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Anumbers station is ashortwaveradio station characterized by broadcasts of formatted numbers, which are believed to be addressed tointelligence officers operating in foreign countries.[1] Most identified stations usespeech synthesis to vocalize numbers, although digital modes such asphase-shift keying andfrequency-shift keying, as well asMorse 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 thehigh-frequency band.[2]
Numbers stations have been reported since at least the start ofWorld 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 and features or origins.
According to the notes ofThe Conet Project,[3][4] which has compiled recordings of these transmissions, number stations have been reported sinceWorld War I with the numbers transmitted in Morse code. It is reported thatArchduke 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 theCold War era. According to an internal Cold War-era report of the Polish Ministry of the Interior, numbers stations DCF37 (3.370 MHz) and DFD21 (4.010 MHz) were transmitted fromWest 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] TheCzech Ministry of the Interior and theSwedish Security Service have both acknowledged the use of numbers stations byCzechoslovakia for espionage,[9][10][11] with declassified documents proving the same. FewQSL responses have been received from numbers stations[12] byshortwave 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]
One well-known numbers station was the E03 "Lincolnshire Poacher",[16] which is thought to have been run by the BritishSecret Intelligence Service.[17] It was first broadcast fromBletchley Park in the mid-1970s but later was broadcast fromRAF Akrotiri inCyprus. It ceased broadcasting in 2008.[18]
In 2001, the United States tried theCuban 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]
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 theWasp 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 intolaptop 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]
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 aone-time pad decoding key is used more than once.[19] If used properly, however, the codecannot be broken.
In 2001,Ana Belén Montes, a senior USDefense Intelligence Agency analyst, was arrested and charged with espionage. The federal prosecutors alleged that Montes was able to communicate with the CubanIntelligence 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 chargedWalter 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 theIllegals 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 thatState 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 July 2016 after a hiatus of sixteen years, a move which some analysts speculated waspsychological war;[25] sixteen such broadcasts occurred in 2017, including unusually timed transmissions in April.[26]
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 aone-time pad to avoid any risk of decryption by the enemy. Writing in 2008, Wallace andMelton described how numbers stations could be used in this way for espionage:[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 theattempted coup of August 1991 in theSoviet Union.[29]
A 1998 article inThe Daily Telegraph quoted a spokesperson for theDepartment of Trade and Industry (the government department that, at that time, regulated radio broadcasting in the United Kingdom) as saying
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 orradio-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, thecryptographic function employed by particular stations is not publicly known, except in one (or possibly two)[a] 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.[32]
High-frequency radio signals transmitted at relatively low power can travel around the world under idealpropagation conditions – which are affected by localRF noise levels, weather, season, andsunspots – 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 shortwavetransmitters using powers from 10 kW to 100 kW.[citation needed]
Amplitude modulated (AM) transmitters with optionally–variable frequency, usingclass-C power output stages withplate modulation, are the workhorses of international shortwave broadcasting, including numbers stations.[citation needed]
Application ofspectrum analysis to numbers station signals has revealed the presence of data bursts,radioteletype-modulatedsubcarriers,phase-shifted carriers, and other unusual transmitter modulations likepolytones.[33] (RTTY-modulated subcarriers were also present on some U.S. commercial radio transmissions during the Cold War.[34])
The frequently reported use of high-tech modulations likedata bursts, in combination or in sequence with spoken numbers, suggests varying transmissions for differing intelligence operations.[35]
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.[36] Being caught with just a shortwave radio has a degree ofplausible deniability, for example, that no spying is being conducted.[citation needed]
The North Korean foreign language serviceVoice of Korea began to broadcast on the E03Lincolnshire Poacher's former frequency, 11545 kHz, 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, E03aCherry Ripe.[37][38]
On 27 September 2006, amateur radio transmissions in the 30 m band were affected by an S06 "Russian Man"[39] numbers station at 17:40 UTC.[38]
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.[38]
ABBC frequency, 7325 kHz, has also been used. This prompted a letter to the BBC from a listener inAndorra. She wrote to theWorld 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 atBush 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.[40]
The Cuban numbers station "HM01" has been known to interfere with shortwave broadcaster Voice of Welt on 11530 kHz.[41]
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'sMossad intelligence agency) being jammed by the "Chinese Music Station" (thought to originate from thePeople's Republic of China and usually used to jam "Sound of Hope" radio broadcasts which are anti-CCP in nature).[42]
Monitoring and chronicling transmissions from numbers stations has been a hobby for shortwave andham radio enthusiasts from as early as the 1970s.[43] Numbers stations are often given nicknames by enthusiasts, often reflecting some distinctive element of the station such as theinterval signal. For example, the "Lincolnshire Poacher" station played the first two bars of the folk song "The Lincolnshire Poacher" before each string of numbers.[44] Sometimes these traits have helped to uncover the broadcast location of a station. The "Atención" station was thought to be fromCuba, because a supposed error allowedRadio Havana Cuba to be carried on the frequency.[45][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.[46]
Portions of the originalENIGMA group moved on to other interests in 2000 and the classification of numbers stations was continued by the follow-on group ENIGMA 2000.[47] 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.[48] This classification scheme takes the form of a letter followed by a number (or, in the case of some "X" stations, more numbers).[49] The letter indicates the language used by the station in question:
There are also a few other stations[31] with a specific classification:
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 forAll India Radio.[50]
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