Definition
An advanced form of secondary surveillance radar in which each aircraft transponder is assigned a unique address, allowing the ground station to interrogate one specific aircraft at a time and exchange data with it selectively. In addition to reporting identity and altitude (as Mode A and Mode C do), Mode S supports two-way data exchange between ground systems and the aircraft, and supplies the position and identity information used by TCAS and ADS-B.
Plain English
A radar system that can talk to one aircraft at a time by name. Each aircraft has its own unique address, so the ground station can ask just that aircraft for information instead of getting replies from every aircraft in the area at once. It can also send and receive data, not just identity and altitude.
Context Anchor
Seen in transponder equipment descriptions, ATC radar surveillance discussions, and aircraft avionics requirements.
Derivation
‘Mode S’ stands for ‘Mode Select’ — the system selects one specific aircraft to interrogate. ‘Secondary’ means the radar relies on a reply from the aircraft’s transponder rather than bouncing a signal off the airframe (which is primary radar). Knowing this helps explain why Mode S is more efficient: instead of every aircraft replying to every sweep, only the selected one answers.
Why Pilots Care
Mode S reduces frequency congestion, supports collision avoidance systems, and provides more reliable tracking in busy airspace.
Analogy
Older transponder systems are more like calling out to everyone in a room. Mode Ssr is more like calling one person by name, so the reply is tied to that specific aircraft.
Intuition Check
Mode here does not mean a flight mode or a pilot technique. It means a specific way the radar and aircraft transponder communicate; the S points to selective addressing.
Example Sentence 1
The aircraft was equipped with a Mode S SSR transponder, which allowed ATC to interrogate it individually and receive its altitude and identification.
Example Sentence 2
Before departure the pilot verified that the transponder was set to Mode S for the flight.