Very low frequency or
VLF refers to
radio frequenciesRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the
radio spectrumRadio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz ....
, only the very simplest signals are used, such as for
radio navigationRadio navigation or radionavigation is the application of radio frequencies to determine a position on the Earth. Like radiolocation, it is a type of radiodetermination.The basic principles are measurements from/to electric beacons, especially...
. Also known as the
myriametre band or
myriametre wave as the wavelengths range from ten to one myriametres (an obsolete metric unit equal to 10 kilometres).
Applications
VLF waves can penetrate
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
to a depth of roughly 10 to 40 metres (30 to 130 feet), depending on the frequency employed and the salinity of the water. VLF is used to
communicate with submarinesCommunication with submarines is difficult because radio waves do not travel well through thick electrical conductors like salt water.The obvious solution is to surface and raise an antenna above the water, then use ordinary radio transmissions. Early submarines had to frequently surface anyway for...
near the
surfaceIn mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...
(for example using the transmitter
DHO38The VLF transmitter DHO38 is a VLF transmitter used by the German Navy near Rhauderfehn, Saterland, Germany. It is used to transmit coded orders to submarines of the German Navy and navies of other NATO countries....
), while
ELFExtremely low frequency is a term used to describe radiation frequencies from 3 to 300 Hz. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz...
is used for deeply-submerged vessels. VLF is also used for radio navigation beacons (alpha) and time signals (beta).
VLF is also used in
electromagneticElectromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
geophysical surveys.
http://www.geonics.com/html/vlfsystems.html
Early in the history of radio engineering attempts were made to use radiotelephone using
amplitude modulationAmplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
and
single-sideband modulationSingle-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
within the band starting from 20 kHz, but the result was unsatisfactory because of the small available bandwidth.
The frequency range below 9 kHz is not allocated by the
International Telecommunication UnionThe International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...
and may be used in some nations license-free. Many natural
radioRadio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
emissions, such as
whistlerA whistler is a very low frequency electromagnetic wave which can be generated, for example, by lightning. Frequencies of terrestrial whistlers are 1 to 30 kHz, with maximum usually at 3 to 5 kHz. Although they are electromagnetic waves, they occur at audio frequencies, and can be...
s, can also be heard in this band.
In the USA, the
time signalA time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...
station WWVL began transmitting a 500 W signal on 20 kHz in August 1963. It used Frequency Shift Keying (
FSKFrequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
) to send data, shifting between 20 kHz and 26 kHz. The WWVL service was discontinued in July 1972.
The very long wave transmitter
SAQThe Grimeton VLF transmitter is a VLF transmission facility at Grimeton close to Varberg, Sweden. It has the only workable machine transmitter in the world and is classified as World Heritage Site....
at Grimeton near Varberg in
SwedenSweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
can be visited by the public at certain times, such as on
Alexanderson DayAlexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2nd of July , is the day of the open house at the Swedish government VLF transmitter Grimeton, call sign...
.
Details of VLF submarine communication methods
High power land-based 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 kilometres), with transmitted power anywhere from 20 kW to 2 MW. Submarines receive the signal using some form of towed antenna which floats just under the surface of the water – for example a BCAA (Buoyant Cable Array Antenna). Modern receivers, such as those produced by
Detica, use sophisticated
digital signal processingDigital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of 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.
Because of the low bandwidth available it is not possible to transmit audio signals, therefore all messaging is done with alphanumeric data at very low bit rates. Three types of
modulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
are used:
- OOK / CWK: On-Off Keying / Continuous Wave Keying. Simple Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
transmission mode where carrier on = mark and off = space. This is the simplest possible form of radio transmission, but it is difficult for transmitters to transmit high power levels, and the signal can easily be swamped by atmospheric noise, so this is only really used for emergencies or basic testing.
- FSK: Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...
. The oldest and simplest form of digital radio data modulation. Frequency is increased by 25 Hz (for example) from the carrier to indicate a binary “1” and reduced by 25 Hz to indicate binary “0”. FSK is used at rates of 50 bit/s and 75 bit/s.
- MSK: Minimum-shift keying
In digital modulation, minimum-shift keying is a type of continuous-phase frequency-shift keying that was developed in the late 1950s and 1960s. Similar to OQPSK, MSK is encoded with bits alternating between quadrature components, with the Q component delayed by half the symbol period. However,...
. A more sophisticated modulation method that uses less bandwidth for a given data rate than FSK. This is the normal mode for submarine communications today, and can be used at data rates up to 300 bit/s- or about 35 8-bit ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
characters per second (or the equivalence of a sentence every two seconds) – a total of 450 words per minute. For comparison, the NCTJ's shorthand requirement is 100WPM for newspapers or 80WPM for magazines.
