Atmospheric noise
Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, there are about 40 lightning flashes per second – ≈3.5 million lightning discharges per day.[1]
Contents
History
In 1925, AT&T Bell Laboratories started investigating the sources of noise in its transatlantic radio telephone service.[2]
Karl Jansky, a 22-year-old researcher, undertook the task. By 1930, a radio antenna for a wavelength of 14.6 meters was constructed in Holmdel, NJ, to measure the noise in all directions. Jansky recognized three sources of radio noise.[3] The first (and strongest) source was local thunderstorms. The second source was weaker noise from more distant thunderstorms. The third source was a still weaker hiss that turned out to be galactic noise from the center of the Milky Way. Jansky's research made him the father of radio astronomy.[4]
Lightning
Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. It is mainly caused by cloud-to-ground flashes as the current is much stronger than that of cloud-to-cloud flashes.[citation needed] On a worldwide scale, 3.5 million lightning flashes occur daily. This is about 40 lightning flashes per second.[1]
The sum of all these lightning flashes results in atmospheric noise. It can be observed,[5] with a radio receiver, in the form of a combination of white noise (coming from distant thunderstorms) and impulse noise (coming from a near thunderstorm). The power-sum varies with seasons and nearness of thunderstorm centers.
Although lightning has a broad-spectrum emission, its noise power increases with decreasing frequency. Therefore, at very low frequency and low frequency, atmospheric noise often dominates, while at high frequency, man-made noise dominates in urban areas.
Survey
From 1960s to 1980s, a worldwide effort was made to measure the atmospheric noise and variations. Results have been documented in CCIR Report 322.[6][7] CCIR 322 provided seasonal world maps showing the expected values of the atmospheric noise figure Fa at 1 MHz during four hour blocks of the day. Another set of charts relates the Fa at 1 MHz to other frequencies. CCIR Report 322 has been superseded by ITU P.372[8] publication.
Random number generation
Atmospheric noise and variation is also used to generate high quality random numbers.[9] Random numbers have interesting applications in the security domain.[10]
See also
Footnotes
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- ↑ Singh 2005, pp. 402–408
- ↑ Singh 2005, pp. 404–405
- ↑ Singh 2005, p. 406
- ↑ Sample of atmospheric noise http://www.ycars.org/EFRA/audio%20files/atmospheric%20noise.mp3
- ↑ Lua error in package.lua at line 80: module 'strict' not found.; first CCIR Report 322 was 1963; revised; second is ISBN 92-61-01741-X.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.; also DTIC
- ↑ ITU, Recommendation P.372: Radio Noise http://www.itu.int/rec/R-REC-P.372/en
- ↑ Lua error in package.lua at line 80: module 'strict' not found., self-published.
- ↑ http://www.random.org/
