Another mode called Mode S (Select) is designed to help avoiding overinterrogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode S transponders are compatible with Modes A & C SSR system.[2] This is the type of transponder that makes the ACAS II (Airborne Collision Avoidance System) and the ADS-B systems function. The format of the messages is documented in ICAO Doc 9688