Showing papers in "Radio Science in 1976"
TL;DR: Wide-angle reflection and refraction (WARR) sounding has been used for mapping the near-surface geological structure and electrical properties of permafrost in the Tuktoyaktuk region of the Mackenzie River delta.
Abstract: A VHF impulse radar system operating on the ground is a viable technique for mapping the near-surface geological structure and electrical properties of permafrost. A fixed antenna configuration transported over the surface yields a reconnaissance map of two-way travel times for subsurface reflectors. Wide-angle reflection and refraction (WARR) sounding determines propagation velocity versus depth when performed in layered areas. To obtain a WARR sounding, one measures travel time versus antenna separation. These techniques were field tested in the Tuktoyaktuk region of the Mackenzie River delta, N.W.T. Data recording was on a graphic display for initial field analysis and on analog magnetic tape for subsequent processing. Reconnaissance surveying has mapped structural features at various depths between 3 and 30 m. The electrical loss of the soils at a site limits the penetration depth. Clays and silts attenuate the radar signal more than sands and gravels. WARR soundings have determined dielectric constant versus depth in layered areas. While impulse radar is useful in delineating geological structure, core drilling is required to determine the geological composition.
231 citations
TL;DR: In this article, a model of equatorial scintillations is developed in the framework of diffraction theory based on measured electron density deviations and other assumed model parameters including a three-dimensional power law form of irregularity spectrum with index 4.
Abstract: In-situ measurements of F-region irregularity amplitude and ambient electron density made by the retarding potential analyzer on OGO-6 near the perigee altitude of 400 km have been utilized to derive the variation of electron density deviation over the equatorial region. Based on these measured electron density deviations and other assumed model parameters including a three-dimensional power law form of irregularity spectrum with index 4, a model of equatorial scintillations is developed in the framework of diffraction theory. The percentage occurrence contours of estimated equatorial scintillations ≥ 4.5 dB at 140 MHz during 1900–2300 LMT for the period November–December 1969 and 1970 have been derived. The model is found to depict a pronounced longitude variation with the scintillation belt width and percentage occurrence being maximum over the African sector. The latitude extent of the spatial scintillation belt narrows over the American sector without much decrease in the scintillation occurrence whereas over the Indian and Far Eastern sectors both the extent and the occurrence are found to decrease. The percentage occurrence of scintillations estimated from this model is found to be consistent with VHF scintillation measurements at Ghana, Huancayo, and Calcutta. In addition, the model was found to be in qualitative agreement with GHz observations at various longitudes made by the COMSAT group. The effects of varying model parameters on scintillation estimates at VHF, UHF, and GHz are discussed. Implications of the observed longitudinal variation of scintillations on current theories of equatorial irregularity formation are indicated.
118 citations
TL;DR: In this article, an exact analytical solution for a plane wave case when the random medium is approximated by A(0) − A(ρ) ∝ ρ2 is presented.
Abstract: Pulse propagation in a random medium is determined by the two-frequency mutual coherence function which satisfies a parabolic equation. In the past, approximate or numerical solutions of this equation have been reported. This paper presents an exact analytical solution for a plane wave case when the random medium is approximated by A(0) − A(ρ) ∝ ρ2. Analytic expressions for two-frequency mutual coherence function, pulse shape, temporal and angular spectra are presented in universal forms, and should be applicable to a large number of practical problems.
99 citations
TL;DR: In this article, a solution for the problem of electromagnetic scattering by a half plane with two different face impedances is presented for both normal and oblique incidence, and a chart relative to the existence of surface wave contributions is presented.
Abstract: The solution for the problem of electromagnetic scattering by a half plane with two different face impedances is presented for both normal and oblique incidence. Illustrative examples are discussed and a chart relative to the existence of surface wave contributions is presented.
94 citations
TL;DR: In this article, an ionospheric volume in the F layer subjected to high power high frequency illumination is observed to be an effective scattering medium for radio signals and a field-aligned scattering geometry is considered.
Abstract: An ionospheric volume in the F layer subjected to high power high frequency illumination is observed to be an effective scattering medium for radio signals. Experimental results are representative of a field-aligned scattering geometry. Scatter of the incident wave into electrostatic waves by these strongly field-aligned density irregularities is considered. This model explains the large decreases in radio wave reflectivity seen during the ionospheric modification.
