Showing papers on "Very low frequency published in 1992"

Microcellular mobile communication system

Abstract: A microcellular mobile radio communication system has a central base station and a plurality of radio base stations interconnected by a shared optical fiber transmission line. In the central and radio base stations, electrical radio signals are frequency-converted to frequencies assigned to the radio base stations and then converted into optical signals. In the central base station, the optical signals are combined for transmission through the optical fiber transmission line to the radio base stations. Alternatively, the frequency-converted electrical radio signals may first be combined and then converted into an optical signal for transmission. In the radio base stations, the electrical radio signals are converted to optical signals, which are combined and transmitted through the transmission line to the central radio station. In each radio base station, the frequency of the radio signal extracted from the received combined signal is frequency-converted to a frequency to be transmitted from its antenna, with the other signals filtered out. The central base station demodulates the received combined signal from the radio base stations into signals for the exchange office. The radio signals in different frequency bands assigned to each radio base station can simultaneously be transmitted through the shared optical fiber transmission lines between the radio and central base stations.

Global Measurements of Low-Frequency Radio Noise

Abstract: We report illustrative results obtained by Stanford University’s global survey of ELF/VLF radio noise (frequencies in the range 10 Hz – 32 kHz). Particular comparison is made between the noise measurements made at high (polar) latitudes with those at lower latitudes. Although most of the natural ELF/VLF noise observed everywhere in the world is lightning-generated, the high-latitude noise often contains additional components that are of magnetospheric origin. In the data we have examined, this noise consists predominantly of polar chorus, which is concentrated in the range 300 Hz–2 kHz. It produces a characteristic signature in the noise statistics. Less frequent occurrences of broad-band (auroral) hiss can occasionally mask most or all of the lightning-generated noise in the ELF/VLF range.

ionospheric modification with a VLF transmitter

Abstract: Detectable heating of the nightime D region by a 28.5 kHz signal is observed in 16 out of 144 cases with events occurring under conditions of moderate to low D region electron densities as represented by the unperturbed VLF signal levels. Three dimensional modeling of the effects of NAU heating on a probe VLF signal predicts values in general agreement with observations and suggests that maximum effects should occur under tenuous D region conditions.

Ulysses observations of escaping VLF emissions from Jupiter

Abstract: The Ulysses URAP experiment has detected Jovian radio emissions in the VLF range at distances from Jupiter in excess of 1.5 AU. The URAP observations represent the first synoptic observations of Jupiter in the VLF band, from 3 to 30 kHz. In this band lie the low-frequency extent of the bKOM emission, the escaping continuum emission, and the Jovian type IIIs. Initial results indicate that the continuum varies in frequency with the solar wind ram pressure at Jupiter, whereas, the Jovian type IIIs appear to be controlled to some extent by the planetary rotation, often appearing when system III longitude 100 deg faces the spacecraft.

The natural background levels of 50/60 Hz radio noise

Abstract: Measurements of the natural background levels of 50- and 60-Hz radio noise by ELF/VLF (extremely low frequency/very low frequency) radiometers located at various sites around the world are reported. The measurements are made on the magnetic fields of the noise using loop antennas, and they cover the first three harmonics of the power line frequencies (frequencies in the range 50-180 Hz). Due to the likelihood of contamination from local electrical power systems, or even from nonlocal systems, the measurements at each power line frequency were made at adjacent frequencies and the desired noise amplitudes obtained by interpolation. It is shown that the magnetic spectral densities of the natural noise typically lie in the range 20-600 fT/ square root Hz or, taking the 50/60-Hz frequencies specifically, in the range 150-600 fT/ square root Hz. These ranges should be typical for most locations on the Earth's surface and for most seasons. >

VLF heating of the lower ionosphere: Variation with magnetic latitude and electron density profile

Abstract: Recent results on the comparative efficiencies of VLF versus HF heating of a typical nocturnal midlatitude ionosphere have been extended. The effects of varying the magnetic latitude and electron density profile of the ionosphere are investigated. Our results indicate that the superior heating efficiency of VLF vs HF radiation and the resulting perturbation on VLF propagation is a latitude-dependent phenomenon, the effects being accentuated at low magnetic latitude and reversed at polar latitudes. The marked superiority of VLF heating is also found to be a feature of nocturnal ionospheres. VLF and HF radiation produce similar heating effects in ionospheres typical of summer daytime conditions. The optimum design of experiments for the study of the perturbation of VLF propagation by VLF heating transmitters is discussed.

