Ultra low frequency (ULF) is the

ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...

designation for the

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

range of electromagnetic waves between 300

hertz The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that on ...

and 3 kilohertz, corresponding to wavelengths between 1,000 to 100 km. In magnetosphere science and

seismology Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other ...

, alternative definitions are usually given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, and 10 mHz to 10 Hz. Many types of waves in the ULF frequency band can be observed in the magnetosphere and on the ground. These waves represent important physical processes in the near-Earth plasma environment. The speed of the ULF waves is often associated with the Alfvén velocity that depends on the ambient magnetic field and plasma mass density. This band is used for communications in

mine Mine, mines, miners or mining may refer to: Extraction or digging * Miner, a person engaged in mining or digging *Mining, extraction of mineral resources from the ground through a mine Grammar *Mine, a first-person English possessive pronoun ...

s, as it can penetrate the earth.

Earthquakes

Some monitoring stations have reported that earthquakes are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the 1989 Loma Prieta earthquake in

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

, although a subsequent study indicates that this was little more than a sensor malfunction. On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 M_w 2010 earthquake. Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes.

Earth mode communications

ULF has been used by the military for secure communications through the ground. NATO

AGARD The Advisory Group for Aerospace Research and Development (AGARD) was an agency of NATO that existed from 1952 to 1996. AGARD was founded as an Agency of the NATO Military Committee. It was set up in May 1952 with headquarters in Neuilly sur Sein ...

publications from the 1960s detailed many such systems, although it is possible that the published papers left a lot unsaid about what actually was developed

secretly "Secretly" is a popular song. It was written by Al Hoffman, Dick Manning, Mark Markwell, and Hugo Peretti and Luigi Creatore and published in 1958. The best-known recording of the song was done by Jimmie Rodgers, which was a gold record. Chart ...

for defense purposes. Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

Radio amateurs An amateur radio operator is someone who uses equipment at an amateur radio station to engage in two-way communication, two-way personal communications with other amateur operators on Frequency, radio frequencies Amateur radio frequency allocatio ...

and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

pairs hammered into the soil. At the receiving end, the signal is detected as a weak electric current between a further pair of electrodes. Using weak signal reception methods with PC-based

DSP DSP may refer to: Computing * Digital signal processing, the mathematical manipulation of an information signal * Digital signal processor, a microprocessor designed for digital signal processing * Yamaha DSP-1, a proprietary digital signal ...

filtering with extremely narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10100W and electrode spacing of around 1050m.

References

External articles

* Tomislav Stimac, "
Definition of frequency bands (VLF, ELF... etc.)
'". IK1QFK Home Page (vlf.it).

* Amateur Radio Below 10 kHz

* Review of Earth Mode Communications
1966 abstract about Earth Mode Comms by Ames, Frazier and Orange
* Radio communications within the Earth's crust
Abstract of article by Burrows written in 1963
{{DEFAULTSORT:Ultra Low Frequency Radio spectrum

Earthquakes

Earth mode communications

See also

References

External articles