80 citations
TL;DR: In this paper, a comparative study of the three spectral analysis techniques was performed using simulated data involving known wave and noise properties and real ULF wave event data from the geosynchronous satellites ATS 1 and ATS 6.
Abstract: Digital power spectral analysis and coherency analysis are powerful techniques for studying ultra-low-frequency (ULF) waves in the earth's magnetosphere. Wave polarization parameters provided by these techniques are important in the development of theoretical models for wave generation. Because of this, it is important to understand the capabilities of the digital analysis techniques. Three different techniques of using the spectral matrix to do wave analysis have been presented in the literature. Because data for wave studies involve measurement in arbitrary coordinate systems, it is necessary to transform the spectral matrix to the principal plane of the wave before coherency analysis can be performed. The fundamental differences in the three techniques lie in how they determine the transformation to the principal plane. A comparative study of these three techniques was done using simulated data involving known wave and noise properties and real ULF wave event data from the geosynchronous satellites ATS 1 and ATS 6. In general, the quality of performance of the three different techniques on both simulated and real wave events was approximately the same.
77 citations
TL;DR: In this paper, the propagation characteristics of pulse waves in rain, fog, and turbulence were determined by a two-frequency mutual coherence function for randomly distributed scatterers and turbulence.
Abstract: Propagation characteristics of pulse waves in rain, fog, and turbulence are determined by a two-frequency mutual coherence function. Parabolic differential equations applicable to strong fluctuation cases are derived for the two-frequency mutual coherence function for randomly distributed scatterers and turbulence. These equations are solved for the plane wave case using the eigenfunction-eigenvalue method. Numerical calculations are given for millimeter waves (100 GHz) and optical waves (0.6943 and 10.6 μm) in rain, fog, and turbulence over the distance of 5 km. The coherence bandwidths are found to be in the range of MHz for millimeter waves (100 GHz) in heavy rain (25 mm/hr) and for optical waves in fog, and considerable distortion of pulse shape may result in these situations. For other cases, the effect on pulse shape distortion may be negligibly small.
76 citations
TL;DR: In this article, the relative wave amplitude with a small receiver antenna scanned around the exciter in a large uniform collisionless magnetized laboratory plasma in the whistler wave regime was measured.
Abstract: Antenna radiation patterns of balanced electric dipoles and shielded magnetic loop antennas are obtained by measuring the relative wave amplitude with a small receiver antenna scanned around the exciter in a large uniform collisionless magnetized laboratory plasma in the whistler wave regime. The boundary effects are assumed to be negligible even for many farfield patterns. Characteristic differences are observed between electrically short and long antennas, the former exhibiting resonance cones and the latter showing dipole-like antenna patterns along the magnetic field. Resonance cones due to small electric dipoles and magnetic loops are observed in both the near zone and the far zone. A self-focusing process is revealed which produces a pencil-shaped field-aligned radiation pattern.
74 citations
TL;DR: Reflector antennas have been used since the radio pioneering era of Lodge, Hertz, and Marconi, but it took the exigent demands of radar in World War II to stimulate a real development in the reflector art as discussed by the authors.
Abstract: Reflector antennas have been used since the radio pioneering era of Lodge, Hertz, and Marconi, but it took the exigent demands of radar in World War II to stimulate a real development in the reflector art. Subsequent interest in the science of radio astronomy and the inception of microwave ground communication links were responsible for a burgeoning growth in the field, so that in the 1940s and 1950s the design principles and requirements for prime focus fed systems were well established. Cassegrain, or secondary focus systems, and horn reflectors came into prominence in the early 1960s with the advent of satellite tracking and communication networks. The desire to maximize the gain, or the gain-temperature ratio, then led to development of sophisticated techniques for properly shaping the illumination over the reflector aperture in order to maximize efficiency and minimize spillover, among them being the shaping of the sub-reflector in Cassegrain systems and the use of multimode and hybrid mode feed horns. Not all reflector antennas utilize paraboloidal surfaces. Some recent developments in line source feeds make the spherical reflector attractive for scanning applications and the conical reflector for deployable, space-borne antennas. The large 1000-foot diameter reflector at Arecibo is a well known example of the former. Although some extremely large spaceborne reflector antennas have been proposed and studied, the largest now in use appears to be the unfurlable 30-foot reflector carried by ATS-6. Finally, some gain comparisons are given for a few of the (electrically) largest reflectors that have been built both for radio astronomy and for space communications. If some milestones in reflector development have been overlooked it is due to the limitations inherent in a review paper.
