Cuban numbers station HM01A recording of The Gong numbers station, run by the National People's Army of the German Democratic Republic, from 1988.
A numbers station is a shortwaveradio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries.[1] 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, some appear to have no discernible pattern and broadcast at random times. Stations may have set frequencies in the high-frequency band.[2]
Numbers stations have been reported since at least the start of World War I and continue in use today. Amongst amateur radio enthusiasts there is an interest in monitoring and classifying numbers stations, with many being given nicknames to represent their quirks or origins.
History
According to the notes of The Conet Project,[3][4] which has compiled recordings of these transmissions, number stations have been reported since World War I with the numbers transmitted in Morse code. It is reported that Archduke Anton of Austria in his youth during World War I used to listen in to their transmissions, writing them down and passing them on to the Austrian military intelligence.[5]
Numbers stations were most abundant during the Cold War era. According to an internal Cold War-era report of the Polish Ministry of the Interior, numbers stations DCF37 (3.370MHz) and DFD21 (4.010MHz) were transmitted from West Germany beginning in the early 1950s.[6]
Many stations from this era continue to broadcast and some long-time stations may have been taken over by different operators.[7][8] The Czech Ministry of the Interior and the Swedish Security Service have both acknowledged the use of numbers stations by Czechoslovakia for espionage,[9][10][11] with declassified documents proving the same. Few QSL responses have been received from numbers stations[12] by shortwave listeners[13] who sent reception reports to stations that identified themselves or to entities the listeners believed responsible for the broadcasts, which is the expected behaviour of a non-clandestine station.[14][15]
In 2001, the United States tried the Cuban Five on the charge of spying for Cuba. The group had received and decoded messages that had been broadcast from the "Atención" number station in Cuba.[19]
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.[19]
The United States government's evidence included the following three examples of decoded Atención messages.[19]
"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."
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 may be possible if a one-time pad decoding key is used more than once.[19] If used properly, however, the code cannot be broken.
Recent cases
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 for the Southern District of Florida[20][which?] stated that "defendants would receive assignments via shortwave radio transmissions".[citation needed]
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.[21][22] As discovered by the FBI up to 2010, one way that Russian agents of the Illegals Program were receiving instructions was via coded messages on shortwave radio.[18] It has been reported that the United States has used number stations to communicate encoded information to persons in other countries.[19] 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 the air for many years.[23][24]
North Korea revived number broadcasts in July2016 after a hiatus of sixteen years, a move which some analysts speculated was psychological war;[25] sixteen such broadcasts occurred in 2017, including unusually timed transmissions in April.[26]
Suspected use for espionage
It has long been speculated, and was argued in one court case, that these stations operate as a simple and fool-proof method for government agencies to communicate with spies working undercover.[27] According to this hypothesis, the messages must have been encrypted with a one-time pad to avoid any risk of decryption by the enemy. Writing in 2008, Wallace & Melton described how numbers stations could be used in this way for espionage:[28]
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.[28]
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.[28]
Evidence to support this theory includes the fact that numbers stations have changed details of their broadcasts or produced special, nonscheduled broadcasts coincident with extraordinary political events, such as the attempted coup of August1991 in the Soviet Union.[29]
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."[30]
Formats
The "Russian Man" station signing off. The numbers read: 83912 83912 10080 10080 46543 46543–257 257 143 143–000 00
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.[citation needed]
The prelude, introduction, or call-up of a transmission (from which stations' informal nicknames are often derived) includes some kind of identifier,[31] 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.[citation needed]
After the prelude, there is usually an announcement of the number of number-groups in the message,[31] 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.[citation needed]
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.[citation needed]
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.[31]
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[32]) 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.[33]
Transmission technology
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 best received with a properly tuned antenna (of adequate, possibly conspicuous size) and a good receiver.[19]
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 10kW to 100kW.[citation needed]
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.[citation needed]
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.[34] (RTTY-modulated subcarriers were also present on some U.S. commercial radio transmissions during the Cold War.[35])
The Speech/Morse generator (pictured here) is a machine that has been used for many well-known numbers stations
