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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, and possibly the first listener was Archduke Anton of Austria. The numbers were transmitted in Morse code. Numbers stations operated mainly during the Cold War, yet many continue to operate and some have even switched 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 behavior of a non-clandestine station.
The best-known of the numbers stations was the E03 "Lincolnshire Poacher", which is thought to have been run by the British Secret Intelligence Service. It was first broadcast out of 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 Belen 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 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.
- 1 Suspected origins and use
- 2 Atención spy case
- 3 Formats
- 4 Transmission technology
- 5 Interference with numbers stations
- 6 Classification
- 7 Recordings and samples
- 8 References in media
- 9 See also
- 10 References
- 11 Further reading
- 12 External links
Suspected origins and use
According to the notes of The Conet Project, which has compiled recordings of these transmissions, numbers stations have been reported since World War I. If accurate, this would count numbers stations among the earliest radio broadcasts.
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 practiced 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.
Others speculate that some of these stations may be related to the smuggling of illegal drugs although this is unlikely and has never shown a case. Unlike government stations, smugglers' stations would need to be lower-powered and irregularly operated, to avoid location by triangulated direction finding. However, numbers stations have transmitted with impunity for decades, so they are generally presumed to be operated or sponsored by governments. Additionally, numbers station transmissions in the international shortwave bands typically transmit at high power levels that might be unavailable to ranches, farms, or plantations in isolated drug-growing regions.
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. Only very large transmitters, perhaps up to 500,000 watts, are guaranteed to get through to nearly any basement-dwelling spy, nearly any place on earth, nearly all of the time. Some governments may not need a numbers station with global coverage if they only send spies to nearby countries.
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."
On some stations, tones can be heard in the background. In such cases, the voice may simply be an aid to tuning to the correct frequency, with the actual coded message being sent by modulating the tones, using a technology such as burst transmission.
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 their interval signal. For example, the "Lincolnshire Poacher", formerly one of the best-known numbers stations (generally thought to be run by SIS, 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.
Several articles in the radio magazine Popular Communications published in the 1980s and early 1990s described hobbyists using portable radio direction-finding equipment to locate numbers stations in Florida and in the Warrenton, Virginia area of the United States. From the outside, they[who?] observed the station's antenna inside a military facility, the Warrenton Training Center. The station hunter speculated that the antenna's transmitter at the facility was connected by a telephone wire pair to a source of spoken numbers in the Washington, D.C. area. The author said the Federal Communications Commission would not comment on public inquiries about numbers stations on American territory.
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 centerpiece 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.
United States government 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" [68 characters]
- "Under no circumstances should [agents] German nor Castor fly with BTTR or another organization on days 24, 25, 26 and 27." [112 characters] (BTTR is the anti-Castro airborne group Brothers to the Rescue)
- "Congratulate all the female comrades for International Day of the Woman." [71 characters] (Probably a simple greeting for International Women's Day on 8 March)
At the rate of one spoken number per character per second, each of these sentences takes more than a minute to transmit.
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, either for the station itself and/or for the intended recipient. 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 an radio-alphabet stations, the prelude can also signify the nature or priority of the message to follow (e.g., [hypothetically] "Charlie India Oscar-2", indicating 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, RTTY-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.
Interference with numbers stations
Numbers stations 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 not one, but three different frequencies, of which 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.
The late "Havana Moon" reported in his own publication The Numbers Factsheet in October 1990 that "one particularly dangerous station has been interfering with air to ground traffic on 6577 kHz, a frequency allocated to international aeronautical communications in the busy Caribbean sector". "On at least one monitored transmission, the air traffic controller at ARINC moved the pilot to an alternate frequency as the numbers transmission was totally blocking the frequency from effective use".
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[when?] 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 sure that this was a numbers station being broadcast on a random frequency.
Attempted jamming of numbers stations
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 mysterious "Chinese Music Station" (thought to originate from the PRC and usually used to jam ROC "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 is called the "ENIGMA Control List", whose 26th edition was published in February 2016. 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.