73 citations
TL;DR: In this article, integro-differential equations are formulated for the general problem of electromagnetic diffraction by an aperture in a planar conducting screen of infinite extent separating two half spaces of different electromagnetic properties.
Abstract: Integro-differential equations are formulated for the general problem of electromagnetic diffraction by an aperture in a planar conducting screen of infinite extent separating two half spaces of different electromagnetic properties. With the aperture specialized to the case of an infinite slot, equations appropriate for TE and for TM illumination are deduced from those for the general aperture. These slot equations are solved numerically, and results are presented for several cases of interest.
69 citations
TL;DR: In this paper, a two-dimensional (altitude, latitude) cross-sectional map of electron densities in the ionosphere is produced by scanning in the geomagnetic meridian plane.
Abstract: The Chatanika, Alaska, incoherent scatter radar has been used to measure the spatial variation of auroral ionization. A two-dimensional (altitude, latitude) cross-sectional map of electron densities in the ionosphere is produced by scanning in the geomagnetic meridian plane. The altitutde variation of ionization is used to infer the differential energy distribution of the incident auroral electrons. The latitudinal variation of this energy distribution and the total energy input are obtained by use of the meridian-scanning technique. Examples are shown of observations made during an active aurora.
TL;DR: In this article, an electrically thin dielectric insulating shell on an antenna composed of cylindrical wires is examined, and the insulation is accounted for entirely through a modification of the symmetric impedance matrix.
Abstract: An electrically thin dielectric insulating shell on an antenna composed of electrically thin circular cylindrical wires is examined. A moment method solution is obtained, and the insulating shell is modeled by equivalent volume polarization currents. These polarization currents are related in a simple manner to the surface charge density on the wire antenna. In this way the insulating shell causes no new unknowns to be introduced, and the size of the impedance matrix is the same as for the uninsulated wires. The insulation is accounted for entirely through a modification of the symmetric impedance matrix. This modification influences the current distribution, impedance, efficiency, field patterns, and scattering properties. The theory is compared with measurement for dielectric coated antennas in air.
TL;DR: In this paper, the electric fields produced by lightning return strokes near Kennedy Space Center, Florida, have been measured simultaneously at distances of about 5 to 25 km and about 200 km.
Abstract: The electric fields produced by lightning return strokes near Kennedy Space Center, Florida, have been measured simultaneously at distances of about 5 to 25 km and about 200 km. Detailed records of the first 12 microsec of waveforms from four strokes are presented, as well as data on the initial field risetimes for 58 first and 92 subsequent strokes. The mean field risetime measured between the 10 to 90% points was about 1 microsec at the close station and about 2 microsec at the distant one.
TL;DR: In this article, a half-space random medium with a non-uniform temperature profile and correlation functions that possess both vertical and lateral variations is derived, and the radiative transfer equations are solved with an iterative integral equation approach for small scattering albedo and with a numerical approach for general cases.
Abstract: Brightness temperatures resulting from microwave thermal emission from a half-space random medium are calculated. The random medium has a nonuniform temperature profile and is characterized by correlation functions that possess both vertical and lateral variations. Radiative transfer equations are derived. They are solved with an iterative integral equation approach for small scattering albedo and with a numerical approach for general cases. New results are illustrated, discussed, and compared with various special cases. Phenomena caused by resonant scattering, which is absent under Rayleigh approximations, are revealed. Scattering in the lateral direction contributes to the decrease of the equivalent reflectivity and the increase of the brightness temperature.
TL;DR: A very long-baseline interferometer system was designed and built for geodetic applications as mentioned in this paper, which can be used to make accurate determinations of vector baselines, radio-source positions, polar motion, and universal time.
