Electronic countermeasures

"Missile guidance locks jammed. Their ECM spoofed the guidance packages, sir."

- Lieutenant Aki Hikowa

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. 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. 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.

History
One of the first examples of electronic countermeasures being applied in a combat situation took place during the Russo-Japanese war. On April 15, 1904, Russian wireless telegraphy stations installed in the Port Arthur fortress and on board Russian light cruisers successfully interrupted wireless communication between a group of Japanese battleships. The spark-gap transmitters in the Russian stations radioed a senseless noise while the Japanese were making attempts to coordinate their efforts in the bombing of a Russian naval base.

In the Halo universe
Covenant vessels such as the Covenant Stealth Corvette 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 Incorruptible, also possess electronic countermeasures for countering enemy Plasma torpedoes, as well as anti-ECM measures.