Showing papers on "Radio wave published in 1982"

Radio wave scintillations in the ionosphere

Abstract: The phenomenon of scintillation of radio waves propagating through the ionosphere is reviewed in this paper. The emphasis is on propagational aspects, including both theoretical and experimental results. The review opens with a discussion of the motivation for stochastic formulation of the problem. Based on measurements from in-situ, radar, and propagation experiments, ionospheric irregularities ate found to be characterized, in general, by a power-law spectrum. While earlier measurements indicated a spectral index of about 4, there is recent evidence showing that the index may vary with the strength of the irregularity and possibly a two-component spectrum may exist with different spectral indices for large and small structures. Several scintillation theories including the Phase Screen, Rytov, and Parabolic Equation Method (PEM) are discussed next. Statistical parameters of the signal such as the average signal, scintillation index, rms phase fluctuations, correlation functions, power spectra, distributions, etc., are investigated. Effects of multiple scattering are discussed. Experimental results concerning irregularity structures and signal statics are presented. These results are compared with theoretical predictions. The agreements are shown to be satisfactory in a large measure. Next, the temporal behavior of a transionospheric radio signal is studied in terms of a two-frequency mutual coherence function and the temporal moments. Results including numerical simulations are discussed. Finally, some future efforts in ionospheric scintillation studies in the areas of transionospheric communication and space- and geophysics are recommended.

Observations of Stimulated Scattering of a Strong High-Frequency Radio Wave in the Ionosphere

Abstract: Electromagnetic sideband spectra induced by a strong monochromatic high-frequency radio wave in an overdense ionosphere have been observed for the first time by means of a new direct observational technique. The observed asymmetry of the spectra, a characteristic feature of stimulated scattering, is tentatively attributed to parametric and linear-mode-conversion processes occurring in the irradiated ionospheric-plasma volume.

An initial critical summary of models for predicting the attenuation of radio waves by trees

Abstract: : This report is a review of models and data that can be used to predict the increase in attenuation due to trees and underbrush that are located between a radio transmitter and receiver. Applications in the 3 MHz to 95 GHz band are addressed with emphasis placed on the 200 to 9200 MHz range. It is shown that the conventional technique for predicting the loss caused by propagation through a small grove of trees is often substantially in error. An improved empirical model is developed to mitigate the problem; a physical justification for the model is presented. (Author)

Low‐level VLF and LF radio emissions observed at Earth and Jupiter

Abstract: Major observational features of magnetospheric VLF-LF radio noise that have been uncovered by the IMP, Hawkeye and ISEE satellites, are examined with emphasis on the capabilities of the noise to diagnose local and remote plasma parameters. The relationship of the radiation to not less than 1-mV/m electrostatic upper hybrid emissions is assessed, and indirect observational evidence suggests that upper noise is associated with the generation of the VLF-LF radiation. Theoretical luminosities of mechanisms including synchrotron radiation, linear mode conversion of upper hybrid waves and nonlinear scattering of upper hybrid waves off plasma density irregularities are estimated, and the wave-wave scattering hypothesis is considered to be a viable mechanism if the presence of low-frequency waves as a scattering agent can be established.

The vertical wave number spectrum of topside equatorial spread F: Estimates of backscatter levels and implications for a unified theory

Abstract: A wave number spectrum for plasma density fluctuations in decaying equatorial spread F covering 5 orders of magnitude in spatial scale and 12 orders of magnitude in spectral power is presented. The outer scale is found, as is the spectral transition between a shallow spectral form (slope < 2) at intermediate scales and a steep spectrum at smaller scales. The characteristic features of the spectrum are discussed and related to the physical processes operating in several regimes. A unified model is discussed involving aeronomic effects and neutral atmospheric dynamics at the largest scales, a generalized Rayleigh-Taylor plasma instability at intermediate scales, and a cascade of wave energy involving drift waves at transitional wavelengths. Open questions still remain concerning free energy sources near 1 km wavelength and at wavelengths less than a meter. Estimates of backscatter levels using the plasma density fluctuation spectra are consistent with Jicamarca and Altair radar observations except in topside regions of low plasma density, where in situ data at very short wavelengths are needed.

