Radio jamming


Radio jamming is the deliberate blocking of or interference with wireless communications. In some cases, jammers work by the transmission of radio signals that disrupt telecommunications by decreasing the signal-to-noise ratio.
The concept can be used in wireless data networks to disrupt information flow. It is a common form of censorship in totalitarian countries, in order to prevent foreign radio stations in border areas from reaching the country.
Jamming is usually distinguished from interference that can occur due to device malfunctions or other accidental circumstances. Devices that simply cause interference are regulated differently. Unintentional "jamming" occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency. Another form of unintentional jamming occurs when equipment accidentally radiates a signal, such as a cable television 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 whereas the term "interference" is used to describe unintentional forms of disruption. 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. Digital wireless jamming for signals such as Bluetooth and WiFi is possible with very low power.
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 is usually some type of noise, such as stepped tones, random-keyed code, pulses, music, erratically warbling tones, highly distorted speech, random noise, and recorded sounds. Various combinations of these methods may be used, often accompanied by regular Morse identification signals to enable individual transmitters to be identified in order to assess their effectiveness. For example, China, which did and does use jamming extensively, plays a loop of traditional Chinese music while it is jamming channels.
The purpose of this type of jamming is to block 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 the FM capture effect, frequency modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier. The receiver locks on to the larger carrier signal, and hence will ignore the FM signal that carries the information.
Digital signals use complex modulation techniques, such as QPSK. These signals are very robust in the presence of interfering signals. But the signal relies on hand shaking between the transmitter and receiver to identify and determine security settings and method of high-level transmission. If the jamming device sends initiation data packets, the receiver will begin its state machine to establish two-way data transmission. A jammer will loop back to the beginning instead of completing the handshake. This method jams the receiver in an infinite loop where it keeps trying to initiate a connection but never completes it, which effectively blocks all legitimate communication.
Bluetooth and other consumer radio protocols such as WiFi have built-in detectors, so that they transmit only when the channel is free. Simple continuous transmission on a given channel will continuously stop a transmitter transmitting, hence jamming the receiver from ever hearing from its intended transmitter. Other jammers work by analysing the packet headers and, depending on the source or destination, selectively transmitting over the end of the message, corrupting the packet.

Types of jammers

  • Portable jammers are phone-sized and low-powered devices. They can block data delivery at a distance up to 15 meters without barriers.
  • Stationary jammers are more expensive and powerful. They usually have a larger jamming radius and wider frequency band. Strong jammers can require additional cooling as they can overheat. Stationary jammers usually have a range of 100 meters and require a power supply of 230 V.
  • Self-made jammers are low-power devices that work over short ranges. However, the coverage can be extended using broadband amplifiers.

    History

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 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 Germany, Israel, Cuba, Iraq, Iran, China, North and South Korea and several Latin American countries, as well as by Ireland against pirate radio stations such as Radio Nova. The United Kingdom government used two coordinated, separately located transmitters to jam the offshore radio ship, Radio North Sea International off the coast of Britain in 1970, to enforce its domestic broadcast licensing law.

World War II

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.
During the Continuation War, after discovering the fact that the mines that the retreating Soviet forces had scattered throughout the city of Viipuri were radio-triggered rather than timer- or pressure-triggered, the Finnish forces played Vesterinen's recording of Säkkijärven Polkka without any pauses from September 4, 1941, to February 2, 1942, as they, to demine the city, needed to block the Soviets from activating the mines through the correct radio wave. The Soviets tried to trigger the mines by changing frequency; the mines had been set up to be able to be triggered by three different frequencies. The Finns countered this by playing Säkkijärven Polkka on all frequencies.
During the Battle of the Beams, Britain jammed navigation signals used by German aircraft.

Cold War era

The Soviet Union started jamming Western radio broadcasts to itself in 1948. The primary targets of jamming have been the BBC External Broadcasting Services, Voice of America and especially RFE/RL. Western nations had allowed jamming prior to World War II, but in the post-War era, the Western view has been that jamming violates the principle of freedom of information, while the Soviet view has been that under the international law principle of national sovereignty, jamming is an acceptable response to foreign radio broadcasts.
During much of 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, utilising highly directional antennas and adding extra frequencies to the already heavily overcrowded shortwave bands to such an extent that many broadcasters not directly targeted by the jammers suffered from the rising levels of noise and interference.
There were also periods when China and the Soviet Union jammed each other's programmes. The Soviet Union also jammed Albanian programmes and its own Warsaw Pact's programmes at times.
Some parts of the world were more affected by these broadcasting practices than others
Meanwhile, some listeners in the Soviet Union and Eastern Bloc devised ingenious methods to hear the Western stations through the noise. Because radio propagation on shortwave can be difficult to predict reliably, listeners sometimes found that there were times when the jamming was particularly ineffective because radio fading was affecting the jamming signals but favouring the broadcasts. At other times, the reverse was the case. There were also times when jamming transmitters were 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 used only in/near major cities throughout the Eastern Bloc. Both types of jamming were less effective on higher shortwave frequencies ; however, many radios sold on the domestic market in the Soviet Union did not tune these higher bands. Skywave jamming was usually accompanied by morse signals in order to enable identification of the jamming station so 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. Previously there had been periods when some individual Eastern Bloc countries refrained from jamming Western broadcasts but this varied widely by time and country. In general, outside of the Soviet Union itself, Bulgaria was one of the most prolific operators of jamming transmitters in the Eastern Bloc with East Germany and Yugoslavia the least.
While western governments 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 Soviet Union 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 Eastern Bloc.
In Francoist Spain, the dictatorship jammed for decades Radio España Independiente, the radio station of the Communist Party of Spain which broadcast from Moscow, Bucharest and East Berlin. It was the most important clandestine broadcaster in Spain and the regime considered it a threat, since it allowed its citizens to bypass the censorship of the local media. Broadcasts from East Germany to South Africa were also jammed.
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 Britain jammed some Egyptian, Greek and Rhodesian stations. During the early years of the Northern Ireland troubles, the British army regularly jammed broadcasts from both Republican and Loyalist paramilitary groups.