Evidence supports popular assumptions that the broadcasts are used to send messages to spies. This usage has not been publicly acknowledged by any government that may operate a numbers station, but in one case, Cuban numbers station espionage has been publicly prosecuted in a United States federal court.
Numbers stations appear and disappear over time (although some follow regular schedules), and their overall activity has increased slightly since the early 1990s. This increase suggests that, as spy-related phenomena, they were not unique to the Cold War.
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 August Coup.
Others speculate that some of these stations may be related to illegal drug smuggling operations. Unlike government stations, smugglers' stations would need to be lower powered and irregularly operated, to avoid location by triangulated direction finding, followed by government raids. However, numbers stations have transmitted with impunity for decades, so they are generally presumed to be operated or sponsored only by governments. Also, numbers station transmissions in the international shortwave bands typically require high levels of electric power that is 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 superb 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.
Although no broadcaster or government has acknowledged transmitting the numbers, 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.
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 MI6 as 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. "Magnetic Fields" plays music from French electronic musician Jean Michel Jarre before and after each set of numbers. The "Atención" station begins its transmission with the Spanish phrase "¡Atención!"
Although it is time-consuming and may require costly global travel to pinpoint the source of a radio transmission in the shortwave band, errors at the transmission site, radio direction-finding, and a knowledge of shortwave radio propagation have provided armchair detective clues to some number station locations.
For example, the "Atención" station was originally presumed to be from Cuba, as a supposed error allowed Radio Habana Cuba to be carried on the frequency. Whether the frequency of Radio Habana Cuba and the frequency of the "Atención" station merely interfered with each other or whether the operator of the station was listening to the radio and it accidentally ended up on the air is unclear. Circa 2000-2001, Atención was officially identified as Cuban by the United States.
Also, 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. From the outside, they spotted the station's antenna inside a military facility. 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 American territory numbers stations.
On some stations, tones can be heard in the background. It has been suggested that in such cases the voice may be an aid to tuning to the correct frequency, with the coded message being sent by modulating the tones, perhaps using a technology such as burst transmission.
Atención 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. (Not reported whether the original clear texts were in Spanish, although the phrasing of "Day of the Woman" would indicate so.):
At the rate of one spoken number per character per second, each of these sentences takes a minute or more to transmit.
The moderator of an e-mail list for global numbers station hobbyists claimed "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.
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 or introduction 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"), characteristic phrases (e.g. "¡Atención!", "1234567890"), and sometimes musical or electronic sounds (e.g. "The Lincolnshire Poacher", "Magnetic Fields"). Sometimes, as in the case of the Israeli 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.
There is usually an announcement of the number of number-groups in the message, then the groups are 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"; "konets"). Some stations, especially those thought to originate from the former Soviet Union, end with a series of zeros, e.g. "000 000"; others end with music or other sundry sounds.
Due to the secretive nature of the messages, the cryptographic function employed by particular stations is not publicly known. It is possible that some stations use a one-time pad that would make the contents of these groups indistinguishable from randomly generated numbers or digits.
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 number 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.)
The frequently reported use of high tech modulations like data bursts, in combination or sequence with spoken numbers, suggest transmissions for differing intelligence operations.
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 more than a civilian shortwave news radio could be construed as evidence of spying. Yet governments' embassies, aircraft, and ships at sea are known to possess complex receiving equipment that could make regular use of encrypted data transmissions from the home country. These probably include charts and photos that require more transmitted data than can be sent efficiently using spoken numbers.
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 station operated by the West German BND agency whose callsign was "Hotel Kilo" () used to transmit on 9450 kHz, interfering with Radio Moscow (now The Voice of Russia) which used the same frequency. A tape recording of the interference was submitted to Radio Moscow which prompted this response:
SW Radio Africa transmits from Meyerton, South Africa, on 4880 kHz and is the "Independent Voice of Zimbabwe". Here you can view a video of the MOSSAD E10 station "Uniform Lima X-Ray" interfering with the African station.
The religious station WYFR transmits from Okeechobee, Florida, USA, on 6855 kHz. It is regularly affected by the Cuban Spanish number station "V2". A video shows the V2 interfering with the American station.-
A BBC frequency, 7325 kHz, has also been used. This prompted a letter to the BBC from a lady listener in Andorra. She wrote to the World Service "Waveguide" program complaining that her listening had been spoiled by a female voice reading out numbers in English and she 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 headquarters) who declared that the voice was reading out nothing more sinister than snowfall figures for the ski-slopes near the listener's home. With more research into this case, short wave enthusiasts are fairly sure that this was a numbers station being broadcast on a random frequency. The likelihood of the broadcast being snow readings is in doubt because it would have been illegal to broadcast on an already used frequency.
