1. Introduction
Sooner or later every Shortwave Listener will hear them, the Number Stations: 5-figure/rarely letter-groups are read by a synthezised male or female voice, are keyed in Morse code or are sent in digital mode. All transmissions start with an identifier/call, usually a 3-figure group, repeated several times. The following figure-groups specify the decryption key/group-count of the message which ends, for example, with a series of "Zero"s (or "out" if spoken).
Every network has its own "fingerprint": transmission schedule, modulation and format are typical for a specific network and will help the listener assigning it to a certain country. An increasing number of utility listener are into the hunt for Number Stations, many of them being members in the ENIGMA2000 (E2k) YAHOO group. Results are published in newsletters, or Ary Boenders "Number & Oddities" Website. For more see here: Links .

2. The story behind Number Stations
Much has been written so far about the purpose of Number Stations. After all we may quite safely assume, that they are a cheap and reliable tool of Secret Services to keep in touch with their "staff" abroad, namely their agents.
The broadcasts can be received everywhere and even with a cheap digital radio, thus hiding perfectly the true addresses. Successfull reception can be confirmed via any means at any time - an agent never will transmit an acknowledgement.

After listening for a while to Number Stations we may ask ourselves: Can we interprete the messages? No, we can't. We can write down (decode) them, but we can't decrypt them. The addressees use One Time Pads for decryption. This is a key which is used only once. To do any statistics you need at least two specimens with comparable properties (e.g. same decryption key has been used), this is not the case with OTPs. Your algorythm will sooner or later produce results, but they are ambiguous; this means that you will get results, but they are not conclusive.

3. Active Number Stations
Number Stations are on the air since many years, in fact soon after the first use of Shortwaves for military, and have had their heyday during the cold war. A variety of voice stations with all sort of musical intros only could be classified with names like:
Cynthia, Magnetic Fields, Edna Sednitzer, Swedish Rhapsody or Tyrolian Music.
That's long ago and our world became less colorful. Only one station, probably from Egypt, still uses a musical intro.

Today (2009) we regularly can hear around 12 voice networks in Russian, English, German, Czech, Arabic and Spanish and less than 10 Morse stations. Not amazingly Russia, Israel and Cuba are the most active players in the Numbers game.
The people from E2k a long time ago started to classify all networks. Morse stations are "M", English voice stations are "E", etc following by a number. As a country may operate several networks in different modes, they are combined in "families" I to XVIII. The current situation is reflected in E2k's "Control List", available there or at "Numbers & Oddities". A Quick Guide to Morse networks I can offer here: Downloads .

Many frequencies and transmission skeds do change regularly and can be predicted, but others change at any time. In most cases we are not sure if all transmissions have been intercepted - and that's the intention.


4. The fascination of Number Stations
We can't understand the messages, we don't know the purpose of a specific message, so why bother? Because it's great fun to find a new frequency, a new sked or even a new network.
We can perform traffic analysis, e.g. try to look for correlations: density of traffic vs. political tensions. We can note the number of addressees (agents)/ decryption keys and the time slots. Maybe we find out, if several transmission sites are involved. And there are the highlights: an operator makes a fault, the wrong tape is played or a faulty equipment reveals how they do it. Even the impossible once became possible: Israels MOSSAD greetings to all of us, the words "GOOD NIGHT" properly separated by dummy figures.
Number Stations have a long tradition and despite the end of the Cold War, the Warsaw Pact and the Wall of Berlin: they still are here. Googling with "Number Station" will lead you to an overwhelming amount of information, sound samples and background information.
Number Stations is not just Morse - and voice transmissions. Sophisticated digital modes increasingly are used. The XPA or XPA2 networks for example are Russian Multitone AM Systems with 20, resp. 15 tones narrowly spaced at 40, resp. 15 Hz. They belong to the same family as M12.
SK01 in turn describe Cuban transmissions, found 2007, in Phase Modulation PSK31. Other modes BPSK125 and BPSK220 are still under investigation.

There is indeed a very wide field for own investigations - Number Stations never are boring.

5. Russian Morse Number Station M12

This is a busy network with constantly changing schedules at any time. Transmissions normally start at the hour and are repeated at h+20 and h+40 min. Some frequency sets are re-used in the following year. M12 keeps local time during Summer Time period.

The 3-figure identifier is sent 3 times followed by the number of messages (1,2 or 000 for "no message"), e.g. 463 463 463 1 (repeated for 2 minutes),
decryption key and group-count follow: 4578 112 4578 112,
the message follows: 77895 44208 56196 34777 ...,
after a pause of a few seconds the end of message bit follows: 000 000 (zero sent as T).

6. The MOSSAD Voice Number Station E10

A big network, based in Israel, with rarely changing identifiers. The messages are 5-letter-groups, English spoken in NATO phonetics by a female voice.
Transmissions take place at h+00 and h+30 and can be, at a certain degree, predicted.
There are many parallel frequencies.
The table with calls and frequencies I have copied from Newsletter 50 produced by ENIGMA2000, where all networks are described in every detail (and where the copyright remains).

The identifier, repeated for 5 min means: 1 message follows,
the identifier plus 1, repeated for 5 min means: test message,
the identifier plus 2, repeated for 10 min means: no message.

Then: "Message, message, group 34, group 34, text, text"
the message follows: "hghfg ruopt lmgnb....",
"End of message, repeat, repeat"....
after the repeated text follows: "End of message, end of transmission"

E10 is, most probably, the most investigated Number Station and that since many years. There are quite interesting findings about the interaction and content of some messages found by Number Station specialists.
It is worth noting that many of E10 transmissions are jammed by a Bubble Jammer. Preliminary RDF results point towards the region of Iran as transmitter site and we certainly agree, that this would make sense...



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