbit convert solar radiation to microwaves, which are beamed to collecting stations on earth in order to provide electric power. An envisioned reference system for such studies is composed of 60 satellites spaced at 1° intervals, each radiating 6.7 GW at 2.45 GHz. Such a system can interact with radio and radar astronomy systems in a number of ways. The power signal and its harmonics, which fall close to radio astronomy bands, can cause overloading of input stages, and mitigation will require development of cryogenically cooled filters. Radiation within radio astronomy bands can result from transmitter‐generated noise, thermal noise from the large solar cell arrays, and possibly from intermodulation, component failures, and turn‐on transients. Noise and harmonics can also be generated by the power‐collecting rectennas. There may be propagation effects resulting from ionospheric heating. For those effects for which the magnitude can be estimated from present data, comparison is made with the interference thresholds for various types of radio telescopes. It is concluded that for any radio telescope, a zone of sky will be centered on the arc of satellites, in which observations with high sensitivity are precluded. The width of this zone, as determined by thermal radiation, is estimated to vary from about 30° for single‐antenna telescopes to a few degrees for high‐resolution arrays and interferometers. There will be some degradation in performance in bands close to the power signal and its second harmonic, and it will be necessary to insure that adequate shielding by terrain exists between radio observatories and the power receiving sites. The interference thresholds are higher for radar astronomy than for radio astronomy, but one of the main frequencies, 2.38 GHz, is very close to the power transmission frequency, and targets are usually close to the ecliptic and therefore at declinations close to the arc of satellites. Coexistence of radio and radar astronomy with the SPS will be crucially dependent upon the quality of engineering and maintenance of the satellites.
Electromagnetic scattering by a layered earth with arbitrarily varying subsurface impedance
Authors: Mahmoud Samir F.
Published: February 2008

The problem of scattering of a plane electromagnetic wave from a two‐layered earth with laterally varying substrate surface impedance is considered. A formal iterative solution which is applicable to any arbitrary profile of the substrate surface impedance is derived and is adapted for use of the fast Fourier transform (FFT) technique to obtain numerical results for the scattered fields and hence for the effective surface impedance and wave tilt. The aim is to study the effect of a finite lateral extent of an underground conducting layer, such as water, on the above quantities, which are measurable at the earth's surface. Numerical examples show that the effective surface impedance and wave tilt are good indicators of the variations in the substrate surface impedance.
Ocean surface current measurements by an L band two‐frequency microwave scatterometer
Authors: Alpers Werner , Schröter Jens , Schlude Franz , Müller Hans‐Jürgen , Koltermann Klaus Peter
Published: February 2008

Real time current measurements with an L band pulsed two‐frequency microwave scatterometer (2FS) were carried out from the German North Sea Research Platform (Forschungsplattform Nordsee) during the 1979 Marine Remote Sensing (Marsen) experiment. The principle of this remote sensing technique consists of measuring the phase velocity of a long ocean wave by resonant scattering of a beat wave produced by two microwave signals. The difference between the measured phase velocity and the phase velocity, as calculated from the dispersion relation for still water, represents a weighted average of the ocean current near the surface. Current measurements over full tidal cycles in various wind conditions (up to 18 m s−1) are presented. At high wind speeds, pronounced differences are found between the 2FS measurements and the in situ current measurements made at depths of 9 m and 23 m. These differences can be explained as the effect of the wind stress acting on the upper layer of the ocean. The measurements indicate that the interaction of the tide‐ and wind‐driven currents strongly depends on the parallel and antiparallel orientation of the currents. The accuracy of the measurements reported here is approximately ±0.12 m s−1, but this can, in principle, be improved.
Ionosphere layer shape from second‐order ATS 6 measurements
Authors: Heron M. L.
Published: February 2008

The carrier phase at 40 MHz on the ATS 6 to Boulder beacon experiment is found to be sufficiently well resolved to allow the measurement of the second‐order term as well as the first‐order total electron content value. The second‐order component of phase path is primarily due to a truncation of the expansion for refractive index when the satellite is within about 45° of zenith, but it is dominated by ray path bending when the satellite is outside that zone. The second‐order value is used to derive scale height at the peak of the F layer for one day of data with absolute accuracy of about ±10% during the daytime, when the plasma densities are generally high. The scale height determined at the peak differs from that derived from ionograms for the underside of the peak and also differs from that of the α‐Chapman layer fitted to total content and peak density. The main advantage of the second‐order evaluation of scale height is its spatial coincidence with total content and F factor observations.
Limits on the accuracy of correction of trans‐ionospheric propagation errors by using ionospheric models based on solar and magnetic indices and local measurements
Authors: Nisbet J. S. , Tyrnov O. F. , Zintchenko G. N. , Ross W. J.
Published: February 2008

