| 词条 | Atmospheric noise |
| 释义 |
HistoryIn 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]LightningAtmospheric 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|date=April 2011}} 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. SurveyFrom 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 CCIR Report 322 has been superseded by ITU P.372[8] publication.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 generationAtmospheric 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
Footnotes1. ^1 {{cite web |title=Annual Lightning Flash Rate Map |series=Science On a Sphere |publisher=NOAA |accessdate=15 May 2014 |url=http://www.sos.noaa.gov/Datasets/dataset.php?id=6}} 2. ^{{Harvnb|Singh|2005|pp=402–408}} 3. ^{{Harvnb|Singh|2005|pp=404–405}} 4. ^{{Harvnb|Singh|2005|p=406}} 5. ^Sample of atmospheric noise {{cite web|url=http://www.ycars.org/EFRA/audio%20files/atmospheric%20noise.mp3 |title=Archived copy |accessdate=2008-03-14 |deadurl=yes |archiveurl=https://web.archive.org/web/20051218032644/http://www.ycars.org/EFRA/audio%20files/atmospheric%20noise.mp3 |archivedate=2005-12-18 |df= }} 6. ^{{Citation |author=International Radio Consultative Committee |title=Characteristics and Applications of Atmospheric Radio Noise Data |id=CCIR Report 322-3 |location=Geneva |publisher=International Telecommunications Union |year=1968 }}; first CCIR Report 322 was 1963; [https://books.google.com/books?id=1ls2AQAAIAAJ revised]; second is {{ISBN|92-61-01741-X}}. 7. ^{{Citation|last=Lawrence |first=D. C. |title=CCIR Report 322 Noise Variation Parameters |date=June 1995 |id=NRaD Technical Document 2813 |url=http://www.spawar.navy.mil/sti/publications/pubs/td/2813 |location=San Diego, CA |publisher=Naval Command, Control and Ocean Surveillance Center, RDT&E Division |doi= |deadurl=yes |archiveurl=https://web.archive.org/web/20091113023506/http://www.spawar.navy.mil/sti/publications/pubs/td/2813/ |archivedate=2009-11-13 |df= }}; also DTIC 8. ^ITU, Recommendation P.372: Radio Noise http://www.itu.int/rec/R-REC-P.372/en 9. ^{{Citation |last=Haahr |first=Mads |title=Introduction to Randomness and Random Numbers |publisher=random.org |accessdate= November 14, 2011 |url=http://www.random.org/randomness/}}, self-published. 10. ^http://www.random.org/ References
|first= Simon |last= Singh |author-link= Simon Singh |title= The Origin of the Universe |publisher= Harper Perennial |year= 2005 |isbn= 978-0-00-716221-5 |doi=}}
|first= Arthur D. |last= Spaulding |first2= James S. |last2= Washburn |title= Atmospheric Radio Noise: Worldwide Levels and Other Characteristics |date= April 1985 |publisher= U. S. Department of Commerce, National Telecommunications & Information Administration, Institute for Telecommunications Sciences |series= NTIA Report TR-85-173 |location= Boulder, CO |url= http://www.its.bldrdoc.gov/pub/ntia-rpt/85-173/ |doi= }}{{Noise}} 1 : Noise (electronics) |
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