Abstract: A very-long-baseline interferometer system was designed and built for geodetic applications. Each interferometer terminal records a 360-kHz spectral band of noise from a compact extragalactic radio source. The center frequency of the spectral band can be selected to sample sequentially bands covering a much wider frequency range to obtain subnanosecond accuracy in group-delay measurements. A tunnel-diode pulse generator is used to calibrate the delays in the receiver. The necessary sets of algorithms and computer programs have been developed to analyze the data and have allowed the system to be employed to make accurate determinations of vector baselines, radio-source positions, polar motion, and universal time.
TL;DR: In this paper, a five-pulse technique was implemented for the 430 MHz incoherent scatter radar at Arecibo Observatory (18.3/sup 0/N) to explore the detailed thermal structure of the E region from 105 to 130 km with an altitude resolution of 3 km.
Abstract: A five-pulse technique was implemented for the 430 MHz incoherent scatter radar at Arecibo Observatory (18.3/sup 0/N) to explore the detailed thermal structure of the E region from 105 to 130 km with an altitude resolution of 3 km. Five days of measurements in Sept-Oct 1970 showed long-period temperature fluctuations having a downward phase progression. The temperature oscillations are interpreted as manifestations of a semidiurnal tide which is quite stable over a 12-day period, together with a superimposed spectrum of shorter-period gravity waves which are randomly phased from day to day. The semidiurnal tide increased to a maximum amplitude of 17 percent of the mean temperature near 115 km and decreased above this altitude as dissipative effects became important. The vertical wavelength, deduced from the altitude variation of semidiurnal tidal phase, showed a smooth increase from about 20 km at an altitude of 109 km to about 50 km at an altitude of 127 km. No ready interpretation of the observed tidal characteristics was possible in terms of present theories for the semidiurnal tide. Altitude profiles of mean daytime temperature and ion-neutral collision frequency were also obtained from the measurements. The mean temperature gradient between 115 and 130 km wasmore » 15 K/km, which is somewhat larger than that given by current atmospheric models.« less
TL;DR: In this article, the basic mechanisms of tunnel propagation and of underground radio systems are briefly reviewed and the TEM approximation of the monofilar mode is compared with available rigorous solutions.
Abstract: The basic mechanisms of tunnel propagation and of underground radio systems are briefly reviewed. The TEM approximation of the monofilar mode is compared with available rigorous solutions. Below the tunnel cutoff frequencies, the leaky feeder techniques can be described by the coupled line methods or, with a simpler physical picture, by the use of two eigenmodes. The only valid explanation of the leaky feeder radiation at higher frequencies is the random diffraction of the feeder leakage fields by the wall irregularities. To avoid the inefficiency of this process, the insertion of leaky stubs in a well-shielded coaxial cable is proposed.
TL;DR: In this article, the theory of P. C. Waterman is applied to the calculation of cross sections and depolarization ratios of spheroids, and drops of the shapes determined by H. R. Pruppacher and R. L. Pitter.
Abstract: With a view to calculating depolarization and differential attenuation of microwaves by rain, with allowance for the variable shape and orientation of raindrops, the theory of P. C. Waterman is applied to the calculation of cross sections and depolarization ratios of spheroids, and drops of the shapes determined by H. R. Pruppacher and R. L. Pitter. Use of Waterman's extended boundary condition facilitates computation. The scattering characteristics of spheroids appear similar in nature to those of spheres, in the regimes considered. Marked singularities in the characteristics of backscattering depolarization appear at optical size 1.5. The Pruppacher and Pitter drops scatter similarly to equivalent oblate spheroids. Implications for weather radar measurements are briefly discussed.
TL;DR: In this paper, a uniform low-pass filter model of the scattering process was proposed to estimate the effective surface roughness versus wavelength, and the asymptotic surface height spectral densities fall at least as fast as an inverse cube of spatial frequency.
Abstract: Radio-wave scattering from natural surfaces contains a strong quasispecular component that at fixed wavelengths is consistent with specular-point theory, but often has a strong wavelength dependence that is not predicted by physical optics calculations under the usual limitations of specular-point models. Wavelength dependence can be introduced by a physical approximation that preserves the specular-point assumptions with respect to the radii of curvature of a fictitious, effective scattering surface obtained by smoothing the actual surface. A uniform low-pass filter model of the scattering process yields explicit results for the effective surface roughness versus wavelength. Interpretation of experimental results from planetary surfaces indicates that the asymptotic surface height spectral densities fall at least as fast as an inverse cube of spatial frequency. Asymptotic spectral densities for Mars and portions of the lunar surface evidently decrease more rapidly.
