Anti-radiation Missile - Air-to-Ground

Air-to-Ground

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 defences 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.