Recordings and samples
- 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.
- Samples from Conet Project recordings, most notably Tcp D1 3 Counting Control Irdial, are used by the Scottish electronic music group Boards of Canada on their 2002 album Geogaddi, most conspicuously on the track "Gyroscope".
- Samples from the tracks "tcp d4 11 konec konec irdial" and "tcp d2 08 the russian man d-va northern russian voice irdial" were used by American electronic music artist Flying Lotus on his 2008 album Los Angeles.
- Samples from the track "tcp d1 1 the swedish rhapsody irdial" are used by the alternative rock band Stereolab in the song "Pause" from their 1993 album Transient Random-Noise Bursts with Announcements.
- Samples from the Lincolnshire Poacher appear in the song "II. The People" by metal artist Norma Jean on the album Polar Similar.
- Samples from E05, an American numbers station, appear in the song "Even Less" by Porcupine Tree at the end of the track.
- Samples from E10, an Israeli numbers station, appear in the song "Poor Places" by Wilco on the album Yankee Hotel Foxtrot.
References in media
- The band Wilco named its album Yankee Hotel Foxtrot (2001) after a segment of a recorded numbers station transmission.
- 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 American television series Fringe has an episode (season 3, episode 6) featuring a numbers station, as do the series Scandal (season 2, episode 6), Covert Affairs (season 1, episode 2), Lost (season 1, episode 18), The Americans (season 1, episode 8 and season 2, episode 2), and Z Nation (season 2, episode 1), along with the podcasts Welcome to Night Vale (episode 6, "The Drawbridge," episode 42, "Numbers," and episode 65, "Voicemail") and Tanis (episode 2, "Radio, Radio").
- 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.
- In Treyarch and Activision's 2010 PC/console game Call of Duty: Black Ops, the primary character Alex Mason is captured and programmed to understand broadcasts from a Russian numbers station.
- In Rocksteady and Warner Bros' 2011 PC/console game Batman: Arkham City, there are a few easter eggs in form of in-game numbers stations that can be listened to by using one of Batman's gadgets.
- 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.
- A BBC Radio 4 drama, Fugue State, focuses on a British government agent investigating a numbers station in a remote village, and features recordings of numbers stations.
- Olivia Sorrel-Dejerine (16 April 2014). "The spooky world of the 'numbers stations'". BBC News.
- "Number stations". Priyom.
- Schaum, Ryan (30 November 2014). "The First Numbers Stations". NSRIC. Archived from the original on 12 March 2016.
- "Lyssna på ett hemligt telegram" [Listen to a secret telegram] (in Swedish). Säkerhetspolisen. 23 January 2015. Archived from the original on 8 March 2016. Retrieved 12 March 2016.
- "The Swedish Security Service Releases Info on a Numbers Station". NSRIC. 24 July 2015. Archived from the original on 12 March 2016. Retrieved 12 March 2016.
- Catinka Mannerfelt Agneskog. "Säpos hemliga radiotelegram" (in Swedish). SvD Nyheter. Retrieved 23 January 2015.
- stations KKN44, BFBX and OLX Mason, Simon. "Shortwave Espionage". Retrieved 28 October 2011.
- Helms, Harry L. (1981). "Espionage Radio Activity". How to Tune the Secret Shortwave Spectrum. Blue Ridge Summit, PA: TAB Books Inc. p. 52. ISBN 0-8306-1185-1.
- "E03 › Priyom.org". priyom.org. Retrieved 2017-06-17.
- "E03 The LincolnShire Poacher". Retrieved 6 September 2014.
- Sokol, Brett (8 February 2001). "Espionage Is in the Air". Miami New Times. Archived from the original on 21 February 2001.
- Rijmenants, Dirk (2013). "Cuban Agent Communications" (PDF). Cipher Machines & Cryptology (PDF). Retrieved 30 December 2013.