Reflection of VLF radio waves at a junction in the Earth-ionosphere waveguide

Abstract: The conditions for the neglect of reflection of VLF modes incident on a junction between the daytime and nighttime Earth-ionosphere waveguides are considered. It is shown that reflections are indeed small but some interesting and possibly significant exceptions may occur at highly oblique incidence. In fact, total reflection may take place for night-to-day transmission at angles within several degrees of grazing. The same analytical model is also relevant to oblique transmission across a flat lying coast line. >

Some characteristics of the ELF/VLF radio noise measured near L'Aquila, Italy

Abstract: Measurements are reported of radio noise by an ELF/VLF radiometer located near L'Aquila, Italy, which at ELF/VLF frequencies can be considered a typical location for the Mediterranean area. The measurements provide information on the average and root mean square (RMS) amplitudes in 16 narrow frequency bands (5% bandwidth) distributed through the range of 10 Hz to 32 kHz. Later processing of these data gives the V/sub d/ statistic, which is a measure of the impulsiveness of the noise. The L'Aquila noise amplitudes tend to be higher during the summer than during the winter, particularly at the higher frequencies covered by the measurements, and the afternoon and evening amplitudes also tend to be higher than those measured at other times during the day. The amplitudes decline with increasing frequency approximately as f/sup -1/, whereas V/sub d/ tends to increase with frequency, indicating increased impulsiveness at the higher frequencies. >

A mechanism responsible for the observation of symmetric lower hybrid sidebands and a low frequency mode in the upper ionosphere

Abstract: A low-frequency ({approximately}500 Hz) mode in conjunction with symmetric sidebands, displaced by approximately 500 Hz off the carrier frequencies of 11.90 and 12.65 kHz, has been observed in experiments conducted with the Aureol 3 satellite at altitudes of 1200 km above the Komsomolsk-on-Amur Alpha Station (50{degrees}N, 135{degrees}E). These observations are examined and interpreted with an alternative mechanism based on the theory of Lee and Kuo. The proposed mechanism suggests that the injected VLF wave parametrically excites a purely growing mode and both the Stokes and anti-Stokes sidebands of the lower hybrid waves. The Doppler-shifted frequencies of these modes resulting from the satellite motion adequately account for both the sidebands and the low-frequency mode observed in the Aureol 3 experiments. 9 refs., 1 fig.

Long-Term Averages of Globally-Measured ELF/VLF Radio Noise

Abstract: : In this report we present the results of a statistical study of a year of data obtained as part of the STAR Laboratory's global survey of ELF/VLF radio noise (frequencies in the range 10 Hz to 32 kHz). The data included in the study were obtained from seven measurement instruments (radiometers) located at both high and low latitudes. Average amplitude and voltage deviation (d) statistics from each of the six stations were first divided into four three- month seasons and then averaged at every measurement frequency to give two separate sets of long-term averages. Long-term averages show characteristic trends in the measurements due to the manner in which noise propagates in the earth-ionosphere waveguide. The average amplitudes of the measurements decrease with frequency, reaching a minimum around 1 kHz, and rise again towards 10 kHz. The voltage deviation amplitudes generally rise with frequency except for a depression of values around 1 kHz.

Excitation of whistler mode signals via injection of polarized VLF waves with the Siple transmitter

Abstract: Whistler mode waves of various polarizations were transmitted by the Siple Station, Antarctica, VLF transmitter and received near the geomagnetic conjugate point at Lake Mistissini, Quebec. Crossed 21-km horizontal dipole antennas on top of the 2-km-thick ice sheet were used to transmit 2- to 4-kHz waves alternately with right-hand circular, left-hand circular, and linear polarizations. Excitation of a multiplicity of magnetospheric propagation paths and the received signal strength were observed to depend on the transmitter antenna polarization. Where whistler mode growth and emission triggering occurred, saturated peak values of received signals were independent of antenna polarization and initial injected power levels, in agreement with previous findings. Propagation paths of ducted Siple signals observed at Lake Mistissini were identified with propagation paths deduced from natural whistlers, from which the L shell values and equatorial number densities for the paths were calculated. A combination of L shell data and models of antenna coupling into the whistler mode may aid in the location of ducts. Dynamics Explorer 1 satellite recordings of unducted Siple signals showed trends similar to the ground data on ducted signals. The observations are discussed in the context of a simplified model of the coupling from the Siple antenna into the ionosphere, which provides reasonable agreement with observations.

Method and system for inferring the radio refractive index structure of the atmosphere from radio measurements

Abstract: A system and method are provided for determining in real time, the radio refractive index structure of the atmosphere. The method may be characterized as comprising the steps of: transmitting a radio frequency signal over a predetermined path; receiving the radio frequency signal; detecting the strength of the radio frequency signal; determining a radio propagation factor from the strength of the radio frequency signal; determining a base height function of the atmosphere; and employing the radio propagation factor in the base height function to determine a trapping layer base height of the atmosphere which represents the radio refractive index structure of the atmosphere.

