A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries. Most identified stations use speech synthesis to vocalize numbers, although digital modes such as phase-shift keying and frequency-shift keying, as well as Morse code transmissions, are not uncommon. Most stations have set time schedules, or schedule patterns; however, other stations appear to broadcast at random times. Stations may or may not have set frequencies in the HF band.
The first known use of numbers stations was during World War I, with the numbers transmitted in Morse code. Numbers stations were most prolific during the Cold War era, yet many continue to broadcast and some long-time stations may have been taken over by different operators. The Czech Ministry of the Interior and the Swedish Security Service have both acknowledged the use of numbers stations by Czechoslovakia for espionage, with declassified documents proving the same. With a few exceptions, no QSL responses have been received from numbers stations by shortwave listeners who sent reception reports to said stations, which is the expected behaviour of a non-clandestine station.
One well-known numbers station was the E03 "Lincolnshire Poacher", which is thought to have been run by the British Secret Intelligence Service. It was first broadcast from Bletchley Park in the mid-1970s but later was broadcast from RAF Akrotiri in Cyprus.
In 2001, the United States tried the Cuban Five on the charge of spying for Cuba. That group had received and decoded messages that had been broadcast from Cuban numbers stations. Also in 2001, Ana Belén Montes, a senior US Defense Intelligence Agency analyst, was arrested and charged with espionage. The federal prosecutors alleged that Montes was able to communicate with the Cuban Intelligence Directorate through encoded messages, with instructions being received through "encrypted shortwave transmissions from Cuba". In 2006, Carlos Alvarez and his wife, Elsa, were arrested and charged with espionage. The U.S. District Court Florida[which?] stated that "defendants would receive assignments via shortwave radio transmissions".
In June 2009, the United States similarly charged Walter Kendall Myers with conspiracy to spy for Cuba, and receiving and decoding messages broadcast from a numbers station operated by the Cuban Intelligence Directorate to further that conspiracy.
It has been reported that the United States used numbers stations to communicate encoded information to persons in other countries. There are also claims that State Department-operated stations, such as KKN50 and KKN44, used to broadcast similar "numbers" messages or related traffic, although these radio stations have been off air for many years.
Monitoring transmissions from numbers stations has become an activity for hobbyists, and Ham radio enthusiasts dating from the 1980s, though it is reported that Archduke Anton of Austria, in his youth used to listen in to their transmissions, writing them down and passing them on to the Austrian military intelligence.
Suspected origins and use
It has long been speculated, and was argued in court in one case, that these stations operate as a simple and foolproof method for government agencies to communicate with spies working undercover. According to this theory, the messages are encrypted with a one-time pad, to avoid any risk of decryption by the enemy. As evidence, numbers stations have changed details of their broadcasts or produced special, nonscheduled broadcasts coincident with extraordinary political events, such as the attempted coup of August 1991 in the Soviet Union.
The one-way voice link (OWVL) described a covert communications system that transmitted messages to an agent's unmodified shortwave radio using the high-frequency shortwave bands between 3 and 30 MHz at a predetermined time, date, and frequency contained in their communications plan. The transmissions were contained in a series of repeated random number sequences and could only be deciphered using the agent's one-time pad. If proper tradecraft was practised and instructions were precisely followed, an OWVL transmission was considered unbreakable. As long as the agent's cover could justify possessing a shortwave radio and he was not under technical surveillance, high-frequency OWVL was a secure and preferred system for the CIA during the Cold War.
High-frequency radio signals transmitted at relatively low power can travel around the world under ideal propagation conditions—which are affected by local RF noise levels, weather, season, and sunspots—and can then be received with a properly tuned antenna (of adequate size) and a good receiver. However, spies often have to work only with available hand-held receivers, sometimes under difficult local conditions, and in all seasons and sunspot cycles.
A 1998 article in The Daily Telegraph quoted a spokesperson for the Department of Trade and Industry (the government department that, at that time, regulated radio broadcasting in the United Kingdom) as saying "These [numbers stations] are what you suppose they are. People shouldn't be mystified by them. They are not for, shall we say, public consumption."
The use of numbers stations as a method of espionage is discussed in Spycraft:
The only item Penkovsky used that could properly be called advanced tradecraft was his 'agent-receive' communications through a one-way voice-link. These encoded messages, known as OWVL, were broadcast over shortwave frequencies at predetermined times from a CIA-operated transmitter in Western Europe. Penkovsky listened to these messages on a Panasonic radio—strings of numbers read in a dispassionate voice—and then decoded them using a one-time pad.
North Korea revived number broadcasts in July 2016 after a hiatus of sixteen years, a move which some analysts regarded as psychological war; sixteen such broadcasts occurred in 2017, including unusually timed transmissions in April.
Identifying and locating
Numbers stations are often given nicknames by enthusiasts, often reflecting some distinctive element of the station such as the interval signal. For example, the "Lincolnshire Poacher", formerly one of the best-known numbers stations (generally thought to be run by the Secret Intelligence Service, because its transmissions have been traced to RAF Akrotiri in Cyprus), played the first two bars of the folk song "The Lincolnshire Poacher" before each string of numbers.