Two alternative character sets may be used: 5-bit ITA2 or 8-bit
ASCIIThe American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
. 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, civilians cannot decode any encrypted messages because they most likely use
one-time padIn cryptography, the one-time pad is a type of encryption, which has been proven to be impossible to crack if used correctly. Each bit or character from the plaintext is encrypted by a modular addition with a bit or character from a secret random key of the same length as the plaintext, resulting...
s since the amount of text is so small.
PC-based VLF reception
PC based VLF reception is a simple method whereby anyone can pick up VLF signals using the advantages of modern computer technology. An aerial in the form of a coil of insulated wire is connected to the input of the soundcard of the PC (via a jack plug) and placed a few metres away from it.
Fast Fourier transformA fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform and its inverse. "The FFT has been called the most important numerical algorithm of our lifetime ." There are many distinct FFT algorithms involving a wide range of mathematics, from simple...
(FFT) software in combination with a sound card allows reception of all frequencies below the
Nyquist frequencyThe Nyquist frequency, named after the Swedish-American engineer Harry Nyquist or the Nyquist–Shannon sampling theorem, is half the sampling frequency of a discrete signal processing system...
simultaneously in the form of
spectrogramA spectrogram is a time-varying spectral representation that shows how the spectral density of a signal varies with time. Also known as spectral waterfalls, sonograms, voiceprints, or voicegrams, spectrograms are used to identify phonetic sounds, to analyse the cries of animals; they were also...
mes. Because CRT monitors are strong sources of noise in the VLF range, it is recommended to record the spectrograms on hard disk with any PC CRT monitors turned off.
These spectrograms show many signals, which may include VLF transmitters, the horizontal electron beam deflection of TV sets and sometimes
superpulseSuperpulse is a term for very strong man-made radio signals. The two most common instances of superpulses are those transmitted by the VLF transmitter GBZ, and occasionally by an unidentified station on 17.8 kHz....
s and twenty second pulses. The strength of the signal received can vary with a
Sudden Ionospheric DisturbanceA sudden ionospheric disturbance is an abnormally high ionization/plasma density in the D region of the ionosphere caused by a solar flare...
. These cause the ionization level to drop in the atmosphere. The result of this is that the VLF signal will reflect down to Earth with greater strength.
List of VLF transmissions
| Callsign |
Frequency |
Location of transmitter |
Remarks |
| - |
11.905 kHz |
Russia (various locations) |
Alpha-Navigation Alpha is a Russian system for long range radio navigation. RSDN in Russian stands for , i.e., radio-technical long-distance navigation system....
|
| - |
12.649 kHz |
Russia (various locations) |
Alpha-Navigation |
| - |
14.881 kHz |
Russia (various locations) |
|
| - |
15.625 kHz |
- |
Frequency for horizontal deflection of electron beam in CRTThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
televisions (576i576i is a standard-definition video mode used in PAL and SECAM countries. In digital applications it is usually referred to as "576i", in analogue contexts it is often quoted as "625 lines"...
) |
| - |
15.734 kHz |
- |
Frequency for horizontal deflection of electron beam in CRTThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
televisions (480i480i is the shorthand name for a video mode, namely the US NTSC television system or digital television systems with the same characteristics. The i, which is sometimes uppercase, stands for interlaced, the 480 for a vertical frame resolution of 480 lines containing picture information; while NTSC...
) |
| GBR |
15.8 kHz |
Rugby, England |
(Regular transmissions ceased April 2003) Many publications listed its frequency as 16 kHz |
| JXN |
16.4 kHz |
HelgelandGildeskål is a municipality in Nordland county, Norway. It is part of the Salten traditional region. The administrative centre of the municipality is the village of Inndyr. Gildeskål was established as a municipality on 1 January 1838 . Beiarn was separated from Gildeskål in 1853...
(Norway) |
|
| SAQ |
17.2 kHz |
Grimeton (Sweden) |
Only active at special occasions (Alexanderson Day Alexanderson Day, named after the Swedish radio engineer Ernst Fredrik Werner Alexanderson and held either on the last Sunday in June, or on the first Sunday in July, whichever comes closer to 2nd of July , is the day of the open house at the Swedish government VLF transmitter Grimeton, call sign...
) |
| - |
ca. 17.5 kHz |
? |
Twenty second pulses |
| NAA The VLF Transmitter Cutler is the United States Navy's very low frequency station at Cutler, Maine. The station provides one-way communication to the United States strategic submarine forces.- History :...
|
17.8 kHz |
VLF station (NAA) at Cutler, Mainehttp://www.random-abstract.com/radio/ |
Transmits occasionally Superpulse Superpulse is a term for very strong man-made radio signals. The two most common instances of superpulses are those transmitted by the VLF transmitter GBZ, and occasionally by an unidentified station on 17.8 kHz....
s |
| RDL/UPD/UFQE/UPP/UPD8 |
18.1 kHz |
Russia (various locations) |
|
| HWU HWU transmitter is a French facility for transmitting orders to submerged submarines of the French Navy. HWU transmitter, situated near Rosnay at 46°42'47"N, 1°14'39"E, is one of the largest radio transmitters in France and is clearly visible on satellite pictures...
