Electronic countermeasure (ECM) radio-frequency jamming equipment is an important military weapon for depriving the enemy of effective radio communications and radar return signals. It can be accomplished by powerful broadband transmitters having the necessary agility to change transmission modes and frequency rapidly in response to enemy reaction. The transmitters can be on ships, aircraft, or vehicles. However, any active jammer, like any active radar, invites retaliatory strikes from enemy missiles capable of homing in on the transmitter's RF emissions and destroying ground-based antennas or air-, sea-, or land-based platforms containing the transmitter.

Passive monitoring by satellites and high-altitude aircraft seeks to locate and identify possible electronic threats to ships or aircraft over or near enemy territory. In addition, most combat aircraft and helicopters carry receivers that permit them to detect such electronic threats as missile guidance beams and determine the bearings of their sources so they can take evasive action.

Specialized electronic countermeasure (ECM) aircraft called weasels are equipped with more extensive broadband radar and radio receivers that can also pinpoint sources of radiation, either defensive countermeasures or offensive guidance beams, as they fly low over enemy territory. These aircraft are fitted with antiradiation missiles that can home in on enemy antennas and transmitters to destroy them. If the targets are electronic countermeasure (ECM) transmitters, the aircraft are performing an electronic counter-countermeasure (ECCM) function.

Radar-guided missiles can be misdirected by transmitting false echoes with the same modulation patterns and frequencies as the missiles' guidance...