From Halopedia, the Halo wiki
Electronic countermeasures (also known as ECM) are a subsection of electronic warfare which includes any sort of electrical or electronic device designed to trick or deceive radar, sonar, or other detection systems like infrared and laser.
Electronic countermeasures 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. That is also true for military ships and on tanks to fool laser or infrared guided weapons. They are frequently coupled with stealth advances so that the ECM system has an easier job.
Often ECM takes the form of jamming.
Usage in the Human-Covenant War
Covenant vessels such as the Covenant stealth ship possess ECM to counter the guidance locks on Archer missiles. One of these used ECM to counter missiles fired by the UNSC Iroquois during the Battle of Sigma Octanus IV. Faced with no choice, Commander Jacob Keyes then rammed the corvette, smashing it and making its remains fall onto the planet. Much larger ships, such as the Varric-pattern heavy cruiser Incorruptible, also possessed electronic countermeasures for countering enemy plasma torpedoes, as well as anti-ECM measures.
UNSC ships, such as frigates, also possess ECM systems. A frigate's countermeasures are capable of jamming an Ceudar-pattern heavy corvette's communications for several minutes, as evidenced by the Paris-class heavy frigate UNSC Savannah disrupting the Ardent Prayer's communications during Operation: UPPER CUT.
List of appearances