|
18.3 kHz |
Le Blanc (France) |
Frequently inactive for longer periods |
| RKS |
18.9 kHz |
Russia (various locations) |
Rarely active |
| GBZ GBZ is the call sign of a British VLF transmitter on 19.6 kHz from Anthorn, Cumbria , which is involved with transmitting encrypted signals to submarines...
|
19.6 kHz |
Anthorn (Britain) |
Many operation modes, even Superpulse Superpulse is a term for very strong man-made radio signals. The two most common instances of superpulses are those transmitted by the VLF transmitter GBZ, and occasionally by an unidentified station on 17.8 kHz....
s. |
| NWC Naval Communication Station Harold E. Holt is located on the northwest coast of Australia, north of the town of Exmouth, Western Australia. The town of Exmouth was built at the same time as the communications station to provide support to the base and to house dependent families of U.S...
|
19.8 kHz |
Exmouth, Western Australia (AUS) |
Used for submarine communication, 1 Megawatt. |
| ICV |
20.27 kHz |
TavolaraTavolara is a small island off the northeast coast of Sardinia, Italy. The island is a limestone massif 5 kilometres long and 1 kilometre wide, with steep cliffs except at its ends. Its highest point, Monte Cannone, is 565 metres above sea level. A cove and beach can be found at each end of the...
(Italia) |
|
| RJH63, RJH66, RJH69 The "Vileyka" VLF transmitter is the site of the 43rd Communications Center of the Russian Navy , situated west of the town of Vileyka in Belarus . The "Vileyka" VLF transmitter is an important facility for transmitting orders to submarines in the very low frequency range...
, RJH77, RJH99 |
20.5 kHz |
Russia (various locations) |
Time signal transmitter Beta Beta is a time signal service in the VLF range in Russia, operated by the Russian navy. There are 6 transmitter stations, which take turns transmitting time signals and other communications....
|
| ICV |
20.76 kHz |
Tavolara (Italia) |
|
| HWU HWU transmitter is a French facility for transmitting orders to submerged submarines of the French Navy. HWU transmitter, situated near Rosnay at 46°42'47"N, 1°14'39"E, is one of the largest radio transmitters in France and is clearly visible on satellite pictures...
|
20.9 kHz |
Le Blanc (France) |
|
| RDL |
21.1 kHz |
Russia (various locations) |
rarely active |
| NPM VLF transmitter Lualualei is a facility of the United States Navy near Lualualei, Hawaii transmitting orders to submerged submarines in the very low frequency range. VLF transmitter Lualualei, which operates under the callsign NPM on 21.4 kHz and 23.4 kHz, uses as antenna two guyed...
|
21.4 kHz |
Hawaii (USA) |
|
| HWU HWU transmitter is a French facility for transmitting orders to submerged submarines of the French Navy. HWU transmitter, situated near Rosnay at 46°42'47"N, 1°14'39"E, is one of the largest radio transmitters in France and is clearly visible on satellite pictures...
|
21.75 kHz |
Le Blanc (France) |
|
| GBZ GBZ is the call sign of a British VLF transmitter on 19.6 kHz from Anthorn, Cumbria , which is involved with transmitting encrypted signals to submarines...
|
22.1 kHz |
Skelton (Britain) |
|
| - |
22.2 kHz |
Ebino (Japan) |
|
| ? |
22.3 kHz |
Russia? |
Only active on 2nd of each month for a short period between 11:00 and 13:00 (respectively 10:00 and 12:00 in winter), if 2nd of each month is not a Sunday |
| RJH63, RJH66, RJH69 The "Vileyka" VLF transmitter is the site of the 43rd Communications Center of the Russian Navy , situated west of the town of Vileyka in Belarus . The "Vileyka" VLF transmitter is an important facility for transmitting orders to submarines in the very low frequency range...
, RJH77, RJH99 |
23 kHz |
Russia (various locations) |
Time signal transmitter Beta |
| DHO38 The VLF transmitter DHO38 is a VLF transmitter used by the German Navy near Rhauderfehn, Saterland, Germany. It is used to transmit coded orders to submarines of the German Navy and navies of other NATO countries....
|
23.4 kHz |
near Rhauderfehn Rhauderfehn is a municipality in the Leer district, in Lower Saxony, Germany....
(Germany) |
submarine communication |
| NAA The VLF Transmitter Cutler is the United States Navy's very low frequency station at Cutler, Maine. The station provides one-way communication to the United States strategic submarine forces.- History :...
|
24 kHz |
Cutler, Maine (USA) |
Used for submarine communication, at 2 megawatts. http://www.globalsecurity.org/military/facility/cutler.htm |
| NLF |
24.8 kHz |
Arlington, Washington (USA) |
Used for submarine communication. http://www.vlf.it/trond2/20-25khz.htmlhttp://ludb.clui.org/ex/i/WA3248/ |
External links