Definition
A secondary surveillance radar system in which each aircraft transponder is assigned a unique 24-bit address, allowing air traffic control radar to interrogate one specific aircraft at a time and exchange data with it, including altitude, identification, and information used by collision-avoidance systems such as TCAS.
Plain English
A radar system that talks to each aircraft individually using a unique address, so controllers and other aircraft get clean, specific data instead of replies from every aircraft in the area at once.
Context Anchor
Seen in discussions of transponders, air traffic control radar, aircraft identification, and modern traffic systems.
Derivation
The 'S' stands for 'select.' Earlier transponder modes (A and C) interrogated every aircraft in the radar beam at once. Mode S adds the ability to selectively address one aircraft using its unique code, which is where the name comes from.
Why Pilots Care
Mode S reduces reply garbling in busy airspace, supports collision avoidance systems, and provides more reliable tracking for controllers.
Intuition Check
MODE S does not mean “special mode” or “secondary mode.” The S means “select,” because the system can address a specific aircraft.
Example Sentence 1
The avionics shop confirmed the Mode S transponder was reporting the correct unique address before signing off the installation.
Example Sentence 2
Before departure the pilot confirmed the Mode S transponder was set to the assigned code and operating normally.