Low-frequency Radio Noise Research Articles

Measurement and Analysis of Low-Frequency Radio Noise in a Typical Southern China Area

Background: External noise is a necessary factor for calculating the signal-to-noise ratio and evaluating communication quality and is one of the important parameters when designing low-frequency communication schemes or evaluating low-frequency signal quality. Methods: To study the radio noise distribution of each frequency point within the low-frequency band from 40kHz to 80kHz in a typical Southern China region at different time periods, the test team conducted a low-frequency radio noise test in Ziyang District, Yiyang City. Results and Discussion: The test data is analyzed and the results show that in this area, the radio noise in the 40kHz to 80kHz frequency band changes significantly day and night. The noise factors of most frequency points are in line with the predicted value given in the predicted value of the Radio Noise Volume of Recommendation ITU-R P.372-10. Conclusion: However, the noise distributions of 50kHz, 54kHz, 60kHz, and 68.5kHz are significantly higher than those of the predicted values.

Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates

Abstract. The transient electromagnetic method (TEM) is widely used for mapping subsurface resistivity structures, but data are inevitably contaminated by noise from various sources. It is common practice to gate signals from TEM systems to reduce the amount of data and improve the signal-to-noise ratio (SNR). Gating acts as a filter, and optimum gating will pass the TEM signal un-attenuated while suppressing noise. In systems based on analog boxcar integrators, the gating corresponds to filtering with a square window. The frequency response of this window shape has large side lobes, which are often insufficient in attenuating noise, e.g., from radio signals in the very low frequency (VLF) 3–30 kHz band. Tapered gates have better side lobe suppression and attenuate noise better, but tapering with analog boxcar integrators is difficult. We propose using many short boxcar gates, denoted sub-gates, and combine the sub-gates into semi-tapered gates to improve noise rejection at late gates where low signal normally leads to poor SNR. The semi-tapering approach is analyzed and tested experimentally on data from a roving TEM system. We quantify the effect of semi-tapered gates by computing an improvement factor as the ratio between the standard error of data measured with boxcar gates and the standard error of data measured with semi-tapered gates. Data from a test survey in Gedved, Denmark, with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centered between 78.7 and 978.1 µs. After inversion of the data, we find that semi-tapering increases the depth of investigation by about 20 % for this specific survey. We conclude that the semi-tapered approach is a viable path towards increasing SNR in TEM systems based on analog boxcar integrators.

Sprites in low‐frequency radio noise

Low‐frequency radio noise is the electromagnetic background radiation which is compared here to the luminosity of 39 sprites recorded with a low‐light video camera. It is found that the sprite luminosities coincide with ∼10–30 ms long sudden enhancements of the electromagnetic background radiation ∼6–8 μV m−1Hz−1/2(∼6–9 dB) with a relative maximum near ∼125 kHz as measured with a wideband (∼1–400 kHz) digital radio receiver. The sprites cluster in 10 groups of 2–5 consecutive sprites which are paralleled by up to ∼1 s long slowly varying enhancements of the electromagnetic background radiation ∼4–5 μV m−1Hz−1/2(∼2–4 dB). The observed electric field strengths place an upper bound on the low‐frequency radiation from the electron multiplication associated with the exponential growth and branching sprite streamers predicted by Qin et al. [2012a]. This upper bound corresponds to a maximum of ∼300–5000 sprite streamers at ∼40 km height above thunderclouds. Some part of the observed electromagnetic background radiation might result from the superposition of low‐frequency radiation emanating from the quick succession of numerous horizontal lightning strokes and/or stepped leaders inside thunderclouds which would constitute a fundamentally novel quasi‐static discharge process inside thunderclouds radiating slowly varying low frequency radio noise.

Analysis of Cloud Droplets Growth and Phase Transition Radiation Process

To have the clouds of steam condensate, there must have nuclei of condensation, which can be attachment with steam and gathered themselves together. The atmosphere contains a large number of small particles, they play a condensate core role in the process of water vapor condenses into water droplets. Hygroscopic condensation nucleus could reduce the saturation vapor pressure. Make environment relative humidity RH <100%, water vapor can condense. On this basis, the paper analyzed the increasing process of the cloud droplets: containing cloud droplets condensation (or sublimation) growth and cloud droplets colliding with each other growth. In the early stages of the cloud droplets growth, condensation growth is given to priority, collision growth for subordination. When the cloud droplets increases to a certain extent, condensation process will puts secondary status, and gravity collisions growth is given to priority. Finally, the paper analyzed the releasing mechanism of the latent heat and sensible heat in the process of clouds steam vapor condensation and cloud droplets growth: there are obvious leaks in atmospheric physics about the thermodynamic mechanism of cloud formation, the way of phase transition energy transferring to the atmosphere around is not by heat transfer, but in the way of emitting electromagnetic radiation, most of them radiating to the space,just a few of them radiating to the ground. Measuring of atmospheric Extremely low frequency and low frequency radio noise indicates that this view, at the same time, while many scholars have measured out infrared radiation in the experiment of water vapor condensation and Melt content crystallization, these have proved the existence of phase change radiation.