According to an internal Cold War-era report of the Polish Ministry of the Interior, numbers stations DCF37 (3370 kHz) and DFD21 (4010 kHz) transmitted from West Germany beginning in the early 1950s.
Atención spy case
The "Atención" station of Cuba became the world's first numbers station to be officially and publicly accused of transmitting to spies. It was the centrepiece of a United States federal court espionage trial, following the arrest of the Wasp Network of Cuban spies in 1998. The U.S. prosecutors claimed the accused were writing down number codes received from Atención, using Sony hand-held shortwave receivers, and typing the numbers into laptop computers to decode spying instructions. The FBI testified that they had entered a spy's apartment in 1995, and copied the computer decryption program for the Atención numbers code. They used it to decode Atención spy messages, which the prosecutors unveiled in court.
The United States government's evidence included the following three examples of decoded Atención messages. (It was not reported whether the original clear texts were in Spanish, although the phrasing of "Day of the Woman" would indicate so.)
- "prioritize and continue to strengthen friendship with Joe and Dennis"
- "Under no circumstances should [agents] German nor Castor fly with BTTR or another organization on days 24, 25, 26 and 27." (BTTR is the anti-Castro airborne group Brothers to the Rescue)
- "Congratulate all the female comrades for International Day of the Woman." (Probably a simple greeting for International Women's Day on 8 March)
The moderator of an e-mail list for global numbers station hobbyists claimed that "Someone on the Spooks list had already cracked the code for a repeated transmission [from Havana to Miami] if it was received garbled." Such code-breaking is possible if a one-time pad decoding key is used more than once. If used properly, however, the code cannot be broken.
Generally, numbers stations follow a basic format, although there are many differences in details between stations. Transmissions usually begin on the hour or half-hour.
The prelude, introduction, or call-up of a transmission (from which stations' informal nicknames are often derived) includes some kind of identifier, for the station itself, the intended recipient, or both. This can take the form of numeric or radio-alphabet "code names" (e.g. "Charlie India Oscar", "250 250 250", "Six-Niner-Zero-Oblique-Five-Four"), characteristic phrases (e.g. "¡Atención!", "Achtung!", "Ready? Ready?", "1234567890"), and sometimes musical or electronic sounds (e.g. "The Lincolnshire Poacher", "Magnetic Fields"). Sometimes, as in the case of radio-alphabet stations, the prelude can also signify the nature or priority of the message to follow (e.g., it may indicate that no message follows). Often the prelude repeats for a period before the body of the message begins.
After the prelude, there is usually an announcement of the number of number-groups in the message, the page to be used from the one-time pad, or other pertinent information. The groups are then recited. Groups are usually either four or five digits or radio-alphabet letters. The groups are typically repeated, either by reading each group twice or by repeating the entire message as a whole.
Some stations send more than one message during a transmission. In this case, some or all of the above process is repeated, with different contents.
Finally, after all the messages have been sent, the station will sign off in some characteristic fashion. Usually, it will simply be some form of the word "end" in whatever language the station uses (e.g., "End of message; End of transmission", "Ende", "Fini", "Final", "конец"). Some stations, especially those thought to originate from the former Soviet Union, end with a series of zeros, e.g., "00000" "000 000"; others end with music or other sounds.
Because of the secretive nature of the messages, the cryptographic function employed by particular stations is not publicly known, except in one (or possibly two) cases. It is assumed that most stations use a one-time pad that would make the contents of these number groups indistinguishable from randomly generated numbers or digits. In one confirmed case, West Germany did use a one-time pad for numbers transmissions.
Although few numbers stations have been tracked down by location, the technology used to transmit the numbers has historically been clear—stock shortwave transmitters using powers from 10 kW to 100 kW.
Amplitude modulated (AM) transmitters with optionally–variable frequency, using class-C power output stages with plate modulation, are the workhorses of international shortwave broadcasting, including numbers stations.
Application of spectrum analysis to numbers station signals has revealed the presence of data bursts, radioteletype-modulated subcarriers, phase-shifted carriers, and other unusual transmitter modulations like polytones. (RTTY-modulated subcarriers were also present on some U.S. commercial radio transmissions during the Cold War.)
For spies in the field, low-tech spoken number transmissions continue to have advantages in the 21st century. High-tech data-receiving equipment is difficult to obtain, and being caught with just a shortwave radio has a degree of plausible deniability that no spying is being conducted.
Interfering with other broadcasts
The North Korean foreign language service Voice of Korea began to broadcast on the E03 Lincolnshire Poacher's former frequency, 11545 kHz, in 2006, possibly to deliberately interfere with its propagation. However, Lincolnshire Poacher broadcasts on three different frequencies, and the remaining two have not been interfered with. The apparent target zone for the Lincolnshire Poacher signals originating in Cyprus was the Middle East, not the Far East, which is covered by its sister station, E03a Cherry Ripe.
