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Very low frequency

About: Very low frequency is a research topic. Over the lifetime, 1540 publications have been published within this topic receiving 24233 citations. The topic is also known as: VLF.


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Journal ArticleDOI
TL;DR: In this article, the absorption of very low frequency (VLF) waves in the whistler mode of propagation at low geomagnetic latitudes of 24° and 15° have been calculated for various ionospheric conditions during sunspot maxima, average and minima.
Abstract: Absorption of very low frequency (VLF) waves in the whistler mode of propagation at low geomagnetic latitudes of 24° and 15° have been calculated for various ionospheric conditions during sunspot maxima, average and minima. The very high absorption values of 60dB, 46dB, 22dB at 24° and 104dB, 90dB, 42dB at 15° of VLF waves (10Kc/s, 5Kc/s and 1Kc/s) for two ionospheric paths between 50-200Km height can explain the non occurrence of whistlers and VLF emissions such as chorus and hiss at low geomagnetic latitudes during day time.The absorption contributed by F-region though generally smaller than that contributed by D and E layers is never negligible and at times of high F-layer concentrations during the magnetic storms the F-layer absorption may even be dominant.The absorption of VLF waves is low at night and as such can not explain the low occurrence of whistlers during night time. However, when the angle between the magnetic field and the wave normal is taken into account it is found that the absorption index increases considerably and hence the whistler energy may not be received on the ground especially at geomagnetic latitudes below 15°.Comparing the computed values of absorption of VLF waves by Altman and Cory at 40° geomagnetic latitude and Laschy at 55° latitude with our values, for low latitudes it is found that the absorption increases rapidly as the latitude decreases which is in conformity with the results of Helliwell and Altman and Cory.

4 citations

Journal ArticleDOI
TL;DR: Analysis of the tracking accuracy is given as a function of the dc offsets of the hard limiter and the flip-flop, the linear gain of the limiter, the signal-to-noise ratios of one or more input signals, and the power consumption of theLimiter.
Abstract: Hard limiters, followed by a D-type flip-flop as a digital-signal-polarity detector, are very effective receiver/phase detectors for low frequency (LF) and very low frequency (VLF) navigation receivers. However the performance not only depends on the signal quality, but also on the specifications of the hard limiter and the flip-flop. Analysis of the tracking accuracy is given as a function of the dc offsets of the limiter and the flip-flop, the linear gain of the limiter, the signal-to-noise ratios of one or more input signals, and the power consumption of the limiter. The results are presented in formulas and graphs for application by circuit designers. A design example of a low-power, high-gain limiter is given.

4 citations

Journal ArticleDOI
TL;DR: In this paper, the day to night shift in reflection height of very low frequency (VLF) waves has been calculated using D-region conductivities derived from IRI models and the results are compared with those obtained from phase variation measurements of VLF transmissions from Rugby (England) made at Visakhapatnam (India).
Abstract: Electron density profiles for the International Reference Ionosphere (IRI) 2001 and 2007 models have been utilized in evaluating the D-region conductivity parameter in earth ionosphere wave guide calculations. The day to night shift in reflection height of very low frequency (VLF) waves has been calculated using D-region conductivities derived from IRI models and the results are compared with those obtained from phase variation measurements of VLF transmissions from Rugby (England) made at Visakhapatnam (India). The values derived from the models are found to be much lower than those obtained from the experimental measurements. The values derived from the IRI models are in good agreement with those obtained from exponential conductivity model.

4 citations

Proceedings ArticleDOI
28 Apr 2009
TL;DR: In this paper, the passage of the day-night terminator across the transmitter-receiver path has a characteristic signature in the amplitude and phase of the signal and principal component analysis (PCA) was applied to distinguish between this regular diurnal pattern and exogenous perturbations.
Abstract: Very Low Frequency (VLF) waves propagate with insignificant attenuation in the Earth-ionosphere waveguide. Attenuation is, however, relatively higher on illuminated paths due to the presence of the ionospheric D region. Sub-ionospheric propagation conditions may be monitored using a narrowband receiver tuned to the stable signals from numerous VLF transmitters distributed across the surface of the Earth. The passage of the day-night terminator across the transmitter-receiver path has a characteristic signature in the amplitude and phase of the signal. Principal Component Analysis (PCA) was applied to distinguish between this regular diurnal pattern and exogenous perturbations.

4 citations

Proceedings ArticleDOI
15 Dec 2009
TL;DR: The receivers developed will contribute to the research of D-layer ionospheric activities and natural radio receiver which is used to detect natural radio signals such as sferics, whistlers emitted by the activities of lightning discharges.
Abstract: VLF (very low frequency) can be used to probe lower atmospheric activities. To detect lower atmospheric activities, two types of VLF receiver were developed in this project. The first type is called SID (sudden ionospheric disturbances) receiver which is used to detect the solar flares activities that affect communication systems. The second one is called natural radio receiver which is used to detect natural radio signals such as sferics, whistlers emitted by the activities of lightning discharges. A wire loop antena was developed to receive VLF signals from outer space for the receivers that has been developed. In the process of developing the VLF receiver, MULTISIM software. It was used to simulate the receiver's circuit developed for this purpose. The simulation results was adopted to design a prototype for the circuit for experiments carried out to measure the performance and the ability of the receivers in receiving VLF signals. Lab equipment, such as signal generators and oscilloscopes, were used to test the performance of the receivers. The receivers developed will contribute to the research of D-layer ionospheric activities.

4 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202320
202232
202156
202048
201942
201852