Wide band attenuation of radio waves caused by powerful HF waves: saturation and dependence on ionospheric variability

Abstract: The phenomenon of wide band attenuation, i.e., anomalous absorption of an HF wave in the F region subjected to modification by a powerful HF wave, has been measured using the Tromso heating facility. Two types of results were found. In a quiet F region situation, the attenuation amounted to approximately 15 dB and did not change when the effective radiated heating power was lowered from 160 to 20 MW. In a variable F region, indicated by a high fading rate of the diagnostic wave, the absorption was considerably less than 15 dB and showed a strong dependence on the heating power. An explanation for both types of results is offered by extending a recent theory of Das and Fejer.

Results from the 19- and 28-GHz COMSTAR satellite propagation experiments at Crawford Hill

Abstract: Strong interactions of radio waves with rain and ice particles in the lower atmosphere significantly affect the performance of earth-space radio links operating at frequencies above 10 GHz. Because of the uncertainties and variabilities in the microstructures of weather phenomena, direct measurements of the effects of rain and ice on radio propagation are needed for the economic design of reliable satellite communications systems. Unique and comprehensive space-earth propagation measurements have been made at Crawford Hill, New Jersey, using the 19- and 28-GHz beacons on the COMSTAR satellites. This paper summarizes these measurements of rain attenuation, rain and ice depolarization, phase and amplitude dispersion, off-path rain-scatter coupling, angle-of-arrival variations, and cloud scintillation. These results provide empirical design data and will serve as checks on theoretical models needed for predicting communication system performance in geographical regions where measurements are not available.

Attenuation of 900 MHz radio waves propagating into a metal building

Abstract: The attenuation of radio waves propagating into buildings is a crucial factor in the feasibility and design of portable radio telephone systems. In order to characterize signal statistics of an approximate worst-case building, signal levels were measured inside a steel shell building at 900 MHz. Cumulative signal level distributions determined for 4-ft square areas at 3, 5, and 7 ft heights within the building are good approximations to the Rayleigh distribution. Attenuation medians for the 4-ft square areas range from 26 dB to 32 dB.

Rain Attenuation 0f Centimeter, Millimeter and Submillimeter Radio Waves

Abstract: The rain attenuation from 1 to 1000 GHz was calculated by using a Weibu11 distribution for raindrop-size, which was assumed to be caused by coalescence, drop break-up and a chain reaction process. The results showed a good agreement with the recent microwave measurements from 8 to 312.5 GHz.

On the analysis and interpretation of spaced-receiver measurements of transionospheric radio waves

Abstract: A correlation analysis method developed by Armstrong and Coles has been applied to spaced- receiver data from the Wideband satellite. The method permits simultaneous estimation of the anisotropy and true drift of the diffraction pattern. The scintillation theory is applied to interpret the measured variations in anisotropy and pattern drift. The results show that in anisotropic media, there is a complicated interplay between apparent drifts and the anisotropy of the diffraction pattern that must be accommodated in spaced-receiver data analyses. The method has been applied to verify the L-shell aligned, cross-field anisotropy of F region auroral zone irregularities.

Applications of Spread Spectrum Radio to Indoor Data Communications

Abstract: This paper describes an operating, radio communication system intended for indoor use. The problems of multipath are discussed and the use of spread spectrum to overcome these problems is described. Two receiver implementations were constructed and a surface acoustic wave correlator version is described in detail. The system operates at a data rate of 100 Kbps over a range of 1000 meters with a power of 50 milliwatts.