- "United States v. Walter Kendall Myers, United States District Court, District of Columbia, no. xxx." (PDF). Retrieved 26 August 2010.
- Helms, Harry L. (1981). "Government and Military Communications". How to Tune the Secret Shortwave Spectrum. Blue Ridge Summit, PA: TAB Books Inc. p. 58. ISBN 0-8306-1185-1.
- Schimmel, Donald W. (1994). The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications (3 ed.). Solana Beach, California: High Text Publications, Inc. pp. 88–95. ISBN 1-878707-17-5.
- Segal, David (3 August 2004). "The Shortwave And the Calling: For Akin Fernandez, Cryptic Messages Became Music To His Ears". The Washington Post. p. C01.
- Mason 1991, pp. 5–6
- Wagner, Thomas (2004). "Chapter 6 "So here she was with a pillow over her head and over the radio ..."". If It Had Not Been for Fifteen Minutes ... a true account of espionage and hair-raising adventure. Retrieved 30 October 2013.
- The Conet Project (included booklet), Irdial-Discs, p. 59.
- Wallace & Melton 2008, p. 438
- "Secret Radio Frequencies" (txt). Archived from the original on 10 February 2016. Retrieved 12 March 2016.
- Pescovitz, David (16 September 1999). "Counting spies". Salon. Archived from the original on 19 January 2000. Retrieved 12 March 2016.
- Wallace & Melton 2008, p. 37
- Sang-Hun, Choe (2016-07-21). "North Korea Revives Coded Spy Broadcasts After 16-Year Silence". NYT. Retrieved 2016-07-30.
- Osbourne, Simon (2017-05-12). "North Korea sends chilling coded radio messages to South Korea amid fears of WW3". Express.
- Mason 1991, pp. 20–21
- William Poundstone, Big Secrets, p. 197.
- Bury, Jan (October 2007). "From the Archives: The U.S. and West German Agent Radio Ciphers". Cryptologia. 31 (4): 343–357. ISSN 0161-1194. doi:10.1080/01611190701578104.
- "Intro to Numbers Stations". NSRIC. Archived from the original on 12 March 2016. Retrieved 12 March 2016.
- In the possible case, the underlying type of encryption might have been stated in the court record of the Atención case when the secretly copied decryption software was introduced into evidence.
- See If It Had Not Been For 15 Minutes, Chapter 7 for a simplified explanation of decoding West German numbers messages without a computer.
- Schimmel, Donald W. (1994). The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications (3 ed.). Solana Beach, California: High Text Publications, Inc. pp. 27–28. ISBN 1-878707-17-5.
- Collins, Barry W. (July 1997). "The day the U.S. Army invaded W4TLV". QST. 81: 48–49. ISSN 0033-4812.
- "NSNL 15: Voice stations". Cvni.net. 3 July 1999. Retrieved 26 August 2010.
- Even a non-standard civilian shortwave radio can be difficult to obtain in a totalitarian state. See If It Had Not Been For 15 Minutes, chapter 6 for the problems of obtaining a numbers station receiving radio in East Germany during the Cold War.
- "E03 › Priyom.org". priyom.org. Retrieved 2017-06-17.
- "E03a › Priyom.org". priyom.org. Retrieved 2017-06-17.
- "Secret Signals". Simonmason.karoo.net. Retrieved 7 November 2015.
- "S06 › Priyom.org". priyom.org. Retrieved 2017-06-17.
- "Secret Signals". Simonmason.karoo.net. Retrieved 26 August 2010.
- "Chinese Music Station" (Windows Media Audio). Retrieved 16 July 2012.
- "ENIGMA: The European Numbers Information Gathering and Monitoring Association". NSRIC. Retrieved 24 February 2015.
- "ENIGMA 2000". Retrieved 1 March 2015.
- "ENIGMA 2000 Active Stations and N&O ident list" (PDF) (Booklet). 1.2. ENIGMA 2000. February 2016. Archived (PDF) from the original on 12 March 2016.
