Mode S is a type of aircraft transponder used in conjunction with Secondary Surveillance Radar. Mode S equipped aircraft transmit an aircraft specific unique code, which can be used for selective interrogation by surveillance equipment.

Mode S solves several problems that users of older surveillance systems face. The use of a unique aircraft ID solves the problem of shortage of 4 digit transponder codes; There are almost 17 million unique Mode S 24 bit codes available. Selective interrogation, meaning that a radar can communicate specifically with one aircraft instead of pinging every transponder in its range, significantly reduces radar garbling and FRUIT. Mode S is backwards compatible with ACAS systems, and can be extended with ADS-B and MLAT technologies. Mode S also allows for additional data to be transmitted between the aircraft and ground equipment. This may include information about actual indicated airspeed, magnetic heading, vertical rate etc., as well as current autopilot settings such as selected flight level.