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Radio jamming is the (usually deliberate) transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without checking that it is in use first, or without being able to hear distant stations on the same frequency. Another form of unintentional jamming occurs when equipment accidentally radiates a signal, such as a cable TV plant that accidentally emits on an aircraft emergency frequency.

Distinction between "jamming" and "interference"

Originally the terms were used interchangeably but nowadays most radio users use the term "jamming" to describe the deliberate use of radio noise or signals in an attempt to disrupt communications (or prevent listening to broadcasts) whereas the term "interference" is used to describe unintentional forms of disruption (which are far more common). However the distinction is still not universally applied. For inadvertent disruptions, see electromagnetic compatibility.

Method

Intentional communications jamming is usually aimed at radio signals to disrupt control of a battle. A transmitter, tuned to the same frequency as the opponents' receiving equipment and with the same type of modulation, can, with enough power, override any signal at the receiver.

The most common types of this form of signal jamming are random noise, random pulse, stepped tones, warbler, random keyed modulated CW, tone, rotary, pulse, spark, recorded sounds, gulls, and sweep-through. These can be divided into two groups – obvious and subtle.

Obvious jamming is easy to detect because it can be heard on the receiving equipment. It usually is some type of noise such as stepped tones (bagpipes), random-keyed code, pulses, music (often distorted), erratically warbling tones, highly distorted speech, random noise (hiss) and recorded sounds. Various combinations of these methods may be used often accompanied by regular morse identification signal to enable individual transmitters to be identified in order to assess their effectiveness. For example, Chinamarker, which used jamming extensively and still does, plays a loop of traditional Chinese music while it is jamming channels (c.f. Attempted jamming of number stations).

The purpose of this type of jamming is to block out reception of transmitted signals and to cause a nuisance to the receiving operator. One early Soviet attempt at jamming western broadcasters used the noise from the diesel generator that was powering the jamming transmitter.

Subtle jamming is jamming during which no sound is heard on the receiving equipment. The radio does not receive incoming signals yet everything seems superficially normal to the operator. These are often technical attacks on modern equipment, such as "squelch capture". Thanks to FM capture effect, Frequency Modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier.

History of Jamming

During World War II ground radio operators would attempt to mislead pilots by false instructions in their own language, in what was more precisely a spoofing attack than jamming. Radar jamming is also important to disrupt use of radar used to guide an enemy's missiles or aircraft. Modern secure communication techniques use such methods as spread spectrum modulation to resist the deleterious effects of jamming.

Jamming of foreign radio broadcast stations has often been used in wartime (and during periods of tense international relations) to prevent or deter citizens from listening to broadcasts from enemy countries. However such jamming is usually of limited effectiveness because the affected stations usually change frequencies, put on additional frequencies and/or increase transmission power.

Jamming has also occasionally been used by the Governments of Germanymarker (during WW2), Israelmarker, Cubamarker, Iraqmarker, Iranmarker (Iraq and Iran war, 1980-1988), Chinamarker, North and South Koreamarker and several Latin American countries, as well as by Irelandmarker against pirate radio stations such as Radio Nova. [139041] The United Kingdommarker government used two coordinated, separately located transmitters to jam the offshore radio ship,Radio North Sea International off the coast of Britain in 1970.[139042].

World War Two

In occupied Europe the Nazis attempted to jam broadcasts to the continent from the BBC and other allied stations. Along with increasing transmitter power and adding extra frequencies, attempts were made to counteract the jamming by dropping leaflets over cities instructing listeners to construct a directional loop aerial that would enable them to hear the stations through the jamming. In the Netherlands such aerials were nicknamed "moffenzeef" (English: "kraut sieve" ) [139043]

Cold War era

During the Cold War Soviet jamming of some Western broadcasters led to a "power race" in which broadcasters and jammers alike repeatedly increased their transmission power, utilised highly directional antennas and added extra frequencies to the already heavily overcrowded shortwave bands to such an extent that many broadcasters not directly targeted by the jammers (including pro-Sovietmarker stations) suffered from the rising levels of noise and interference. . [139044]

A further method used was operating transmitters for domestic radio stations on the same or nearby frequencies for example, for many years the GDRmarker operated at Wiederaumarker a transmitter on the same mediumwave frequency that Mühlacker radio transmitter used with an output power of 100 kW, which made it difficult to receive the AFN Mühlacker radio transmitter in much of the GDR.

