Number Stations are shortwave radio stations or a high frequency that transmit a call sign, usually letters from the phonic alphabet or strings of numbers, usually sent in groups of five – sometimes they feature music.
They are usually broadcasted on or around the top of the hour, and most of the time stick to a rigid schedule.
There has never been any official explanation for these broadcasts, however it is strongly believed that they are one way messages from the intelligence agencies to spies in the field.
The messages can be heard throughout the day and through the night on high or medium bands, commonly between 2 and 25 MHz and can be heard in and around the ‘utility’ bands on the edge of the international broadcast area.
Number Stations are broadcasting in many different languages from many different locations around the world. They are unlicensed and will often interfere with legitimate radio broadcasts, including air traffic controls and shipping broadcasts.
Number Stations are believed to have started at the time of the First World War. They have most certainly bee around since the cold war started, and for some reason are still being broadcast, despite the end of the cold war.
They probably started off as Morse Code (CW Mode) and then moved to voice in AM mode around the time of the cold war – Many are now using Single Side Band (SSB).
Originally numbers were hand keyed or used recording of real people. This probably carried on until the mid 1980s when the recordings were replaced by computerized and automated voices.
How do they work?
The sender (or the agency) will encrypt the message using a cryptography key that’s known as a One Time Pad. This ‘key’ is a dictionary for a language that’s only used once.
The message will then be broadcast to a pre-arranged schedule, most likely from a military base or an embassy, usually located in a different country.
An agent or spy who is out in the field is issued with a normal, standard portable radio that is capable of receiving shortwave, a copy of the transmission schedule and a book, which will be used for the OTP (one time pad).
If the message is his, he will note down the numbers and then use the key to decode the message. As soon as this is done the key is destroyed.