Numbers stations are shortwave radio stations of uncertain origin. They generally broadcast people reading streams of numbers, words, or letters (sometimes using a phonetic alphabet).
The voices that can be heard on these stations are often mysterious: mechanically generated; spoken in a wide variety of languages; usually female, but sometimes male or those of children.
Evidence supports popular assumptions that the broadcasts are channels of communication used to send messages to spies. This has not been publicly acknowledged by any government that may operate a numbers station, but in one case, numbers station espionage has been publicly prosecuted by a US 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 when they were spy-related phenomena, they were not unique to the Cold War.
Suspected origins and use
According to the notes of The Conet Project (see below), numbers stations have been reported since World War I. If accurate, this would make number stations among the earliest radio broadcasts.
It has long been speculated, and was charged in one case, that these stations operate as a simple and foolproof method for government agencies to communicate with spies working under cover (sometimes literally). 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 Russian constitutional crisis of 1993.
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. Numbers stations have transmitted with impunity for decades, so they are presumed to be operated or sponsored only by governments. Also, global numbers station transmissions typically require high wattage electricity that is unavailable to ranches, farms, or plantations in isolated drug-growing regions.
Radio transmitter signals under 40 watts can travel around the world under ideal conditions of frequency band, local RF noise level, weather, season, sunspots, big receiving antenna, and a superb receiver. However, spies have to work with available hand held receivers, sometimes under pressured local conditions in all seasons and sunspot years. 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 agency 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." Listening to numbers stations in the UK is illegal under the Wireless Telegraphy Act 1949, since it is unlikely that it is possible to get official permission to listen to them, however it is unlikely that the legislation would be used to prosecute those who listen to the stations privately.
Numbers stations are often given nicknames by enthusiasts, often reflecting some distinctive element of the station. For example, "The Lincolnshire Poacher", one of the best known numbers stations (supposed by many to be run by MI6), plays the first two bars of the folk song of that name 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! ¡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 USA.)
Also, a radio magazine article published during the 1980-'90s, described hobbyists using portable radio direction-finding equipment to locate a numbers station in Florida (or Warrenton, VA, - Smolinski reported by Mays, 2005) 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, DC area. The author said the Federal Communications Commission would not comment on public inquiries about USA 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.
The Atención spy case evidence
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 USA 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.
USA government evidence included the following three examples of decoded Atención messages. (Not reported whether the original clear texts were in Spanish.):
* "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 May 8, International Women's Day)
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. (Chris Smolinski of Spooks to Miami New Times reporter Brett Sokol, 2001)
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 phonetic "code names" (e.g. "Charlie India Oscar", "250 250 250"), characteristic phrases (e.g. "¡Atención! ¡Atención!", "1234567890"), and sometimes musical or electronic sounds (e.g. "The Lincolnshire Poacher", "Magnetic Fields"). Sometimes, as in the case of the Israeli phonetic alphabet stations, the prelude can also signify the nature or priority of the message to follow (e.g. "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 phonetic letters. The groups are typically repeated, either by reading each group twice, or by repeating the entire message as a whole. These messages are commonly thought to be encrypted with a one-time pad; hence, the contents of these groups are indistinguishable from randomly generated numbers or digits.
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"; "konec"). 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.
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.
The USSR and superpower number stations
* During the Cold War there was substantial and substantive evidence that the USSR may have used 500 kW transmitters on the other side of the Urals to reach agents in Western Europe, North Africa and possibly North America.
* HF direction finding evidence that was collected by many different sets of amateurs in Europe, Africa and the Americas during the Cold War substantiates number stations broadcasting from the East of the Urals.
* Existing USSR technical literature shows that the USSR pioneered HRS 8/8/1 directional HF antennas for shortwave news and information broadcasting in the late 1960s - mid 1970s. Thus it is possible that lower transmitter powers (like 100 kw) were used in the 1980s -- the later part of the Cold War.
* Superpower number station broacasting from the USSR cannot be concretely proven to this day: The USSR jammed HF broadcasts from the west making many HF direction finding attempts nearly impossible. The HF bands in the European region were very crowded during most of the Cold War making good HF direction problematic.
Amplitude modulated, frequency agile Class-B (Push-Pull) modulated HF transmitters are the workhorses of numbers stations. Polyphase and PDM modulators were not used by numbers stations because energy costs were not an issue in the running of these stations.
This may change in the future with the introduction of Digital Radio Mondiale's (DRM) multimedia capabilities.
Each data service broadcast within the DRM multiplex is signalled in the SDC data field and has an 11 bit unique identifying code allowing for up to 2048 different data services. This data is subsequently transmitted as part of the MSC datagram.
A summation of DRM's data capabilities is available at this external link: 
The Deutsche Welle DRM PAD service has a data rate of approximately 800 bit/s. The MSC radio text data is sent out at a rate of 80 bit/s. For the transmission of messages to agents in the field, 80 bit/s is more than adequate.
Application of spectrum analysis to number station signals has revealed the presence of data bursts, RTTY-modulated subcarriers, and phase-shifted carriers. RTTY-modulated subcarriers were also present on some U.S. commercial radio transmissions during the Cold War.