ENIGMA 2000 Newsletter - Issue 2

January 2001
Articles, newsreports and Items of interest : e2k_news@hotmail.com

Enigma 2000 article

Jamming

Regular listeners to numbers stations will be aware that some of these stations are heavily jammed, sometimes to the point of making the transmission totally unreadable. It is obvious that a lot of effort goes into the jamming of numbers stations.

For example, the station known as Lincolnshire Poacher [E3] simultaneously transmits on three separate frequencies at a time, more often than not, all three of these frequencies will suffer jamming to some degree, sometimes to the point that the transmission is totally unreadable. This would point to the existence and use of at least three jamming transmitters of appreciable power, and their associated equipment.

So what's it all about ?

Jamming can be defined as the deliberate interference of an electromagnetic communications system, with the intent to prevent, or to make difficult, the reception of the information transmitted by the targeted communications system.

Well, that's all a bit heavy, so what does it mean? As we know, numbers stations transmit information, encrypted in the form of numeric groups, some thought to be produced by the use of one time pads, and believed to carry messages to intelligence personnel or 'illegals' operating inside a hostile foreign country. In the case of some stations, similar information outside the remit of normal diplomatic communications channels is transmitted to embassies. If you put yourself in the position of the target country, it is obviously in your country's interests to prevent the information being received and/or be acted upon by those persons who may be operating within your country - most likely with a purpose detrimental to the interests of your country.

There are two ways you could go about this - you could concentrate your efforts on breaking the encryption which is being used to secure the message, or taking steps to prevent the instructions or information within the message being acted upon.

In the case of numbers stations transmissions the encryption provided by the one time pad system is believed to be currently unbreakable as long as certain rules regarding their use are followed. Since there is no way to ascertain which, if any, of the many transmissions contain actual traffic and which are 'null', attempting to decrypt the transmissions would be a time and resource consuming act, and would most likely prove fruitless. The only option left therefore is to prevent the reception of the transmissions by the intended recipients in the first instance. This may be accomplished by jamming.

To jam a transmission, equipment no more complex than a transmitter of suitable power operating on the target frequency and a suitable antenna system are all that is required.

Some form of modulation is normally added to the carrier to increase the effectiveness of the jamming, but other than equipment to produce this modulation, no particularly exotic equipment is required.

Taking into account the frequencies and powers required, the equipment used in jamming numbers stations would not be so different from commercial broadcast equipment, and indeed in the case of the frequencies used by some numbers stations, suitably modified commercial broadcast, aviation or marine transmitters could be put to use.

An interesting development in dedicated jamming technology is the 'intelligent' or 'look through' jammer, which consists of a linked transmitter and receiver. In this set up, the equipment is tuned to the frequency that is to be jammed, and the receiver then monitors the frequency. When it detects a signal on the frequency, it will activate the transmitter portion of the equipment, which will then proceed to jam the frequency. After a set time, the jamming power is reduced, or switched off, and the receiver 'looks through' to see if a transmission is still present on the frequency. If a transmission is present, it reactivates the jamming transmitter, if not the transmitter remains off, the receiver recommencing its watch. Although it is unknown if such equipment is deployed against numbers stations, certain jamming sounds would give rise to possibility that such equipment might be in use, although perhaps not by those who jam the Lincolnshire Poacher.

Those attempting to affect the Lincolnshire Poacher sometimes commence jamming on the pre 1800Z frequencies after the 1800Z schedule changeover! It is quite possible that some jammers, which jam stations having a definite schedule, operate under the control of something as simple as a timer switch.

There are two main forms of jamming technique - Spot and Splash.

Spot jamming is the form utilised against numbers stations, where a single frequency is jammed by a single transmitter, and is usually quite effective, as can be seen from the disruption caused to many numbers transmissions.

Splash jamming, sometimes called barrage jamming, is where a single wide bandwidth transmitter is used to jam a specific range of adjacent frequencies, [eg. 5.0 MHz to 5.2 MHz].

This is normally utilised against multi channel communications systems having several closely spaced channels, a single transmitter being used to jam all the channels simultaneously. It must be noted however, that although some numbers stations which operate on several parallel frequencies are often jammed simultaneously, this is still individual spot jamming of each frequency, as they are too widely separated to be Splash jammed.

As mentioned above, some form of modulation is often added to the jamming transmitter carrier to increase the effectiveness of the jamming. Although an unmodulated carrier could be used to disrupt communications, it is not the most effective manner of jamming. There are several forms of modulation commonly used in conjunction with jamming. A few of these are listed below, but it should be noted that any conceivable form of modulation may be used.

White noise

White noise can be deployed as a jamming modulation, and is insidious in the fact that it may not be recognised as jamming at all, being simply dismissed as natural atmospheric interference. Sometimes the white noise may itself be modulated, often with a 'pulsing' quality, which is quite effective, having a high 'annoyance factor'.

Bubble

Bubble jamming gets its name from the particular sound which it makes - if you listen carefully, you will notice that the sound is similar to that of water boiling in a pot. Often used in the jamming of numbers stations.

Tones

Tones are another common jamming modulation, some being single tones applied to the frequency for several seconds, some sounding like two alternating tones, some with rising and falling tones, all of various pitches.

As mentioned above, this list is very far from exhaustive, and only the most common have been listed. It may be useful, however, if you ever come across an unusual form of jamming, that you note it in your log.

In early October, a curious jamming signal was heard on 5059kHz in the late evening, which consisted of rapidly changing 'tune' of random tones, distinct from any data mode which then became the ubiquitous bubble jamming. Test of a jamming transmitter?

Another point to note is that although the jamming may not have a great effect on your reception at your position, the jamming experienced by the intended recipient may be much greater. This being due to his position in relation to that of the jamming transmitter, and also the fact that he will probably not have the luxury of a large antenna and a professional receiver with audio filtering. In fact, he will probably have not much more than a few feet of wire and an inexpensive broadcast type receiver.

Perhaps at this juncture, we should consider that numbers stations are not the only transmissions to suffer from jamming, indeed some broadcast stations are subjected to jamming. A quick look through the "Passport to World Band Radio", or a similar publication, will reveal that some stations, particularly those deemed by certain regimes to be subversive, are affected in this way. A large scale example of this would be jammers deployed in Russian cities, during the Soviet era, to prevent the reception, by the general public, of Western radio broadcast stations. A more recent example of this would in 1998, when Voice of Tibet was allegedly jammed by the Chinese authorities, who suddenly decided to broadcast a Radio Canada International programme on the same frequency.

The Chinese authorities have been known to jam other external broadcasters, including BBC World Service and Voice of America.

In 1996, around the time of the Balkans Conflict, the independent Belgrade radio station Radio B92 was jammed by several jamming transmitters, restricting its reception area to only 40% of its usual coverage.

Perhaps one of the most interesting recent developments in jamming technology is the mobile phone jammer, developed by an Israeli company. The unit prevents the use of mobile telephones, in transmission and reception.

Whilst intended to be utilised in concert halls, theatres, churches and other premises where it may be inappropriate for mobile phones to ring, it could obviously have other applications.

Anon of NI