Radio Free Europe and its sister service Radio Liberty were the main target of Sovietmarker jammers followed by Voice of America and the BBC World Service.

Other stations targeted by the Sovietmarker jammers (but not to the same extent as RFE/RL/VOA/BBC) included Deutsche Welle and occasionally Radio Vaticanamarker, Kol Yisrael and Radio Canada International. The jamming usually only took place during programming in languages widely spoken in Eastern Bloc countries (e.g., Russian, Polish, Czech, Lithuanian, etc.). Programmes in English or other major Western languages were rarely (if ever) jammed intentionally. [139045]

There were also periods when Chinamarker and the USSRmarker jammed each other's programmes. The USSRmarker also jammed Albanian programmes at times.

Some parts of the world were more impacted by these broadcasting practices than others

Soviet Union

Meanwhile some listeners in the Soviet union and Eastern bloc devised ingenious methods (such as homemade directional loop antennas) to hear the Western stations through the noise. Because radio signal propagation on shortwave can be difficult to predict reliably listeners sometimes found that there were days/times when the jamming was particularly ineffective because radio fading (due to atmospheric conditions) was affecting the jamming signals but favouring the broadcasts. On other days of course the reverse was the case. There were also times when jamming transmitters were (temporarily) off air due to breakdowns or maintenance. The Soviets used two types of jamming transmitter. Skywave jamming covered a large area but for the reasons described was of limited effectiveness. Groundwave jamming was more effective but only over a small area and was thus only used in/near major cities throughout the Eastern bloc. Both types of jamming were less effective on higher shortwave frequencies (above 15 MHz) however many radios in the USSR didn't cover the higher bands. Skywave jamming was usually accompanied by morse signals in order to enable (coded) identification of the jamming station in order that Soviet monitoring posts could assess the effectiveness of each station.

In 1987 after decades of generally refusing to acknowledge that such jamming was even taking place the Soviets finally stopped jamming western broadcasts with the exception of RFE/RL which continued to be jammed for several months into 1988.

While western governmentsmarker may have occasionally considered jamming broadcasts from Eastern Bloc stations, it was generally accepted that doing so would be a pointless exercise. Ownership of shortwave radios was less common in western countries than in the USSR where due to the vast physical size of the country many domestic stations were relayed on shortwave as it was the only practical way to cover remote areas. Additionally western governments were generally less afraid of intellectual competition from the communist bloc.

However in Latin America there were instances of communist radio stations such as Radio Venceremos being jammed, allegedly by the CIA, while there were short lived instances where Britainmarker jammed some Egyptianmarker (during the Suez crisis), Greek (Prior to Cyprusmarker gaining independence) and Rhodesian stations. [139046]

Post Cold War (1989 -- Present)

People's Republic of China

In 2002, Chinamarker acquired standard short-wave radio-broadcasting equipment designed for general public radio-broadcasting and technical support from Thales Broadcast Multimedia, a former subsidiary of the Frenchmarker state-owned company Thales Group.
  • It is assumed that China is using ALLISS technology for jamming foreign radio stations broadcasting into China.
  • Thales jamming technology only operates at power levels below 500kW (for its shortwave jamming products).
  • Adele Milna (BSEE) of Continental Electronics (in an audio file held at shortwave.org) claims that China has duplicated his companies 100 kW, 250kw shortwave transmitters. It is unclear if these products were indeed duplicated or if broadcast jamming (as opposed to future product sales) were a reason for the duplication.


Other Countries

  • Since the early 1960s, the practice of radio jamming has been very common in Cubamarker, blocking not only American government funded radio stations (Such as VOA) but also radio stations owned and/or operated by (or selling airtime to) Cuban exile groups transmitting from Miami, Floridamarker, such as La Cubanisima, Radio Mambi, and Cadena Azul. The same practice has been applied to Radio Marti and TV Marti, operated by the U.S. Information Agency since 1985.
  • North Koreamarker and South Koreamarker still regularly jam some of each other's radio (and sometimes television) stations. (See: Radio jamming in Korea)
  • Several middle eastern countries (particularly Iranmarker) jam shortwave broadcasts (and even occasionally attempt to jam satellite TV signals [139047] [139048]) targeted at their countries.
  • During 2007 Indiamarker allegedly jammed some broadcasts from Pakistanmarker #
  • Pakistan has recently stated its intention to begin jamming clandestine radio stations operated by the Taliban #


See also



References

External links




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