- Friesen, Christopher (2014). "Spy 'Numbers Stations' still enthrall". Radio World. 38 (2): 14. ISSN 0274-8541.
- "E2K 32 - E22 is not what it seems". www.cvni.net. Retrieved 2016-04-05.
- "The Conet Project - Recordings of Shortwave Numbers Stations [ird059]". Internet Archive (archive.org). Retrieved 16 November 2016.
- "Boards of Canada's "Geogaddi"". Discogs. Retrieved 9 May 2012.
- "Flying Lotus's "Los Angeles"". Discogs. Retrieved 25 March 2014.
- "Stereolab's "Transient Random-Noise Bursts with Announcements"". discogs. Retrieved 14 November 2014.
- "Norma Jean - Polar Similar at Discogs".
- "F.A.Q.". Porcupine Tree. Retrieved 4 February 2015.
- Shachtman, Noah (23 June 2004). "Wilco Pays Up for Spycasts". Wired. Archived from the original on 21 April 2008. Retrieved 12 March 2016.
- Dowd, A. A. (25 April 2013). "Movie Review: The Numbers Station". The A.V. Club. Chicago. Archived from the original on 12 March 2016.
- Murray, Noel. "Fringe: '6995 kHz'". The A.V. Club. Retrieved 1 October 2012.
- Film Review: 'Banshee Chapter' Dennis Harvey, Variety, 4 January 2014, Retrieved 21 July 2015
- Farrimond, Dorian (1 December 2010). "Call of Duty: Black Ops – campaign story". Blitter and Twisted. Retrieved 22 January 2014.
- "Easter Eggs - Batman: Arkham City Wiki Guide - IGN". IGN. Retrieved 2017-01-13.
- Forbes, Malcolm (27 August 2015). "Book review: Mai Jia’s In the Dark shines a light on China’s secret world". The National. Archived from the original on 3 March 2016. Retrieved 12 March 2016.
- "Fugue State". BBC Radio 4. Retrieved 26 February 2016.
- Mason, Simon (1991). Secret Signals: The Euronumbers Mystery. Lake Geneva, WI: Tiare Publications. ISBN 0-936653-28-0. Retrieved 24 December 2013.
- Wallace, Robert; Melton, H. Keith (2008). Spycraft: The Secret History of the CIA's Spytechs, from Communism to al-Qaeda.
- Havana Moon (1987). Uno, Dos, Cuatro: A Guide to the Numbers Stations (PDF). Lake Geneva, WI: Tiare Publications. ISBN 0-936653-06-X. Retrieved 24 December 2013.
- Schimmel, Donald W. (1994). "1. Numbers Stations". The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications (3rd ed.). Solana Beach, California: High Text Publications, Inc. pp. 1–28. ISBN 1-878707-17-5.
- Pierce, Langley (1994). Intercepting Numbers Stations. Perth, UK: Interproducts. ISBN 0-9519783-4-9.
- Smolinski, Chris (February 1998). "Spy Numbers Stations: Have you heard them?". Popular Communications. Hicksville, NY: CQ Communications: 8–10. ISSN 0733-3315.
- Beaumont, Paul (November 2012). "Numbers Stations: A Modern Perspective (Part 1)". Radio User. Poole, UK: PW Publishing Ltd: 50–53. ISSN 1748-8117.
- Beaumont, Paul (January 2013). "Numbers Stations: A Modern Perspective (Part 2)". Radio User. Poole, UK: PW Publishing Ltd: 50–55. ISSN 1748-8117.
- Bury, Jan (October 2007). "From the Archives: The U.S. and West German Agent Radio Ciphers". Cryptologia. 31 (4): 343–357. ISSN 0161-1194. doi:10.1080/01611190701578104.
- Friesen, Christopher (15 January 2014). "Spy 'Numbers Stations' still enthrall". Radio World. 38 (2): 12, 14. ISSN 0274-8541.
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