(Date: Sun, 28 Jun 1998 22:43:09 +0200)
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Here is the promised jamming article. The article gives some info about jamming in general and the various types of jammers. In next month's column I will publish the frequencies and findings of the few readers who were so kind to send me their jammer-logs and comments.
Everyone must have heard them, those odd sounding signals. Pulse jammers, and bubble jammers with their typical 'woo woo woo' sound, are the most common types of jammers on SW. The amount of jammers is amazing in the 4, 5 and 6 MHz bands, especially in the evening between 1900-2100 UTC and at night between 0200-0400 UTC, equalling the late evening and early morning in the Middle and Far East, where most of the jamming nowadays originates. Top of the list of suspects are Iran, Iraq and Cuba. The USA may be responsible for the jamming of Iraqi and Iran based stations. Possibly using jammers in EW-aircraft or vessels. Cuba jams Radio Miami, Radio Marti, WRMI, and stations which relay La Voz de la Fundacion (WHRI, maybe some others). Radio Free Asia reported jamming of its Vietnamese and Chinese services by possibly China, North Korea or Vietnam. Aeronautical, maritime and SAR stations suffer from these activities. Tony Orr, WUN's aeronautical editor wrote in his column: ''Many of you have by now either heard or heard about the troubles with jammers in the 5 MHz range of frequencies lately. These jammers, targeting a certain clandestine station broadcasting from the Middle East, are literally all over the band, causing the aeronautical ground stations operating there to scramble for new frequencies to use in lieu of their regular 5 MHz homes. Especially hard hit was Gander Radio on 5649 kHz, which has seen fit to move down to 4675 kHz for it's operations on NAT-C.''
Tony is right, I heard it myself. Recently Dutch Coast Guard aircraft had to change frequency because of this problem. SAR frequency 5680 kHz is also a victim. SAR monitor Alan Gale from the UK sent me the following comments:
« The interefrence on 5680 kHz first appeared around the 2 August 1997. It fades in here in the UK around 1600 UTC every evening. The AM station comes on air, and quickly gets jumped on by a 'Bubble Jammer'. After a while the station stops broadcasting and the jammer also stops, but then seconds after the broadcast restarts again the jammer also returns.
The clandestine station which was heard on 5680 kHz in the clear before the jammer caught up with it had a YL presenter. Shortly afterwards what sounded to be the same presenter was heard on 5670, 5660 and 5630 kHz. The pattern seemed to be transmit on one channel until the jammer commenced, and then QSY to another channel on steps a multiple of 10 kHz away. As soon as the jammer caught up with them they stopped transmitting, though in many cases a blank carrier remained on channel and the jamming continued. The jammer only ceased when the carrier appeared to be switched off.
No Station IDs were heard, but the words 'Iran' and 'Rafsanjani' were heard on the station on 5680 with the YL presenter. On another occasion the name 'Khomeni' was heard several times, and if as seems likely, this is 'The Voice of the Mohajed' operating from the Iraqi border into Iran this would make sense. There may well be a set pattern as to which channel the station changes to when the jamming commences, it would be difficult for listeners if there wasn't. A regular pattern would make life easier for the jammers though, so a further study of this might well prove interesting. »
Jamming was first used by the Germans during World War 1. In the early 1920s, competing broadcasters in the United States jammed rival radio programs. During the 1930s, jamming became a political weapon. World War II demonstrated that a jamming network operating against military circuits, was a potent wartime weapon.
"Knickebein" was an early guided weapon system. The Germans invented this system and used it in WWII. It used two beams; one transmitted from Kleve for guidance, and a second cross-beam from Bredstedt. The bombers followed the first beam until it intersected the second, directly over the target, and dropped their bombs.
Documents retrieved from downed German bombers showed that the beams operated on a frequency of 30 MHz. In those days the only receiver that was capable of detecting the beams was the Hallicrafters S-27. Fitted to a search aircraft, the beam was detected. The Germans made this work easier by testing their system over England instead of Germany.
Knickebein was called "headache" by the British, and jammers dubbed "aspirins" were developed. Soon German bombing accuracy diminished due to the interference of the British jammers.
The War's end saw jamming continued but on an even larger scale, especially with the advent of the Cold War. Stalin decided in 1948 to launch massive jamming campaigns against the West. At first, the commitment was made using a dozen jammers operating against Russian-language broadcasts of the Voice of America. By 1956 about 3,000 Soviet block jammers were operating against Western broadcasts in all languages.
Their jamming system was administered by a secret department in the Ministry of Communications, privately known as the Krestyaninova Section. It was named after Natalia Krestyanoniva who ran the department for more than twenty-five years.
The purpose of all jamming is to interfere with the enemy's effective use of the electromagnetic spectrum. Use of the spectrum involves the transmission of information from one point to another. This information can take the form of voice or non-voice (e.g., video or digital format) communications, command signals to control remotely located assets, data returned from remotely located equipment or the location and motion of friendly or enemy assets (land, sea or air).
For many years, jamming has been called electromagnetic countermeasures (ECM), but it is now referred to in most literature as electronic attack (EA). EA also includes the use of high levels of radiated power or directed energy to physically damage enemy assets. Jamming is sometimes called "soft kill" because it temporarily makes an enemy asset ineffective but does not destroy it.
The basic technique of jamming is to place an interfering signal into an enemy receiver along with the desired signal. Jamming becomes effective when the interfering signal in the receiver is strong enough to prevent the enemy from recovering the required information from the desired signal, either because the information content in the desired signal is overwhelmed by the power of the jamming signal or because the combined signals (desired and jamming) have characteristics that prevent a processor from properly extracting or using the desired information.
Communications jamming (COMJAM) is the jamming of communications signals.
