What is a mode S transponder?
Mode S
See also: Air traffic control radar beacon system#Mode S
Another mode called Mode S (Selective) 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.[2] This is the type of transponder that makes the ACAS II (Airborne Collision Avoidance System) and the ADS-B (Automatic dependent surveillance-broadcast) systems function.