The response of a radio-astronomy synthesis array to interfering signals

Abstract: Multiplying interferometers, and the more complex synthesis arrays that have been developed from them, possess the advantage of greater discrimination against interfering signals than can be obtained with single-antenna radio telescopes. The major contribution to this effect results from the relative changes of the phases of the signals received in spaced antennas, associated with the sidereal motion of a cosmic source across the sky. Signals that do not show this predictable phase behavior are substantially suppressed in the data processing. In this paper an approximate general expression for the interference response is derived and compared with results of an experimental test. The major uncertainty results from the variability of the gain of the antenna sidelobes in which the interferenee is received. Threshold levels at which interference becomes harmful are derived for the very large array (VLA) of the National Radio Astronomy Observatory. In the case of broad-band interfering signals, further rejection occurs because inequalities in the time delays of the signal paths via the individual antennas result in decorrelation. The magnitude of the decorrelation depends upon the position of the source of interference, and as an example, computations are given for broad-band signals from a satellite in geostationary orbit. General thresholds for harmful interference to radio astronomy are given in International Radio Consultive Committee (CCIR) Report 224, and the present results are not intended to supercede them. Rather, the intention is to provide data specific to synthesis arrays to allow more accurate coordination in certain frequency ranges shared with active spectrum users. A possible application to the search for extraterrestrial intelligence is mentioned.

Radio wave heating of the corona and electron precipitation during flares

Abstract: Electron-cyclotron masers, excited while energy release is occurring in a flaring magnetic loop, are likely to generate extremely intense radiation at decimeter wavelengths. The energy in the radiation can be comparable with that in the electrons associated with hard X-ray bursts, i.e., a significant fraction of the total energy in the flare. Essentially all of the radio energy is likely to be reabsorbed by gyroresonance absorption, either near the emitting region or at some distance away in neighboring loops. Enhanced diffusion of fast electrons caused by the maser can lead to precipitation at the maximum possible rate, and hence account for hard X-ray emission from the footpoints of the magnetic loops.

Phase changes induced in a diagnostic radio wave passing through a heated region of the auroral ionosphere

Abstract: Some results of recent high-latitude ionospheric heating experiments are presented. Phase changes were induced in a diagnostic radio wave reflected from the lower F region of the ionosphere as it passed through a region of the ionosphere which was being illuminated by a high-power electromagnetic wave. From the results obtained there are indications that anomalous absorption contributes much more than deviative absorption to ionospheric heating at high latitudes.

The influence of localized precipitation‐induced D‐region ionization enhancements on subionospheric VLF propagation

Abstract: A two-dimensional mode propagation model has been used to investigate the effect of localized ionospheric perturbations on the propagation of VLF radio waves in the earth-ionosphere waveguide. Computations have been performed for the NSS transmissions at 22.3 kHz from Annapolis, Maryland to Eights Station, Antarctica, where anomalous short-duration signal amplitude changes, in coincidence with whistlers, were first noted. The calculations suggest that two ionization regions, one very near the transmitter and another near the receiver, may need to be present at the same time in order to obtain the large amplitude increases (>3 dB) that are observed to occur on occasion. It is suggested that the former is due to transmitter-induced electron precipitation, while the latter is due to precipitation associated with the whistler. We also find that small amplitude changes can be obtained under less restrictive ionospheric perturbation conditions.

The commons In the sky: the radio spectrum and geosynchronous orbit as issues in global policy

Abstract: Radio waves, which are used to transmit information electronically, are a portion of the larger spectrum of electromagnetic waves. They are used for purposes as diverse as telephone, AM and FM radio, UHF and VHF television, air and marine navigation, radar, radio astronomy, meteorology, data transmission, and electronic mail. Some uses of the spectrum entail transmitting or receiving information via satellites orbiting the planet. In order to communicate without interference at any given time, a user must have exclusive access to a frequency over a geographical area determined by the distance that the signals travel to targeted receivers.

Decimeter radio wave propagation in the turbulent plasma near the sun, using Venera 10 spacecraft

Abstract: The 1976 Venus superior solar conjunction resulted in the occultation of the spacecraft Venera 10 by the sun at solar minimum. At the conjunction time, decimeter radio waves passed through the solar corona. The broadening of the spectrum line, as well as amplitude and frequency fluctuations, was observed over extensive distances of the radio link from the sun. The fluctuation characteristics of the radio waves are used to determine solar wind parameters. The fluctuations of the electron concentration have been shown to decrease rapidly with the increasing distance from the sun, but in the region between 12 and 20 solar radii (Rs) the irregularities have increased values. The shape of the spatial spectrum is closely described by the power function ΦN(q) ∝ q−p, but the spectral index may have different values in different parts of the spectrum. The spectral index of the small-scale part of the spectrum decreases with the decreasing distance from the sun, and at radial distances shorter than 12 Rs its value is close to 3.0 ± 0.2. The inner scale of turbulence of the near-sun plasma is of the order of several kilometers at distances between 6 and 10 Rs. Typical values of the velocity of motion of the inhomogeneities are 30–40 km/s at radial distances from 2 to 6 Rs, while at distances greater than 18 Rs the velocity exceeds 400 km/s.

