Definition
A transponder that supports Mode S (Select), a surveillance and communication protocol allowing air traffic control radar to interrogate one specific aircraft individually using its unique 24-bit address, rather than triggering replies from every transponder in range. Mode S transponders also exchange data such as altitude, aircraft identification, and information used by traffic and collision-avoidance systems.
Plain English
A transponder that can be addressed by ATC radar individually, like being called by name instead of everyone replying at once. It also sends extra information about the aircraft, not just an altitude and code.
Context Anchor
Seen in instrument flying, traffic information system discussions, and aircraft equipment requirements for traffic display features.
Derivation
The 'S' stands for 'Select.' Earlier transponder modes (A and C) replied to every interrogation pulse from any radar within range. Mode S was developed so radar could selectively interrogate one aircraft at a time, reducing congestion and allowing data exchange. Knowing 'S = Select' makes the behavior easy to remember.
Why Pilots Care
Enables more precise air traffic control identification and supports collision avoidance in busy airspace.
Intuition Check
Do not read the S as standby or as a speed setting. In this term, S means selective: the system can communicate with a specific aircraft.
Example Sentence 1
Because the aircraft was equipped with a Mode S transponder, the pilot was able to receive Traffic Information Service advisories from ATC radar.
Example Sentence 2
The S-mode transponder allowed air traffic control to identify the specific aircraft among many others in the area.