Radio Mediterranee Int.(Medi 1) transmits on 9575 kHz from Nador, Morocco. On 11 SEP 08, the English language number station E11a sent a message on 9576 kHz, which was hidden in the upper sideband of the Moroccan station. You can view the interference caused in this video clip:
Numbers station transmissions have often been the target of jamming attempts. Despite this targeting, many number stations continue to broadcast unhindered. Several theories exist that aid in explaining the inability to effectively jam the transmissions. With only a finite number of jamming transmitters available at any given time, it may be more efficient to block clandestine stations intended for a large audience rather than a message intended for a single person. Another theory is that there may be a "gentlemen's agreement" in place; i.e. "we won't jam yours if you don't jam ours". In addition, the haphazard nature of some stations, e.g. not having a fixed schedule or frequency, also makes jamming more difficult because the broadcast may go undetected.
Historical examples of jamming:
For example, the well known, defunct Lincolnshire Poacher station has the designation E3 (or E03), the Cuban "Atencion" station has designation V2 (or V02). The most recent station to be given a designation is E27, which was a station heard broadcasting on several occasions in late 2006 and early 2007.
Some stations have also been stripped of their designation if they are 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.
In 1997, The Conet Project: Recordings of Shortwave Numbers Stations, a four CD set of recordings of numbers stations was released by England's Irdial-Discs record label. This has now been made available on archive.org .
Recordings of numbers stations sometimes find their way onto records by other musicians via sampling, such as Stereolab's song "Pause", Porcupine Tree's "Even Less", Chroma Key's "Even the Waves", or various songs by Wilco, whose album Yankee Hotel Foxtrot is named after a message sampled on it. Pere Ubu's drummer Scott Krauss is an avid fan of numbers stations, and has featured recordings in several of the group's songs. The track "On the Lamb" by 310, from the album Aug 56 lasts almost half an hour, and features samples from a numbers station throughout.
The reclusive Scottish duo Boards of Canada were influenced by numbers stations at an early age. The track "Gyroscope" on the Geogaddi album contains a sample of a numbers station with a child counting.
U.S. punk band Heather Hates You references numbers stations in their song "I've Got A Secret Decoder Ring" from their 2006 disc "A Scar is Born". The album ends with a morse code message akin to certain numbers station transmissions.
Italian progressive metal band Madwork sampled a numbers station in the introduction to their track entitled "Null" in 2005.
The electronica song "Lifelight" by Andy Hunter° on the album "Life" features a sample of a Spanish number station in the background midway through the track at 5:07.
Yankee Hotel Foxtrot is the fourth album by Chicago-based rock band Wilco. The album took its name from a track on The Conet Project: Recordings of Shortwave Numbers Stations.
The final track, "Found Wanting" on Australian musician David Bridie's 2000 album Act Of Free Choice begins, ends and is interspersed with what sounds like a Numbers Station recording.
Turkish electronic music band Etnik Sentetik sampled several transmissions between 2000-2001. Numbers Station recording used at the intro of their track entitled "Kısa Dalga" in 2004.
The West German film Der Westen Leuchtet shows an agent called Harald Liebe receiving a number station transmission via a Sony ICF-7800 radio. He is then shown decoding the message using his one-time pad. See
A transcript of numbers from transmissions of the Lincolnshire Poacher station were printed on the set of a series of the UK TV Series: Mark Thomas Comedy Product. In the special episode The Secret Map of Britain number station samples were used as link music between segments.
The numbers 4, 8, 15, 16, 23, and 42 were transmitted from a numbers station on the island in ABC's drama series Lost. This message was changed after an unspecified amount of time to a distress call made by Danielle Rousseau, which played for over 16 years, after which it was shut down.
One of the clues that appears during the playing of the Lost Experience alternative reality game ("ARG") is the website http://persephone.thehansofoundation.org. The website features looping audio meant to very closely resemble a numbers station.
In the 1991 film Toy Soldiers, the Colombian forces utilize number stations to communicate between the school and their home base. There are multiple scenes which depict the encoding and decoding of information transmitted over portable numbers stations.
In the 1984 film Red Dawn, a band of high school guerrilla fighters hears two code phrases (each repeated twice) broadcast over the radio as they hide out in the wilderness. The phrases are: The chair is against the wall and John has a long mustache (the latter of which was actually used as a code-signal by the French Resistance during World War II).
In the 1959 book and 1962 film The Longest Day, members of the French resistance listen to a radio station broadcasting nonsense phrases (such as "I like Siamese cats") for particular phrases (such as "John has a long mustache," and "Wounds my heart with dolorous languour," a line from a Paul Verlaine poem), which are coded messages regarding D-Day.
In parody of this, the British TV series Allo Allo has Rene Artois (Nighthawk), Edith Artois (Mrs Nighthawk) and others operating a secret radio in wartime Nouvion. They broadcast in codephrase, usually with the codephrases having an alternative humorous or risque interpretation, with predictably hilarious results from eavesdroppers.