Science Fair Project Encyclopedia
Jamming is an electronic warfare (EW) technique to limit the effectiveness of an opponent's communications and/or detection equipment.
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; Wobbler; Random Keyed Modulated CW; Tone; Rotary; Pulse; Spark; Recorded Sounds; Gulls; and Sweep-through. All of these can be divided into two groups - obvious and subtle.
Obvious jamming is easy to detect as it can be heard on the receiving equipment. It is some type of noise such as stepped tones (bagpipes), random-keyed code, pulses, erratically warbling tones, and recorded sounds. The purpose of this type of jamming is to block out reception of transmitted signals and to cause a nuisance to the receiving operator.
Subtle jamming is that 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'.
Radar jamming is the intentional emission of radio frequency signals to interfere with the operation of a radar by saturating its receiver with false information. There are two types of radar jamming: noise jamming and deception jamming.
A noise jamming system is designed to delay or deny target detection. Noise jamming attempts to mask the presence of targets by substantially adding to the level of thermal noise received by the radar. Noise jamming usually employs high power signals tuned to the same frequency of the radar. The most common techniques include barrage, spot, swept spot, cover pulse, and modulated noise jamming. Noise jamming is usually employed by stand-off jamming (SOJ) assets or escort jamming assets.
Deception jamming systems (also called repeat jammers) are designed to offer false information to a radar to deny specific information on either bearing, range, velocity, or a combination of these. A deception jammer receives the radar signal, modifies it and retransmits the altered signal back to the radar.
Initially, the challenge was simple: tune in to the fixed frequencies of the radar, and then start jamming on those frequencies. However, as radars became more sophisticated they used irregular noise superimposed on the radar signal to cloak it, and the signals were broken up into short bursts, and the frequencies used were changed rapidly and constantly.
Radar jamming for the purposes of defeating speed detection radar is simpler than for military application, although it is often illegal.
E-mail jamming is used by some civil rights activists to thwart government spy networks such as ECHELON. Activists deliberately include "sensitive" words and phrases in otherwise innocuous emails to ensure that these are picked up by the monitoring systems. The theory is that the senders of these emails will eventually be added to a "harmless" list and their emails no longer intercepted, thus allowing them to regain some privacy.
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