Radio jamming is the deliberate blocking or interference with wireless communications. In the United States, radio jamming devices (known as "jammers") are illegal and their use can result in large fines.
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 under different regulations. 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.
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.
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 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, China, which used jamming extensively and still does, plays a loop of traditional Chinese music while it is jamming channels (c.f. "Attempted jamming of numbers stations).
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 onto the larger carrier signal and hence will ignore the FM signal with information.
Digital signals use complex modulation techniques such as "QPSK. These signals are very robust in the presence of interfering signals. However, 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 transmit over the end of the message, corrupting the packet.
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 Germany (during WW2), "Israel, Cuba, Iraq, Iran (Iraq and Iran war, 1980–1988), 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.
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").
During much of the "Cold War, "Soviet (and "Eastern Bloc) 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 (known as "barrage" or "frequency diversity" broadcasting) to the already heavily overcrowded "shortwave bands to such an extent that many broadcasters not directly targeted by the jammers (including pro-Soviet stations) suffered from the rising levels of noise and interference.
Other stations targeted by the Soviet jammers (but not to the same extent as RFE/RL/VOA/BBC) included "Deutsche Welle and occasionally "Vatican Radio, "Kol Yisrael and "Radio Canada International. The jamming usually took place only 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.
Some parts of the world were more impacted by these broadcasting practices than others
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 "radio 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 (a phenomenon sometimes dubbed "twilight immunity"). 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 (and most of their Eastern bloc allies) 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 (above 15 MHz); however, many radios in the Soviet Union 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. 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 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 (1941–1955) and Bucharest (1955–1977). It was the most important pirate radio in Spain and the regime considered it a threat, since it allowed its citizens to skip the censorship of the local media. Money and technological assistance for the jamming came from the "United States.
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 Britain jammed some Egyptian (during the "Suez Crisis), Greek (prior to Cyprus gaining independence) and "Rhodesian stations.
In 2002, China 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 French state-owned company "Thales Group.
Debates have been raised in Iran regarding the possible health hazards of satellite jamming. Iranian officials including the health minister have claimed that jamming has no health risk for humans. However, the minister of communication has recently admitted that satellite jamming has 'serious effects' and has called for identification of jamming stations so they can put a stop to this practice. The government has generally denied any involvement in jamming and claimed they are sent from unknown sources. According to some sources "IRGC is the organization behind satellite jamming in Iran.
The Russian armed forces have, since the summer of 2015, begun using a multi-functional "EW weapon system in "Ukraine, known as "Borisoglebsk 2. It is postulated that this system has defeated communications in parts of that country, including mobile telephony and "GPS systems.
Icasa summarised its position on the issue of cellphone signal jamming as follows:
The use of jamming devices by any entity other than National Security Cluster Departments is not authorised and/or permitted.
The National Security Cluster departments may, where supported by relevant security legislation, deploy the use of jammers in relation to, amongst others, State security functions.line feed character in
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