An anti-radiation missile (ARM) is a missile which is designed to detect and home on an enemy radio emission source. Typically these are designed for use against an enemy radar, although jammers and even radios used for communication can also be targeted in this manner.
Most ARM designs to date have been intended for use against ground-based radars. Commonly carried by specialist aircraft in the SEAD (Suppression of Enemy Air Defense) role (known to the USAF as "Wild Weasels"), the primary purpose of this type of missile is to degrade enemy air defenses in the first period of a conflict in order to increase the chances of survival for the following waves of strike aircraft. They can also be used to quickly shut down unexpected SAM sites during a raid. Aircraft which fly with strike aircraft to protect them from enemy air defenses often also carry cluster bombs and are known as a SEAD escort. The cluster bombs can be used to ensure that after the ARM disables the SAM system's radar, the command post, missile launchers, and other components or equipment are also destroyed to guarantee the SAM site stays down.
Early ARMs such as the AGM-45 Shrike were not particularly intelligent; they would simply home in on the source of radiation and explode when they got near. SAM operators learned to turn their radar off when an ARM was fired at them then turn it back on later, greatly reducing the missile's effectiveness. This led to the development of more advanced ARMs like the AGM-78 Standard ARM and AGM-88 HARM missiles, which have inertial guidance systems (INS) built-in. This allows them to remember the radar's location if it is turned off and continue to home into it. It is less likely to hit the radar if it is turned off quickly, as the longer the radar is off (and assuming it never turns back on), the more error is introduced into the missile's course. The ALARM even has an added loiter mode, with a built in parachute, enabling it to descend slowly until the radar lights up, when the rocket motor will re-ignite. However, the high speed and smokeless motors of the more recent missiles such as the HARM mean that they will probably close the distance significantly before anyone even realizes one has been fired. This gives the missile a much higher probability of destroying the radar even in this circumstance. In any case, even a temporary shut down of the enemy's missile guidance radar can be of a great advantage to friendly aircraft during battle.
Several surface-to-surface missiles, like the P-700 Granit, P-500 Bazalt, MM40 Exocet and OTO Melara Teseo include a home-on-jam capability wherein the receiver component of their active radar seeker is used to home in on enemy radar, ECM or communications. This makes these missiles significantly harder to defeat with ECM and distraction countermeasures, and makes the use of semi-active missiles against them dangerous.
Due to experiences with jamming during Vietnam and Middle Eastern wars in late 1960s by US-built aircraft, the Soviets have designed an alternative guidance mode for their S-75 (SA-2) missiles, which allowed them to home onto the source of the jamming if normal forms of navigation failed. In cases of heavy jamming, missiles were often launched exclusively in this mode, ironically because passive tracking allowed the SAM operators to prevent American anti-radiation missiles from being fired at them in retaliation. Recently, The People's Republic of China developed the FT-2000 system to counter AEW and AWACS targets, based on the HQ-9 which is in turn based on the S-300PMU. The anti-radiation missile systems has been marketed to Pakistan and various countries.
More recently, air-to-air ARM designs have begun to appear, notably the Russian Vympel R-27P. Such missiles have several advantages over other missile guidance techniques; they do not trigger radar warning receivers (conferring a measure of surprise), and they can have a longer range (since battery life of the seeker head is the limiting factor on the range of most active radar homing systems).
In the 1970s, Hughes Aerospace had a project called BRAZO (Spanish for ARM). Based on a Raytheon AIM-7 Sparrow, it was meant to offer an air-to-air capability against proposed Soviet AWACS types and also some other types with extremely powerful radar sets such as the MiG-25. The project was not proceeded with.