Anti-radiation missile
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An anti-radiation missile (ARM) is a missile which is designed to detect and home in on the emissions of an enemy radar.
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 role (known to the USAF as "Wild Weasels"), the primary purpose of this type of missile is to degrade enemy air defences 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 etc. are also destroyed to make sure the SAM site stays down.
Early ARMs such as the AGM-45 Shrike weren't particularly intelligent; they would simply home in on the source of radiation and explode when they got near. Smart 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' where the radar last was if it is turned off and continue to home into it. It is less likely to hit the radar in this case as the earlier the radar is turned off (and assuming it never turns back on), the more error is introduced into its course, however the high speed of the HARM and its smokeless motor means that it will probably close the distance significantly before anyone realises it has been fired and gives it a good chance of hitting even in this circumstance.
The British ALARM has an extra ability - loitering. A parachute allows it to slow its descent until the radar reactivates. The AGM-136 Tacit Rainbow would have operated in the same way. This was a small, air launched cruise missile, which could be fired either at a transmitting radar, or an area in which SAM radars are believed to be hiding. In the event of the radar deactivating, the Tacit Rainbow would loiter, using its small jet engine, until the radar reappeared - a Persistent Anti-radiation Missile. The project was cancelled in 1991, for budget reasons.
Another design point of modern ARM missiles, other than the range (which is hopefully greater than that of the SAM systems it will be targeted at) is their speed. Some SAM systems utilise huge missiles which are able to accelerate up to incredible speeds (some as high as Mach 10), which means that if the ARM is to be useful in a 'duel' between an aircraft and a SAM site, the ARM should be able to fly to and hit the SAM site faster than the SAM can fly to and hit the aircraft. The AGM-88 HARM mostly succeeds in this area; its top speed of around Mach 4 is partly due to its altitude and speed advantage at launch over the SAM (which has to climb from rest at ground level), and partly due to its powerful rocket motor. This means that if the SAM launches a missile at the aircraft first, unless it is one of the fastest SAM systems such as the SA-10/S-300 or SA-20/S-400, the aircraft is likely to win.
Air-to-Air
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.
References
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