Satellite propagation data have been used to estimate the limits on the accuracy of models of the ionospheric electron content used in calculating refractive corrections. In the first series of comparisons, electron content measurements have been compared with the Penn State Mark I Ionospheric Model. The probability of making errors in estimating electron content has been derived for stations at different latitudes and different time periods. It is shown that with this model there is a 10% probability of making errors, ranging from 2.2 × 1017 m2 at an equatorial station in the time period 1600–2200 hours to less than 7 × 1016 m2 at a midlatitude station. In the second series of comparisons, the same satellite data base was used, but the comparisons were made to an ideal model for each station, obtained by fitting each data set, using a least squares analysis, to functions of the geophysical indices, local time, and day number. These models typically produced only a factor of 2 improvement in the accuracy for given error probabilities. In a third series of comparisons, horizontal gradient measurements were used to estimate the errors that would result if electron content measurements were available along a ray path separated from the desired ray path by 500, 1000, and 1500 km. Error probabilities are given for an equatorial station and a midlatitude station for conditions when the mean horizontal gradients are assumed to be unknown and for the limiting case, when the mean horizontal gradient can be assumed to be known exactly. It is concluded that major advances in ionospheric modeling for predictive purposes will require a much better understanding of the factors controlling the day‐to‐day variability.
Characteristics of modulated periodic media: 1. Passive structures
Authors: Jaggard D. L. , Warhola G. T.
Published: February 2008

We examine the filter characteristics of modulated periodic structures through the use of vector coupled‐mode equations and the corresponding matrix Riccati equation. Simple expressions are found for several quantities of interest and the effect of loss, modulation index, and modulation wave number are examined. We find excellent agreement between this approximate theory and the exact electromagnetic formulation if the structure contains many cycles of each pertinent periodicity and if the perturbation is small.
Multimode optical fibers with noncircular cross section
Authors: Brenci M. , Checcacci P. F. , Falciai R. , Scheggi A. M.
Published: February 2008

An experimental investigation is presented to verify some theoretical results on the existence of two classes of modes (whispering and bouncing modes) in elliptical step index fibers. By utilizing a technology previously developed, silica plastic fibers with quasi elliptical cross section have been fabricated. The used noncircular preforms have been obtained by machining the circular cross section rods with a CO2 laser. The near field measurements performed on fibers with different ellipticity are interpreted on the basis of the theoretical results.
Fluctuations in intermodal dispersion of concatenated multimode fibres
Authors: Iyer Ramakrishna S. , Javed Alauddin
Published: February 2008

Dispersion characteristics of a fibre link that consist of several lengths of fibres jointed together cannot be related deterministically to the (nominally) specified dispersion values of the individual sections because of the random effects introduced at the fibre joints, and statistical differences between the specified and actual dispersion values of the sections. In this paper, the fibre link dispersion is modelled as a statistical quantity, and preliminary results that relate the link dispersion to the first and second order moments of the rms dispersion of each section are obtained.
Prediction of attenuation due to rainfall on earth‐space links
Authors: Brussaard G.
Published: February 2008

A review is given of the current state of the art in the prediction of rain attenuation on slant paths. Different types of research effort are identified and discussed. A critical assessment of data analysis methods and theoretical models is made to identify areas where more effort is needed. Various types of data available are discussed (direct attenuation measurements, radiometric data, rain rate, and radar reflectivity) as well as the models and methods available to date to relate these to attenuation on a satellite path. Finally, prediction methods for worldwide use are reviewed. While great progress is being made in the physical modeling of the atmosphere, statistical prediction methods are still very general and coarse. In the future, the emphasis should be on the development of detailed models to predict the variability of attenuation statistics within a coverage area. In this connection there is a need for coordination in the comparison of experimental data. In Europe, projects of international cooperation for this purpose are being carried out.