Automatic activation device

Automatic Activation Device in skydiving terminology refers to an electronic-pyrotechnic or mechanical device that automatically opens the main or reserve parachute at a preset altitude or after a preset time.AADs are typically used to open the reserve parachute container below a preset altitude when they detect that the user's descent rate exceeds the preset activation speed. This indicates that the user has not opened his parachute, or that the parachute is malfunctioning and is not slowing the descent rate sufficiently.
See more at Wikipedia.org...