TL;DR: In this paper, an expansion of the nonsingular solutions of the vector wave equation in cylindrical coordinates in terms of non-singular solutions in spherical coordinates is presented, where both perfectly conducting and dielectric spheres are treated.
Abstract: An expansion of the nonsingular solutions of the vector wave equation in cylindrical coordinates in terms of the nonsingular solutions of the vector wave equation in spherical coordinates is presented. As an example application of the expansion, the scattering of a particularly simple gaussian beam of electromagnetic radiation by a spherical obstacle is discussed. Both perfectly conducting and dielectric spheres are treated.
TL;DR: In this paper, a general scheme for calculating the change (both the real part and the imaginary part) of the propagation constant of a surface-wave mode on a general open waveguide structure is presented.
Abstract: A general scheme for calculating the change (both the real part and the imaginary part) of the propagation constant of a surface-wave mode on a general open waveguide structure is presented. By carefully keeping track of lower-order terms in the analysis, it is shown how approximations commonly made in other analyses can result in quite significant errors in the determination of the radiation loss. The resulting formulas require knowledge only of the fields and propagation constant of the corresponding straight waveguide mode, and the value of the radius of curvature of the waveguide axis. As a simple example, the curvature loss of a Goubau line is calculated.
TL;DR: In this paper, the effects of horizontal refractivity gradients are investigated by ray tracing through spherically symmetric and three-dimensional profiles constructed from radiosonde data, and the results indicate that the horizontal gradients introduce an rms error of approximately 3 cm when the satellite is near 10 deg elevation.
Abstract: Numerous formulas have been developed to partially correct laser ranging data for the effects of atmospheric refraction. All the formulas assume the atmospheric refractivity profile is spherically symmetric. The effects of horizontal refractivity gradients are investigated by ray tracing through spherically symmetric and three-dimensional refractivity profiles. The profiles are constructed from radiosonde data. The results indicate that the horizontal gradients introduce an rms error of approximately 3 cm when the satellite is near 10 deg elevation. The error decreases to a few millimeters near zenith.
TL;DR: In this article, it was shown that an FM-CW radar can be used as a Doppler radar to measure winds, and an analysis of scattering by refractive-index fluctuations showed that, under certain conditions, the radar should be able to continuously measure winds continuously in the boundary layer.
Abstract: It is shown that an FM-CW radar can be used as a Doppler radar to measure winds. A description of the processing used to measure Doppler shifts is included. An analysis of scattering by refractive-index fluctuations shows that, under certain conditions, a Doppler FM-CW radar should be able to measure winds continuously in the boundary layer. Experimental results comparing horizontal winds measured by a tethered balloon and the FM-CW radar show very good agreement.
TL;DR: In this paper, a half-space random medium with three-dimensional correlation functions is used to measure the emissivity of electromagnetic waves and the Born approximation is used for various cases over a wavelength range smaller or larger than the correlation lengths.
Abstract: Scattering of electromagnetic waves by a half-space random medium with three-dimensional correlation functions is studied with the Born approximation. The emissivity is calculated from a simple integral and is illustrated for various cases. The results are valid over a wavelength range smaller or larger than the correlation lengths.
TL;DR: In this article, the amplitude distributions of the 1.5-GHz and 4-GHz scintillations at Tangua, Brazil, were analyzed under weak scattering conditions and it was shown that the scintillation index varies approximately as the wavelength.
Abstract: Analysis of simultaneously recorded 1.5- and 4-GHz scintillations at Tangua, Brazil, has shown that the scintillation index varies approximately as the wavelength between 1.5 and 4 GHz under weak scattering conditions. The amplitude distributions, especially those of the 1.5-GHz data, are somewhat irregular in shape and cannot be represented by a simple mathematical expression. The power spectra of the amplitude fluctuations on a log-log plot show a slope of −3 and have low wave number rolloffs near 10−2 m−1. This paper discusses some possible implications of these findings. Half of the 4-GHz power spectra show the minima which are predicted by the thin-screen weak scattering theory. The l/e falloff of the autocorrelation corresponds to irregularities between 100 and 200 m. The cross correlation between the 1.5- and 4-GHz scintillations is about 0.3.
