Electronic countermeasures
From Wikipedia, the free encyclopedia
Electronic countermeasures (ECM) are a subsection of electronic warfare which includes any sort of electrical or electronic device designed to fool radar, sonar, or other detection systems. It may be used both offensively or defensively in any method to deny targeting information to an enemy. The system may make many separate targets appear to the enemy, or make the real target appear to disappear or move about randomly. It is used effectively to protect aircraft from guided missiles. Most air forces use ECM to protect their aircraft from attack.
Offensive ECM often takes the form of jamming. Defensive ECM includes using blip enhancement and jamming of missile terminal homers.
ECM is practiced by nearly all military units—land, sea or air. Aircraft, however, are the primary weapons in the ECM battle because they can "see" a larger patch of earth than a sea or land-based unit. When employed effectively, ECM can keep aircraft from being tracked by search radars, surface-to-air missiles or air-to-air missiles.
[edit] Principles
ECM is a function of the electromagnetic spectrum or frequency of energy that is directed toward a target. Every type of radar functions in the electromagnetic spectrum and can be sensed by the proper equipment. Since radar works very much like an echo you hear if you yell into a chasm or canyon, it can be timed and identified by the frequency at which it operates. For example, if you strum an open low "A" string on a guitar, it resonates at a frequency of 110 hertz. The sound waves travel out like ripples in a pond and in all directions. When the waves reach the other side of the canyon, they are reflected or bounced back toward the source. The sound arrives at your ear after a short time and you hear the "echo". If someone on the other side of the canyon has a guitar tuned to 112 Hz and they strum it as soon as they hear the sound from your guitar, their sound will reach you at approximately the same time as your echo. This combination of sounds will sound strange to you. A radar system is much more accurate (in detecting frequencies) while it listens for the echo. If it sends out 110 Hz, it expects to "hear" a reflected 110 Hz. If the sound comes back at 112 or 115 Hz, then the radar receiver or "ear" can be confused.
ECM deals with jamming, covering (concealing) from and eliminating threats. Many sophisticated methods of jamming an electromagnetic signal exist. Covering involves overpowering a signal so that the radar receiver is overwhelmed and cannot process the echo. Elimination consists of anti-radiation missiles and bombs that home in on the radar signal and destroy the source.