Observation of local radio emission associated with type III radio bursts and Langmuir waves

Abstract: The first clear detection of fundamental and harmonic radiation from the type III radio source region is presented. This radiation is characterized by its lack of frequency drift, its short rise and decay times, its relative weakness compared to the remotely observed radiation and its temporal coincidence with observed Langmuir waves. The observations were made with the radio and plasma frequency (URAP) receivers on the Ulysses spacecraft between about 1 and 2 AU from the Sun.

Very low-frequency seismo-acoustic noise below the seafloor.

Abstract: The low‐frequency acoustic seismic experiment (LFASE), conducted in the Blake Bahama Basin in August and September of 1989, was the first experiment to measure VLF ambient ocean noise on a seismic array within a DSDP borehole. Four three‐component geophones were clamped to the borehole wall at 10, 40, 70, and 100 m below the seafloor. The frequency response of the acquisition system was flat from 4.5 to 40 Hz but meaningful ambient noise measurements were acquired down to 0.3 Hz. Both discrete sources (interpreted as local microearthquakes) and a continuous distributed source (identified as a mid‐Atlantic storm) were observed. The earthquakes are characterized as sudden arrivals of high‐amplitude (at least 10 dB above background), high‐frequency (above 20 Hz) signals with a coda decaying to background levels within 30 s. Within the upper 10 m of the seafloor these events have dominantly horizontal motion. Vertical motion becomes dominant at 70 m and by 100 m the events are indistinguishable from backgroun...

Very low frequency (VLF) chorus emissions: A topical survey

Abstract: In this paper we have tried to present, coherently, a review of the observations of natural VLF radio emissions, termed “chorus”, that have been made on the ground and aboard satellites, and also their dependence on various geomagnetic phenomena. We have highlighted current theoretical interpretations of the generation and propagation of chorus signals.

Absorption of very low frequency (vlf) waves in the whistler mode propagation in jovian ionosphere

Abstract: Making use of currently available theory of wave absorption, an attempt has been made to estimate the refractive indices and absorption coefficients for different wave frequencies during day and night times in the Jovian ionosphere. The results obtained have striking similarity with the corresponding results in the case of the Earth's ionosphere. It is concluded that VLF signals can be observed more easily during night times.

The effect of variability of ground conductivity on the calculation of atmospheric noise using thunderstorm‐based models

Abstract: Radio emissions from lightning discharges in thunderstorms are the primary source of atmospheric radio noise at very low and low frequencies (10–60 kHz). Attenuation of the lightning-induced signal between a source in a thunderstorm and a receiver occurs mainly by absorption in the ionosphere and in poorly conducting ground. The attenuation in poorly conducting ground depends primarily on the conductivity so uncertainty in this conductivity contributes directly to uncertainty in calculating the level of atmospheric noise. A thunderstorm-based computer model of atmospheric noise is used to evaluate the effect of this uncertainty in ground conductivity on calculation of atmospheric noise. It is found that this model produces only small variations in the level of atmospheric noise when the conductivity of the ground is varied. This result occurs because it employs large numbers of sources which tend to mask variations in the signal level on paths which cross low-conductivity ground.

Very Low Frequency Radio Astronomy from Lunar Orbit

Abstract: This paper discusses the use of very low frequency aperture synthesis as a probe of astrophysical phenomena. Specifically, the science achievable with the Lunar Observer Radio Astronomy Experiment (LORAE) is discussed. The density distribution and the degree of turbulence of the galactic plasma can be determined from its effect on background radio sources. These same absorption phenomena can be used to determine magnetic field strengths within astrophysical plasmas. Detection of unabsorbed synchrotron emission can provide information on the relativistic plasma component. The detection of new pulsars may prove feasible because of their steep spectra. Observations of planets in the solar system will be used to study their interaction with the solar wind. VLF studies of the Sun will provide new information on the outer layers of its atmosphere and magnetic field as well as provide insight on the propagation of MHD disturbances through the corona. The plasma in the earth's magnetic field can be observed in detail and as a function of time. This will prove invaluable in understanding auroral phenomena and reconnection events.

Effects of energetic particle events on VLF/LF propagation parameters/1989. Interim report. [VLF/LF (very low frequency/low frequency)]

Abstract: This report is a summary of disturbance effects of three major energetic particle events (PCA's) on VLF/LF propagation parameters as observed by the Phillips Laboratory High Resolution Oblique VLF/LF Ionosounder located in northern Greenland. Disturbance effects on ionospheric reflectivity parameters, including deduced reflection heights and plane wave reflection coefficients, are presented along with riometer, magnetometer, and satellite particle detector data. VLF/LF propagation, Ionospheric disturbances, Polar cap absorption events, Oblique ionosounding.

Application of wave guide propagation to selection of transmitter power and frequency

Abstract: This paper is concerned with developing reasonable criteria for the selection of frequency and power for very low and low-frequency (VLF/LF) transmitting stations. The approach uses a wave guide model for low-frequency propagation and accounts for the variability of the ionosphere. A sample problem involving a hypothetical transmitter is described.