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.