The frequently reported use of high-tech modulations like data bursts, in combination or in sequence with spoken numbers, suggests varying transmissions for differing intelligence operations.[36]
Those receiving the signals often have to work only with available hand-held receivers, sometimes under difficult local conditions, and in all reception conditions (such as sunspot cycles and seasonal static).[19] However, in the field low-tech spoken number transmissions continue to have advantages even in the 21st century. High-tech data-receiving equipment can be difficult to obtain and even a non-standard civilian shortwave radio can be difficult to obtain in a totalitarian state.[37] Being caught with just a shortwave radio has a degree of plausible deniability, for example, that no spying is being conducted.[citation needed]
Interference
Interfering with other broadcasts
The North Korean foreign language service Voice of Korea began to broadcast on the E03 Lincolnshire Poacher's former frequency, 11545kHz, in 2006, possibly to deliberately interfere with its propagation.[citation needed] 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.[38][39]
On 27 September 2006, amateur radio transmissions in the 30m band were affected by an S06 "Russian Man"[40] numbers station at 17:40 UTC.[39]
In October 1990, it was reported that a numbers station had been interfering with communications on 6577kHz, 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.[39]
A BBC frequency, 7325kHz, has also been used. This prompted a letter to the BBC from a listener in Andorra. She wrote to the World ServiceWaveguide 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.[41]
The Cuban numbers station "HM01" has been known to interfere with shortwave broadcaster Voice of Welt on 11530kHz.[42]
Attempted jamming
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 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 and usually used to jam "Sound of Hope" radio broadcasts which are anti-CCP in nature).[43]
Identification and classification
Monitoring and chronicling transmissions from numbers stations has been a hobby for shortwave and ham radio enthusiasts from as early as the 1970s.[44] 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" station played the first two bars of the folk song "The Lincolnshire Poacher" before each string of numbers.[45] Sometimes these traits have helped to uncover the broadcast location of a station. The "Atención" station was thought to be from Cuba, because a supposed error allowed Radio Havana Cuba to be carried on the frequency.[46][full citation needed]
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.[47]
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.[48] 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 of the list was published in September 2017.[49] This classification scheme takes the form of a letter followed by a number (or, in the case of some "X" stations, more numbers).[50] The letter indicates the language used by the station in question:
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[31] 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.[51]
Recordings
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.
In the British television spy drama Spooks episode "Nuclear Strike", a Russian sleeper agent is awoken by a numbers station broadcast to detonate a nuclear suitcase bomb in central London. The radio broadcast states in Russian, "2.5.0.0.2.5, Finland Red, Egypt White, It is twice blest, It is twice blest, rain from heaven, rain from heaven."[citation needed]
The American science fiction series Fringe has an episode, "6955 kHz", featuring a numbers station that induces amnesia.[54]
In the British mystery series Endeavour episode "Quartet", a spy ring in Oxford communicates using a numbers station, which has a female voice that speaks German and uses "London Bridge Is Falling Down" as an interval signal.[55]
The first section 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. The novel has been adapted into a TV series and a movie.[57]
Music
American band Wilco named its 2001 album Yankee Hotel Foxtrot after a segment of a recorded numbers station transmission. Samples from E10, an Israeli numbers station, appear in the album's song "Poor Places".[58]
Icelandic composer Jóhann Jóhannsson sampled tape recordings of German numbers station broadcasts in the track "A Song for Europa" on his 2016 album Orphée.[60]
American metalcore band Norma Jean's 2016 album Polar Similar includes a track titled "II. The People" that samples a recording of the Lincolnshire Poacher.[61]
Radio and podcasts
Several BBC Radio 4 dramas have incorporated numbers stations:
The standalone 2015 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.[62]
The 5-part 2022 drama Dead Hand by Stuart Drennan features a numbers station in Northern Ireland broadcasting the voices of individuals who have mysteriously disappeared.[64]
In a 2015 episode of Welcome to Night Vale, a numbers station called WZZZ begins broadcasting words along with its numbers.[65]
The Magnus Archives' 2019 episode "Decrypted" features a numbers station that appears on an iPod, attached to the entity The Extinction.[66]
A 2008 episode of Skeptoid discusses numbers stations.[67]
In Signalis, the player uses in-game radio signals transmitting numbers to solve puzzles. Some frequencies feature samples from historical German number stations.[69]
In Call of Duty: Black Ops, the main character Alex Mason is brainwashed in the Soviet Gulag of Vorkutlag, and receives orders from a numbers station broadcast in Cuba.[70]
Signals intelligence (SIGINT) is the act and field of intelligence-gathering by interception of signals, whether communications between people or from electronic signals not directly used in communication. As classified and sensitive information is usually encrypted, signals intelligence may necessarily involve cryptanalysis. Traffic analysis—the study of who is signaling to whom and in what quantity—is also used to integrate information, and it may complement cryptanalysis.
Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz ; above the medium frequency band (MF), to the bottom of the VHF band.
International broadcasting consists of radio and television transmissions that purposefully cross international boundaries, often with then intent of allowing expatriates to remain in touch with their countries of origin as well as educate, inform, and influence residents of foreign countries. Content can range from overt propaganda and counterpropaganda to cultural content to news reports that reflect the point of view and concerns of the originating country or that seek to provide alternative information to that otherwise available as well as promote tourism and trade. In the first half of the twentieth century, international broadcasting was used by colonial empires as a means of connecting colonies with the metropole. When operated by governments or entities close to a government, international broadcasting can be a form of soft power. Less frequently, international broadcasting has been undertaken for commercial purposes by private broadcasters.
A radiotelephone, abbreviated RT, is a radio communication system for conducting a conversation; radiotelephony means telephony by radio. It is in contrast to radiotelegraphy, which is radio transmission of telegrams (messages), or television, transmission of moving pictures and sound. The term is related to radio broadcasting, which transmit audio one way to listeners. Radiotelephony refers specifically to two-way radio systems for bidirectional person-to-person voice communication between separated users, such as CB radio or marine radio. In spite of the name, radiotelephony systems are not necessarily connected to or have anything to do with the telephone network, and in some radio services, including GMRS, interconnection is prohibited.
JJY is the call sign of a low frequency time signal radio station located in Japan.
Radio Havana Cuba is the official government-run international broadcasting station of Cuba. It can be heard in many parts of the world, including the United States, on shortwave frequencies. Radio Havana Cuba, along with Radio Rebelde, Cubavision Television, and other Cuban radio and television, broadcasts to North, Central and South America via free-to-air programming from the Hispasat 30W-6 satellite over the Atlantic Ocean and worldwide via Internet streaming.
A secret broadcast is, simply put, a broadcast that is not for the consumption of the general public. The invention of the wireless was initially greeted as a boon by armies and navies. Units could now be coordinated by nearly instant communications. An adversary could glean valuable and sometimes decisive intelligence from intercepted radio signals:
Shortwave listening, or SWLing, is the hobby of listening to shortwave radio broadcasts located on frequencies between 1700 kHz and 30 MHz (30 000 kHz). Listeners range from casual users seeking international news and entertainment programming, to hobbyists immersed in the technical aspects of long-distance radio reception and sending and collecting official confirmations that document their reception of remote broadcasts (DXing). In some developing countries, shortwave listening enables remote communities to obtain regional programming traditionally provided by local medium wave AM broadcasters. In 2002, the number of households that were capable of shortwave listening was estimated to be in the hundreds of millions.
A one-way voice link (OWVL) is typically a radio based communication method used by spy networks to communicate with agents in the field typically using shortwave radio frequencies.
Radio jamming is the deliberate blocking of or interference with wireless communications. In some cases, jammers work by the transmission of radio signals that disrupt telecommunications by decreasing the signal-to-noise ratio.
Tradecraft, within the intelligence community, refers to the techniques, methods, and technologies used in modern espionage (spying) and generally as part of the activity of intelligence assessment. This includes general topics or techniques, or the specific techniques of a nation or organization.
Letter beacons are radio transmissions of uncertain origin and unknown purpose, consisting of only a single repeating Morse code letter. They have been classified into a number of groups according to transmission code and frequency, and it is supposed that the source for most of them is Russia and began during the Soviet Union.
DXing, taken from DX, the telegraphic shorthand for "distance" or "distant", is the hobby of receiving and identifying distant radio or television signals, or making two-way radio contact with distant stations in amateur radio, citizens band radio or other two-way radio communications. Many DXers also attempt to obtain written verifications of reception or contact, sometimes referred to as "QSLs" or "veries".
The B-Dienst, also called xB-Dienst, X-B-Dienst and χB-Dienst, was a Department of the German Naval Intelligence Service of the OKM that dealt with the interception and recording, decoding and analysis of the enemy. In particular, it focused on British radio communications before and during World War II. B-Dienst worked on cryptanalysis and deciphering (decrypting) of enemy and neutral states' message traffic and security control of Kriegsmarine key processes and machinery.
Voice of Korea is the international broadcasting service of North Korea. It broadcasts primarily information in Chinese, Spanish, German, English, French, Russian, Japanese and Arabic. Until 2002 it was known as Radio Pyongyang. The interval signal is identical to that of Korean Central Television.