This is normally considered the jamming of tactical HF, VHF and UHF signals using noise-modulated cover jamming, but it can also mean the jamming of point-to-point microwave communications links or command and data links to and from remote assets.
The effectiveness of a jammer is calculable only in the context of the enemy receiver that it jams. The most common way to describe that effectiveness is in terms of the ratio of the effective jammer power (i.e. the jamming signal power that gets into the heart and soul of the receiver) to the signal power (that the receiver really wants to receive). This is called the "jamming to signal ratio," or the "J-to-S ratio," or simply the "J/S."
Jamming signals are, by their nature, one-way transmissions. In general, the performance of the jamming signal is the same whether its target is a communications receiver or a radar receiver. Its acceptance by the receiver differs from that of the desired signal in two ways.
Every type of receiver must have an adequate signal-to-noise ratio (SNR) in order to properly process the signals it is designed to receive. The SNR is the power ratio of the desired signal to the noise power in the receiver's bandwidth. The received desired-signal power is a function of the transmitter power, the length of the transmission path, the operating frequency and (for radars) the radar cross section (RCS) of the target. Cover jamming injects additional noise into the receiver, which has the same effect as increasing the transmission-path length or decreasing the RCS of a radar's target.
When the jamming noise is significantly higher than the receiver's thermal noise, we speak of the jamming-to-signal (J/S) ratio rather than the SNR, but the effect on signal reception and processing is the same. If cover jamming is increased gradually, the operator or the automatic processing circuitry following the receiver may never become aware that jamming is present - only that the "SNR" is becoming extremely low. The required RCS depends on the nature of the received signal and the way it is processed to extract its information. For voice communications, the SNR will depend on the skill of the speaker and the listener and the nature of the messages being passed. Effective communication ceases when the SNR rises to the point at which no information can be received. For digital signals, inadequate SNR causes bit errors and communication ceases when the bit error rate is too high to pass messages.
If frequency hopping is employed in either radar or communications applications, the frequency band accepted by the receiver is a "moving target"
When other types of spread-spectrum techniques are used, the signal is spread over a wide frequency range that the receiver can reverse to achieve the sensitivity appropriate to the signal before it was spread. The problem for the jammer is that to be effective, it must spread its available power over the entire frequency that the receiver might be receiving - over all the angular space that might contain the receiving antenna - during all of the time that the receiver might be accepting signal energy. Still, it is only the amount of power that gets through all of the receiver's defenses, that contributes to J/S. Since a jammer's transmitter power is directly related to its size, weight, prime power availability and cost, the answer is seldom just to increase the jammer output power until there is enough effective jammer power. The more the jammer knows about the operation of the receiver, the more narrowly it can focus its jamming power to what the receiver will notice Jammer energy-focusing is called "power management," and it can only be as good as the information available about the jammed receiver. The bottom line is that the jammer can concentrate its power where it will do the most good.
Many dxers -especially the 'spook hunters-, often report carriers on many frequencies between 4 and 6 MHz. Sometimes a station pops up after a while, but most of the times nothing happens. At least that's what you think. Although I have no solid proof, I am pretty sure that these carriers have a purpose, namely to attract jammers. When the jammers are busy jamming the carriers, the station itself can broadcast without being jammed.
A few examples:
The 'chase': 5721 17.03 jammer, stopped at 17.03; jumped to 5680 at 17.05 because there was an carrier. Both carrier and jammer stopped at 17.06. The jammer jumped from 5680 to 5729 and stopped after one minute. Then to 5768 at 17.07 and stayed there also for one minute. Back again to 5729 where it was active for quite a while.
Another one: 5660 17.45 UTC a Clandestine? stn with a marching song and a male voice in unid language came on. Its signal was good. About 30-40 seconds later, the jammer on 5658 jumped to 5660 kHz. 10 seconds later two other jammers joined in, all power houses. Very loud. The station often changed frequency, hopping up and down the dial with no obvious "strategy" in 10, 20 or 30 kHz steps. It was chased by 3 jammers. It took the jammers not much time to catch up with the station.
''Is someone listening to all these frequencies so that he can switch on the jamming device at the right moment?'', you may ask. No, not really. But it's close....... This is how it works:
The various types of jamming signals are described in the US Army Field Manual 24-33 chapter 3. This is an exact quote of the relevant
parts of the text, hence the 'we', 'us' and 'our' expressions :-)
Jamming is an effective way for the enemy to disrupt our command, control, and communications on the battlefield. All the enemy needs to jam us is a transmitter tuned to our frequency with enough power to override friendly signals at our receivers. Jammers operate against receivers--not transmitters. There are two modes of jamming: spot and barrage.
Jamming can be difficult, if not impossible to detect. For this reason, we must always be aware of the possibility of jamming and be able to recognize it. The two types of jamming most commonly encountered are obvious and subtle jamming.
Radio operators must be able to recognize jamming. Again, this is not always an easy task. Threat jammers may employ obvious or subtle jamming techniques. Also, interference may be caused by sources having nothing to do with enemy jamming. Interference may be caused by the following:
The enemy can use two types of jamming signals: powerful unmodulated or noise-modulated signals. Unmodulated jamming signals are characterized by a lack of noise. Noise-modulated jamming signals are characterized by obvious interference noises.
Adjust the receiver.
When jamming is experienced, we should always check to ensure the receiver is tuned as precisely as possible to the desired incoming signal. A slight readjustment of the receiver may provide an improved signal-to-jamming ratio. Depending on the radio being used, some of these methods are:
Special thanks to the following dxers for their logs and comments: Tony Orr, Alan Gale, Iron Eagle, Markus Buttinger, Day Watson, Alec Muffett, Clarence Thompson, John Maky, Roger Preston, Simon Denneen, and various anonymous dxers.
|See also Newsletter 5 Jamming section.|
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