Editing Very low frequency (section)

== Applications ==
[[Image:Grimetonmasterna.jpg|thumb|right|[[T antenna|Flattop antenna]] towers of the [[Grimeton Radio Station|Grimeton VLF transmitter]], Varberg, Sweden]]

=== Early wireless telegraphy ===
Historically, this band was used for long distance transoceanic radio communication during the [[wireless telegraphy]] era between about 1905 and 1925. Nations built networks of high-power LF and VLF [[radiotelegraphy]] stations that transmitted text information by [[Morse code]], to communicate with other countries, their colonies, and naval fleets. Early attempts were made to use radiotelephone using [[amplitude modulation]] and [[single-sideband modulation]] within the band starting from 20&nbsp;kHz, but the result was unsatisfactory because the available bandwidth was insufficient to contain the [[sideband]]s.

In the 1920s the discovery of the [[skywave]] (skip) radio propagation method allowed lower power transmitters operating at [[high frequency]] to communicate at similar distances by reflecting their radio waves off a layer of [[ionization|ionized]] atoms in the [[ionosphere]], and long-distance radio communication stations switched to the [[shortwave]] frequencies. The [[Grimeton VLF transmitter]] at Grimeton near Varberg in [[Sweden]], one of the few remaining transmitters from that era that has been preserved as a historical monument, can be visited by the public at certain times, such as on [[Alexanderson Day]].

=== Navigation beacons and time signals ===
Due to its long propagation distances and stable phase characteristics, during the 20th&nbsp;century the VLF band was used for long range [[hyperbolic navigation|hyperbolic]] [[radio navigation]] systems which allowed ships and aircraft to determine their geographical position by comparing the phase of radio waves received from fixed VLF [[navigation beacon]] transmitters.

The worldwide [[Omega (navigation system)|Omega]] system used frequencies from 10 to 14&nbsp;kHz, as did Russia's [[Alpha (navigation)|Alpha]].

VLF was also used for [[standard time and frequency signal|standard time and frequency]] broadcasts. In the US, the [[time signal]] station [[WWVL]] began transmitting a 500&nbsp;W signal on 20&nbsp;kHz in August&nbsp;1963. It used frequency-shift keying ([[Frequency-shift keying|FSK]]) to send data, shifting between 20&nbsp;kHz and 26&nbsp;kHz. The WWVL service was discontinued in July&nbsp;1972.

=== Geophysical and atmospheric measurement ===
Naturally occurring signals in the VLF band are used by [[geophysicist]]s for long range lightning location and for research into atmospheric phenomena such as the aurora. Measurements of [[Whistler (radio)|whistlers]] are employed to infer the physical properties of the [[magnetosphere]].<ref>{{Cite web |title=AWDANet |url=http://plasmon.elte.hu/awdanet.htm |website=plasmon.elte.hu}}</ref>

[[Geophysicists]] use VLF-[[electromagnetic]] receivers to measure conductivity in the near surface of the Earth.<ref>{{Cite web |title=Geonics Limited - VLF Receivers |url=http://www.geonics.com/html/vlfsystems.html |access-date=13 June 2014}}</ref>

VLF signals can be measured as a [[Geophysics|geophysical]] [[electromagnetic]] survey that relies on transmitted currents inducing secondary responses in conductive geologic units. A VLF anomaly represents a change in the attitude of the electromagnetic vector overlying conductive materials in the subsurface.

=== Mine communication systems ===
VLF can also penetrate soil and rock for some distance, so these frequencies are also used for [[through-the-earth mine communications]] systems.

=== Military communications ===
Powerful VLF transmitters are used by the military to communicate with their forces worldwide. The advantage of VLF frequencies is their long range, high reliability, and the prediction that in a [[nuclear war]] VLF communications will be less disrupted by nuclear explosions than higher frequencies. Since it can penetrate seawater VLF is used by the military to [[Communication with submarines|communicate with submarines]] near the surface, while [[Extremely low frequency|ELF]] frequencies are used for deeply submerged subs.

Examples of naval VLF transmitters are
* Britain's [[Skelton Transmitting Station]] in Skelton, Cumbria
* Germany's [[VLF transmitter DHO38|DHO38]] in [[Rhauderfehn]], which transmits on 23.4&nbsp;kHz with a power of 800&nbsp;kW
* U.S. [[Jim Creek Naval Radio Station]] in [[Oso, Washington]] state, which transmits on 24.8&nbsp;kHz with a power of 1.2&nbsp;MW
* U.S. [[VLF Transmitter Cutler|Cutler Naval Radio Station]] at [[Cutler, Maine]] which transmits on 24&nbsp;kHz with 1.8&nbsp;MW.

Since 2004 the [[US Navy]] has stopped using ELF transmissions, with the statement that improvements in VLF communication has made them unnecessary, so it may have developed technology to allow submarines to receive VLF transmissions while at operating depth.

High power land-based and aircraft transmitters in countries that operate submarines send signals that can be received thousands of miles away. Transmitter sites typically cover great areas (many [[acres]] or square kilometers), with transmitted power anywhere from 20&nbsp;kW to 2,000&nbsp;kW. Submarines receive signals from land based and aircraft transmitters using some form of towed antenna that floats just under the surface of the water – for example a [[Buoyant Cable Array Antenna]] (BCAA).

Modern receivers use sophisticated [[digital signal processing]] techniques to remove the effects of atmospheric noise (largely caused by lightning strikes around the world) and adjacent channel signals, extending the useful reception range. Strategic nuclear bombers of the United States Air Force receive VLF signals as part of hardened nuclear resilient operations.

Two alternative character sets may be used: 5&nbsp;bit [[ITA2]] or 8&nbsp;bit [[ASCII]]. Because these are military transmissions they are almost always [[encrypted]] for security reasons. Although it is relatively easy to receive the transmissions and convert them into a string of characters, enemies cannot decode the encrypted messages; military communications usually use unbreakable [[one-time pad]] [[cipher]]s since the amount of text is so small.