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.

A synchronization system of very low-frequency interferometers

Abstract: A solution to the problem of synchronization of spaced autonomous data-acquisition systems and obtaining stable reference-frequency signals for measuring phases of very low-frequency (VLF) radio signals is described. The expediency of using GPS receivers and tunable crystal oscillators in the synchronization system of VLF interferometers is grounded. Measurement results of the phase stability of the reference signal, received from the designed device, are presented. The short-term frequency stability of a 200-kHz pulse signal at the output of the designed unit is no more than 1.5 × 10−7, allowing one to measure the phase of VLF signals with an accuracy of no worse than ±0.81°.

An Architecture for High Data Rate Very Low Frequency Communication

Abstract: Very low frequency (VLF) communication is used for long range shore-to-ship broadcasting applications. This paper proposes an architecture for high data rate VLF communication using Gaussian minimum shift keying (GMSK) modulation and low delay parity check (LDPC) channel coding. Non-data aided techniques are designed and used for carrier phase synchronization, symbol timing recovery, and LDPC code frame synchronization. These require the estimation of the operative Eb/N0 for which a kurtosis based algorithm is used. Also, a method for modeling the probability density function of the received signal under the bit condition is presented in this regard. The modeling of atmospheric radio noise (ARN) that corrupts VLF signals is described and an algorithm for signal enhancement in the presence of ARN in given. The BER performance of the communication system is evaluated for bit rates of 400 bps, 600 bps, and 800 bps for communication bandwidth of ~200 Hz. Defence Science Journal, 2013, 63(1), pp.25 -33 , DOI:http://dx.doi.org/10.14429/dsj. 63.3760

The great earth storms in november 1960 as observed at kiruna geophysical observatory

Abstract: : The great earth storms caused by the solar flares, which occurred during the period 1020 November 1960, situated at the southern boundary of the auroral zone, are discussed. Possible physical connections between the various parameters recorded by means of a riometer, a VLF receiver, ionospheric sounder equipments, an oblique auroral reflection receiver, magnetometers and cosmic ray telescopes are discussed. Special attention is given to the following features: (1) the good correlation between disturbances of the geomagnetic field and ionospheric absorption of HF radio waves; (2) the world wide cosmic ray increases, the responsible solar proton energies and the extension of the polar cap; (3) unusual propagation conditions for VLF radio waves; (4) absence of auroral echoes at the time, when the strongest geomagnetic activity has been observed; and (5) observation of unusually low daytime reflection heights by means of a 3 Mc vertical ionospheric recorder. (Author)

Geophysical investigation of antique iron furnaces: insights from modelling magnetic and VLF data

Abstract: Magnetic and very low frequency electromagnetics (VLF) surveys at an Antique iron mine near Saales (France) revealed several large magnetic anomalies (of the order of a few hundred nT), strong electrical conductivity contrasts (10‐1000 Ω m) and out-of-quadrant VLF phases (φ >90°). To define both the location and the vertical extent of the underground artefacts, we interpret our magnetic data with Euler deconvolution and a 2D wavelet transform that combines maps of the total field and its gradient. We also explain out-of-quadrant VLF phases using simple 3D EM forward modelling. A joint interpretation of magnetic, VLF and topography data enables us to locate iron-ore exploitation trenches, Antique iron furnaces and their surrounding slags at depths between 1 and 3 m, in good agreement with results obtained during a recent road construction.

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.