An anti-radiation missile (ARM) is a missile designed to detect and home in on an enemy radio emission source. Typically, these are designed for use against an enemy radar, although jammers and even radios used for communications can also be targeted in this manner. The earliest known anti-radiation weapon is a variant of the Blohm & Voss BV 246 radar guided bomb. Air-to-surface Most ARM designs to date have been intended for use against ground-based radars. Commonly carried by specialist aircraft in the Suppression of Enemy Air Defenses (SEAD) role (known to United States Air Force 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 chance of survival for the following waves of strike aircraft. They can also be used to quickly shut down unexpected surface-to-air missile (SAM) sites during an air raid. Often, SEAD escort aircraft also carry cluster bombs, which can be used to ensure that, after the HARM disables the SAM system's radar, the command post, missile launchers, and other components or equipment are also destroyed to guarantee that 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 it. 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 such as the AGM-78 Standard ARM, AGM-122 Sidearm, and AGM-88 HARM missiles, which have inertial guidance systems (INS) built-in. This allows them to remember the radar's direction if it is turned off and continue to fly towards it. ARMs are less likely to hit the radar if the radar is turned off shortly after the missile is launched, 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 activates, whereupon the rocket motor will re-ignite. Even a temporary shut down of the enemy's missile guidance radar can be of a great advantage to friendly aircraft during battle. Defence Research and Development Organisation is working on the Rudram