TL;DR: In this paper, a theory for the input impedance of a short, cylindrical dipole antenna immersed in a warm anisotropic plasma which is described by the Boltzmann equation is presented.
Abstract: A theory is presented for the input impedance of a short, cylindrical dipole antenna immersed in a warm anisotropic plasma which is described by the kinetic theory (Boltzmann equation). The plasma is assumed to be homogeneous with collisions included. The input impedance is based on the induced EMF technique and a quasistatic approximation. The dipole is assumed to have a triangular current distribution along its axis. For a dipole oriented parallel to the static magnetic field, the input impedance is derived as a one-dimensional integral suitable for numerical integration. Under certain conditions, this integral can be evaluated giving analytical formulas valid near the second and third electron cyclotron harmonics. The results show new contributions to the input impedance due to the excitation of cyclotron harmonic waves which propagate near the harmonics of the electron cyclotron frequency. These harmonic effects are not predicted by either the cold or hydrodynamic theories.
TL;DR: In this paper, it was shown that the results of the two results are in fact identical, and that they are not at first glance dissimilar, but they are actually identical.
Abstract: Approximate solutions to Tatarskii's equation for the intensity scintillations of a wave propagating through a thin, highly turbulent layer have recently been developed by both the optical and radio astronomy communities. It is demonstrated here that these results, which are at first glance dissimilar, are in fact identical.
TL;DR: In this paper, the field scattered by a homogeneous isotropic dielectric of finite extent illuminated by a low-frequency plane electromagnetic wave is expressed in terms of a single polarizability tensor which is a function of only the geometry of the body and a parameter τ representing either the relative permittivity or permeability of the dielectrics.
Abstract: The field scattered by a homogeneous isotropic dielectric of finite extent illuminated by a low-frequency plane electromagnetic wave is expressed in terms of a single polarizability tensor which is a function of only the geometry of the body and a parameter τ representing either the relative permittivity or permeability of the dielectric. Integral equations from which to compute the individual tensor elements are derived. Some of the properties of the tensor, including bounds on its diagonal elements, are also developed, and data are presented for the special case of a spheroidal body.
TL;DR: In this paper, a theoretical model was developed to explain the unexpectedly long communication ranges of over 300 m obtained, and the marked polarization effects observed, during recent radio transmission measurements in a conductor-free area in a coal mine at frequencies in the range of 57.5 to 920 kHz.
Abstract: A theoretical model has been developed to explain the unexpectedly long communication ranges of over 300 m obtained, and the marked polarization effects observed, during recent radio transmission measurements in a conductor-free area in a coal mine at frequencies in the range of 57.5 to 920 kHz. The model is based on a dipolar cylindrical TEM mode of propagation, with E vertical and H horizontal, in a low-conductivity horizontal coal seam bounded by high-conductivity rock. The best overall fit to the experimental data over the frequency range 57.5 to 920 kHz is found for coal and rock conductivities of 1.4 × 10−4 mho/m and 1.0 mho/m, respectively, for an assumed coal dielectric constant of 7. A better fit to the data at 57.5 kHz can be obtained if, in the model, the conductivity of the surrounding rock is allowed to be high for only a short distance above and below the coal seam and of a substantially lower value elsewhere. If the coal conductivity is changed from 1.4 × 10−4 mho/m, which is near the lower end of the range of expected values for bituminous coals, to 1.0 × 10−2 mho/m, which is near the upper bound of values for such coals, the attenuation is markedly increased with corresponding reductions in communication range.
TL;DR: Averages and distributions of the rain parameters, cancellation and degree of orientation, were determined from observations of power and correlation with a polarization diversity radar using circular polarization at 16.5 GHz during the period 1971-1974 as mentioned in this paper.
Abstract: Averages and distributions of the rain parameters, cancellation and degree of orientation, are presented. The quantities were determined from observations of power and correlation with a polarization diversity radar using circular polarization at 16.5 GHz during the period 1971–1974. Data are given for a related quantity, “maximum cancellation.” The fact that this quantity is not invariant with range is shown to be evidence for the occurrence of incoherent transmission processes at 16.5 GHz.