Definition
A Secondary Surveillance Radar transponder mode that allows air traffic control to interrogate (query) a specific aircraft individually by its unique 24-bit aircraft address, rather than interrogating all aircraft in range at once. Mode S supports selective addressing, two-way data exchange (datalink), altitude reporting, and unique aircraft identification, and is the foundation for ADS-B and TCAS coordination.
Plain English
A type of radar transponder that lets controllers talk to one specific aircraft at a time using a unique ID, instead of pinging every aircraft in the area together. It also carries extra information like altitude and identity, and supports modern systems such as ADS-B and traffic alerting.
Context Anchor
Seen in instrument procedure and equipment discussions where aircraft surveillance and transponder capability are part of operating in controlled airspace or flying certain procedures.
Derivation
The 'S' in Mode S stands for 'Select' — the key feature is that the radar can selectively address one aircraft. Earlier modes (A and C) had to interrogate every transponder in range simultaneously, which became unworkable as airspace grew busier.
Why Pilots Care
It enables more reliable tracking, supports TCAS, and reduces frequency congestion in busy terminal airspace.
Intuition Check
Do not read “Mode S” as just another general radar setting. The “S” means selective: the system can address a specific aircraft rather than treating all aircraft replies the same.
Example Sentence 1
The aircraft's Mode S transponder allowed ATC to query it directly without affecting nearby traffic on the same frequency.
Example Sentence 2
Mode S transponders allow nearby aircraft to receive altitude and position data for collision avoidance.