Science Fair Project Encyclopedia
An anti-radiation missile is a missile which is designed to detect and home in on the emissions of an enemy radar installation. Commonly carried by specialist "Wild Weasel" aircraft in the SEAD role, 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 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-45 Shrike 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.
Another design point of modern ARM missiles, other than the range (which is hopefully greater than that of the SAM systems it will be targetted 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 very fastest SAM systems (SA-10/S-300 or SA-20/S-400), the aircraft is likely to win.
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