Natural atmospheric noise statistics from VLF measurements in the eastern Mediterranean

[1] Very low frequency (VLF) (3–30 kHz) and extremely low frequency (ELF) (3–3000 Hz) electromagnetic transient signals and noise are generated by various natural and anthropogenic processes. On a global basis by far the most significant source is ELF/VLF radiation from lightning propagating in the Earth-ionosphere waveguide. This atmospheric “noise,” originating essentially from lightning discharges, is the main source of interference for VLF/LF telecommunications. One of the statistical measures that is used to define the properties of low-frequency radio noise is the voltage deviation Vd, which is a measure of the impulsiveness of the noise that is widely used to characterize radio noise, particularly in the International Radio Consultative Committee reports. In this paper we present atmospheric noise statistics based on VLF measurements at different temporal resolution (from minutes to seasonal variations). For the first time we present analysis of the statistical parameters of Vd from continuous broadband VLF measurements for a period extending more than 1 year. Our analysis shows that the long-term observed Vd characteristics can be reasonably estimated as the sum of two Gaussians distribution functions, while the hourly and seasonal distributions of Vd values can be fitted using a single Gaussian distribution with different mean and variance values.

Remote sensing of the termination of the solar wind via in situ plasma measurements

Since 1983 the Plasma Wave experiments on the two Voyager spacecraft have detected low frequency radio noise in the outer heliosphere which has been postulated to emanate from the terminal shock of the solar wind or, possibly, from the heliopause itself. We have examined the solar wind data from the Plasma Science experiment on the Voyager spacecraft to search for correlations with these radio emissions. We find that two anomalous high speed streams passed Voyager 2 in late 1982 and early 1983 and suggest that the interaction of the streams with the heliospheric terminal shock is responsible for the generation of the most intense radio noise observed later in the same year. If the stream speeds did not decrease in traveling to the interaction region, that region is ∼135 A.U. from the sun. This is consistent with previous estimates of the distance to the inner heliospheric shock.

Spread-Spectrum Multiple-Access Performance of Orthogonal Codes: Linear Receivers

This paper analyzes a direct-sequence, spread-spectrum, multiple-access (SSMA) communication system which assigns a set of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> orthogonal sequences to each user. With all direct sequence SSMA systems, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</tex> users share a channel by phase modulating their transmissions with signature sequences. However, the users of our system transmit <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">log_{2}M</tex> bits of information/sequence. This contrasts classical SSMA schemes which use a pair of antipodal sequences and transmit 1 bit/sequence. In this paper, we assume that the channel noise is a combination of additive white Gaussian noise (AWGN) and multiple-access interference. We employ the optimum (single-user) demodulator for orthogonal signals in Gaussian noise. The multiple-user performance of this receiver is analyzed. We obtain approximations for the multiuser probability of error by using a Gaussian approximation for the multiple-access interference. We also obtain an upper bound on the exact probability by using characteristic functions. Our SSMA system is Well suited for application at the lower radio frequencies. Therefore, a companion paper describes a realistic model for low-frequency radio noise, modifies the receiver to include a zero-memory nonlinearity, and studies the performance of the nonlinear receiver.

Hook Whistler—a New Equatorial Whistler observed by Injun 3

THIS communication reports a new type of whistler discovered in the very low frequency radio noise data from the Injun 3 satellite. (See ref. 1 for details on the Injun 3 very low frequency experiment.)

The theory of the guiding of radio waves in the upper ionosphere

In a horizontally stratified magneto-ionic medium in which the electron density varies monotonically with height, tracks exist in which radio waves may be guided to great distances. This kind of propagation is studied using wave-guide mode theory. When one of the component waves in a guided signal gives rise to another by reflection, the phase is sometimes advanced and sometimes retarded by π 2 . An expression for the velocity of energy propagation in a mode is derived. The results of computations using this theory are presented and discussed. It is suggested that some low frequency radio noise, observed by satellites in the upper ionosphere, may originate at points remote from the satellite, and travel a large distance before being observed.

Rocket observations of very low frequency radio noise at night

Diffuse whistlers, with sometimes a single echo, were observed during a night-time rocket flight; none were observed on the ground at the same time. The whistlers were not ducted, but were loosely guided along the magnetic field. From the modulation of the whistlers and their echoes by the spinning vehicle, it is concluded that the wave normal angles were not within the limits required for penetration of the ionosphere. A band of hiss, centred at about 2.8 kc/s, 2 kc/s wide, and deeply modulated by spin, was also observed at the rocket but not on the ground. It is suggested that, even at night, many more V.L.F. phenomena can be observed above the ionosphere than on the ground.

Directional observations of 5 kc/s radiation from the Earth's outer atmosphere

Low-frequency radio noise bursts associated with geomagnetic disturbances have been observed with a network of direction-finding receivers in southeastern Australia during September and October 1959. Over a range of longitudes from 135°E to 155°E, 18 noise bursts came from apparent sources at latitudes greater than 42°S. On 8 occasions, isolated, discrete noise sources with an average geographical size of 550 km were detected at latitudes between 34°S and 42°S.