The Lincolnshire Poacher was a powerful British shortwave numbers station that transmitted from HM Government Communications Centre near Gawcott in Buckinghamshire, England, and later from Cyprus, from the mid-1960s to July 2008. The station gained its commonly known name as it uses bars from the English folk song "The Lincolnshire Poacher" as an interval signal. The radio station was believed to be operated by the British Secret Intelligence Service. Amateur direction finding linked it with the Royal Air Force base at Akrotiri, Cyprus, where several curtain antennas were identified as being its transmitter. It consisted of a pre-recorded English-accented female voice reading groups of five numbers: e.g., '0-2-5-8-8'. The final number in each group was spoken at a higher pitch. It is likely that the station was used to communicate to undercover agents operating in other countries, to be decoded using a one-time pad.
UVB-76, also known by the nickname "The Buzzer", is a shortwave radio station that broadcasts in Upper Side Band mode on the frequency of 4625kHz. It broadcasts a short, monotonous, repeating at a rate of approximately 25tones per minute, 24hours per day. Sometimes, the buzzer signal is interrupted and a voice transmission in Russian takes place.
Cherry Ripe is the nickname of a discontinued shortwave numbers station that used several bars from the folk song "Cherry Ripe" as an interval signal. The station, which appears to have commenced transmissions in the late 1960s, is believed to have been controlled by the British Secret Intelligence Service. It is thought to have originally broadcast from a base on Guam, but moved to Australia in 2009.
Before the development of radar and other electronics techniques, signals intelligence (SIGINT) and communications intelligence (COMINT) were essentially synonymous. Sir Francis Walsingham ran a postal interception bureau with some cryptanalytic capability during the reign of Elizabeth I, but the technology was only slightly less advanced than men with shotguns, during World War I, who jammed pigeon post communications and intercepted the messages carried.
The Pip is a shortwave radio station that broadcasts on the frequency 5448 kHz by day, and 3756 kHz during the night. It broadcasts short, repeated beeps at a rate of around 50 per minute, for 24 hours per day. The beep signal is occasionally interrupted by voice messages in Russian. The Pip has been active since 1986, when its distinctive beeping sound was first recorded by listeners, and is a sister station to UVB-76.
↑ "OLX". 24 January 2015. Archived from the original on 24 January 2015. Retrieved 23 September 2020.
↑ Helms, Harry L. (1981). "Espionage Radio Activity". How to Tune the Secret Shortwave Spectrum. Blue Ridge Summit, Pennsylvania: TAB Books. p.52. ISBN0-8306-1185-1.
↑ Helms, Harry L. (1981). "Government and Military Communications". How to Tune the Secret Shortwave Spectrum. Blue Ridge Summit, Pennsylvania: TAB Books]. p.58. ISBN0-8306-1185-1.
↑ Schimmel, Donald W. (1994). The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications (3ed.). Solana Beach, California: High Text Publications. pp.88–95. ISBN1-878707-17-5.
↑ 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.
↑ Schimmel, Donald W. (1994). The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications (3ed.). Solana Beach, California: High Text Publications, Inc. pp.27–28. ISBN1-878707-17-5.
↑ Collins, Barry W. (July 1997). "The day the U.S. Army invaded W4TLV". QST. 81: 48–49. ISSN0033-4812.
Wallace, Robert; Melton, H. Keith (26 May 2009). Spycraft: The secret history of the CIA's spytechs, from communism to al-Qaeda (illustrated, printed.). New York, NY: Plume. ISBN978-0452295476.
Further reading
Beaumont, Paul (November 2012). "Numbers Stations: A Modern Perspective (Part1)". Radio User. Poole, UK: PW Publishing: 50–53. ISSN1748-8117.
Beaumont, Paul (January 2013). "Numbers Stations: A Modern Perspective (Part2)". Radio User. Poole, UK: PW Publishing: 50–55. ISSN1748-8117.
Schimmel, Donald W. (1994). "1. Numbers Stations". The Underground Frequency Guide: A Directory of Unusual, Illegal, and Covert Radio Communications (3rded.). Solana Beach, CA: High Text Publications. pp.1–28. ISBN1-878707-17-5.
Smolinski, Chris (February 1998). "Spy Numbers Stations: Have you heard them?". Popular Communications. Hicksville, NY: CQ Communications: 8–10. ISSN0733-3315.
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