Atmospheric noise

CCIR 322 atmospheric noise relationship. The standard has tables and maps that determine the noise figure at 1 MHz according to the season and the time of day. This graph converts that noise figure to other frequencies. Notice that the graph is spaced in 10 dB increments at 1 MHz.

Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, eight million lightning discharges per day — about 100 lightning flashes per second.

History

In 1925, AT&T Bell Laboratories started investigating the sources of noise in its transatlantic radio telephone service^[1].

Replica of Jansky's radio telescope, now at the National Radio Astronomy Observatory. 38°25′54″N 79°48′59″W / 38.431659°N 79.816253°W / 38.431659; -79.816253

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^[2]. 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^[3].

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 for cloud-to-cloud flashes.^{[citation needed]} On a worldwide scale, eight million lightning flashes occur daily. This is about 100 lightning flashes per second.^{[citation needed]}

The sum of all these lightning flashes results in atmospheric noise. It can be observed^[4] 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 rural areas.

Survey

From 1960s to 1980s, a worldwide effort was made to measure the atmospheric noise and variations. This has been documented in the CCIR 322^[5] publication. CCIR 322 provided seasonal world maps showing the expected values of the atmospheric noise figure F_a at 1 MHz during four hour blocks of the day. Another set of charts relates the F_a at 1 MHz to other frequencies. CCIR Report 322 has been superseded by ITU P.372^[6] publication.

Random number generation

Atmospheric noise and variation is also used to generate high quality random numbers.^[7] Random numbers have interesting applications in the security domain.^[8]

See also

• Radio atmospheric

References

• Singh, Simon (2005), Big Bang: The Origin of the Universe, Harper Perrennial, ISBN 978-0-00-716221-5 
• Spaulding, Arthur D.; Washburn, James S. Washburn (April, 1985), Atmospheric Radio Noise: Worldwide Levels and Other Characteristics, NTIA Report TR-85-173, Boulder, CO: U. S. Department of Commerce, National Telecommunications & Information Administration, Institute for Telecommunications Sciences, http://www.its.bldrdoc.gov/pub/ntia-rpt/85-173/