The Electronic warfare reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Electronic warfare

Electronic Warfare (EW) includes, but is not limited to, the following:

• Electronic Counter Measures (ECM)
  This is the active use of the electromagnetic spectrum to deny its use by an adversary. Most ECM activity is in the form of jamming.

• Electronic Protective Measures (EPM)
  This is all activities related to making enemy ECM activites less successful. Active EPMs include technical modifications to radio equipment (such as frequency-hopping spread spectrum), while passive EPM include education of operators (enforcing strict discipline) and modified battlefield tactics. An older term for EPM is ECCM (Electronic Counter Counter Measures).

• Electronic Support Measures (ESM)
  This is the passive use of the electromagnetic spectrum to gain intelligence about other parties on the battlefield for immediate tactical action. This intelligence might be used directly as fire missions for artillery or air strike orders, or as the basis of ECM/EPM actions.

ECM and active EPM can be detected by an adversary by the nature of their active use of transmissions. ESM, however, can be conducted without the enemy ever knowing it. Its strategic counterpart, SIGINT is continuously perfomed by most of the world's countries in order to gain intelligence about potential enemies' electronic equipment and tactics.

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