Nonlinear reflectivity of high-power radio waves in the ionosphere

Abstract: An important result of earlier ionospheric modification experiments performed at Platteville, Colorado, was the discovery that the reflectivity of the ionosphere was greatly reduced in the reflection region of a high-power radio (pump) wave1. Because the reflectivity of low-power diagnostic waves passing through the modified region with frequencies from just below the pump wave frequency up to the F region critical frequency were reduced, the phenomenon was termed wide-band absorption (WBA). Subsequent experimental and theoretical work (for review see ref. 2) has led to the conclusion that WBA is due to the scattering of radio waves from field-aligned irregularities into plasma waves. These irregularities are generated by the interaction of the pump wave and the ionospheric plasma but the mechanism by which this occurs is not completely understood. It is evident, however, that in the process the pump wave itself undergoes WBA. In a recent campaign using the Max-Planck Institute ionospheric modification facility at Ramfjordmoen, near Tromso, Norway, the existence of WBA at high latitudes was confirmed by experiments using low-power diagnostic waves passing through the reflection region of the high-power radio wave3,4. As we report here, during an experiment to measure the effect of WBA on the pump wave itself, the essentially nonlinear nature of the interaction between the high-power radio waves and the ionosphere was demonstrated in a new and striking way.

Spatial irregularities in Jupiter's upper ionosphere observed by voyager radio occultations

Abstract: Radio scintillations (at 3.6 and 13 cm) produced by scattering from ionospheric irregularities during the Voyager occultations are interpreted using a weak-scattering theory. Least squares solutions for ionospheric parameters derived from the observed fluctuation spectra yield estimates of (1) the axial ratio, (2) angular orientation of the anisotropic irregularities, (3) the power law exponent of the spatial spectrum of irregularities, and (4) the magnitude of the spatial variations in electron density. It is shown that the measured angular orientation of the anisotropic irregularities indicates magnetic field direction and may provide a basis for refining Jovian magnetic field models.

Attenuation with distance and wind speed of HF surface waves over the ocean

Abstract: An HF monostatic experiment is performed in order to evaluate the relative attenuation with distance of radio waves propagating in the ground wave mode over the sea surface, at 7 and 14 MHz. Clear evidence of the wind speed (causing the sea roughness) influence arises from the data, particularly at 14 MHz. The results are compared to Barrick's theoretical calculations, and a good agreement is found for the values of attenuation rates (in decibels per kilometer) and their variation with the wind speed in the upwind/downwind case. Some discrepancies in the data are partially interpreted as due to a fetch influence upon the radar cross section.

Backward and forward scattering of microwaves by ice particles: A review

Abstract: Approximate and exact mathematical techniques for the calculation of the scattering properties of a single ice particle are first reviewed. Then, for dealing with practical applications such as radar meteorology or radio wave transmission along earth-space links, the problem of the scattering by an assembly of particles is examined. This problem involves the knowledge of the particle size distribution and of the orienting mechanisms which can affect the ice crystals in the clouds. Although the T matrix formalism has been successfully used in a greater range of scattering problems than the other techniques, the Rayleigh approximation may be used with confidence for calculating the cross-polarization properties of radio waves at frequencies up to at least 25 GHz. Alignment of ice particles is not expected to occur in high-altitude cirrus clouds while alignment is observed in strong thunderstorms. Thus different models are to be used in order to predict the cross polarization along earth-space links.

The optical activity of intergalactic space

Abstract: We obtain an experimental bound on the optical activity of intergalactic space for radio waves, and discuss the expected magnitude in some simple models.