In October 1990, it was reported that a numbers station had been interfering with communications on 6577 kHz, a frequency used by air traffic in the Caribbean. The interference was such that on at least one monitored transmission, it blocked the channel entirely and forced the air traffic controller to switch the pilot to an alternative frequency.
A BBC frequency, 7325 kHz, has also been used. This prompted a letter to the BBC from a listener in Andorra. She wrote to the World Service Waveguide programme in 1983 complaining that her listening had been spoiled by a female voice reading out numbers in English and asked the announcer what this interference was. The BBC presenter laughed at the suggestion of spy activity. He had consulted the experts at Bush House (BBC World Service headquarters), who declared that the voice was reading out nothing more sinister than snowfall figures for the ski slopes near the listener's home. After more research into this case, shortwave enthusiasts are fairly certain that this was a numbers station being broadcast on a random frequency.
The Cuban numbers station "HM01" has been known to interfere with shortwave broadcaster Voice of Welt on 11530 kHz.
Numbers station transmissions have often been the target of intentional jamming attempts. Despite this targeting, many numbers stations continue to broadcast unhindered. Historical examples of jamming include the E10 (a numbers station thought to originate from Israel's Mossad intelligence agency) being jammed by the "Chinese Music Station" (thought to originate from the People's Republic of China (PRC) and usually used to jam Taiwan "Sound Of Hope" radio broadcasts which are anti-PRC in nature).
Although many numbers stations have nicknames which usually describe some aspect of the station itself, these nicknames have sometimes led to confusion among listeners, particularly when discussing stations with similar traits. M. Gauffman of the ENIGMA numbers stations monitoring group originally assigned a code to each known station.
Portions of the original ENIGMA group moved on to other interests in 2000 and the classification of numbers stations was continued by the follow-on group ENIGMA 2000. The document containing the description of each station and its code designation was called the "ENIGMA Control List" until 2016, after which it was incorporated into the "ENIGMA 2000 Active Station List"; the latest edition the list was published in September 2017. This classification scheme takes the form of a letter followed by a number (or, in the case of some "X" stations, more numbers). The letter indicates the language used by the station in question:
- E indicates a station broadcasting in English.
- G indicates a station broadcasting in German.
- S indicates a station broadcasting in a Slavic language.
- V indicates all other languages.
- M is a station broadcasting in Morse code.
- X indicates all other transmissions, such as polytones, in addition to some unexplained broadcasts which may not actually be numbers stations.
There are also a few other stations with a specific classification:
- SK: Digital mode
- HM: Hybrid mode
- DP: Digital-pseudo polytone
Some stations have also been stripped of their designation when they were discovered not to be a numbers station. This was the case for E22, which was discovered in 2005 to be test transmissions for All India Radio.
- The Conet Project: Recordings of Shortwave Numbers Stations is a four-CD set of recordings of numbers stations. It was first released in 1997 by the Irdial-Discs record label. A broadcast of these recordings can be heard and downloaded at the Internet Archive.
References in media
Film and TV
- The British–American action thriller The Numbers Station, released in 2013 and starring John Cusack and Malin Åkerman, features a CIA‑run numbers station in the British countryside.
- The 2013 American horror film Banshee Chapter, starring Ted Levine and Katia Winter, features a numbers station transmitting from the Black Rock Desert in Nevada.
- The American television series Fringe has an episode (season 3, episode 6) featuring a numbers station,
- Endeavour (prequel to the Inspector Morse series on PBS in the United States) 2018 Series 5, Episode 5 Quartet at 01:03:00. The discovery of a hidden radio tuned to a Numbers Station leads to the uncovering of a Cold War spy ring in Oxford. Voice is female - probably German, musical identifier is "London Bridge is Falling Down." However, the producers err by playing the identifying tune between each set of numbers. Also each set includes only 4 spoken word numbers . 
- The first story of In the Dark, a Chinese novel by Mai Jia, focuses on a cryptographer in Special Unit 701, part of China's effort to track down and decode enemy number stations. It has been made into a TV series and a movie.
- The band Wilco named its album Yankee Hotel Foxtrot (2001) after a segment of a recorded numbers station transmission.. Samples from E10, an Israeli numbers station, appear in the song "Poor Places" by Wilco on the album Yankee Hotel Foxtrot.
- American musician and "internet person"  Neil Cicierega created the track "Transmission" which parodies number stations and sounds like a transmission from one, for his album Mouth Silence.
- Icelandic composer Jóhann Jóhannsson sampled tape recordings of German numbers station broadcasts in the track A Song For Europa on his 2016 album Orphée.
Radio and podcasts
- A BBC Radio 4 drama, Fugue State, written by Julian Simpson focuses on a British government agent investigating a numbers station in a remote village, and features recordings of numbers stations.
- Numbers stations are incorporated into BBC Radio 4's 2019 adaptation of H. P. Lovecraft's The Whisperer in Darkness by the same writer.
- Secret broadcast
- Letter beacon
- Yosemite Sam (shortwave)
- Warrenton Training Center
- Radio Londres
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