Definition
A secondary surveillance radar transponder mode that gives each aircraft a unique 24-bit address, allowing ground radar and other Mode S equipment to interrogate that specific aircraft individually rather than broadcasting to every aircraft in the area. Mode S also supports two-way data exchange between the aircraft and ground stations, and is the foundation for services such as the Traffic Information Service (TIS) and ADS-B.
Plain English
Mode S is a smarter type of transponder. Instead of every aircraft replying to every radar sweep, each aircraft has its own ID code, so the radar can talk to one aircraft at a time. It can also send and receive small amounts of data, not just a position reply.
Context Anchor
Seen in Traffic Information System discussions, transponder equipment descriptions, and aircraft surveillance information.
Derivation
The 'S' stands for Select. Earlier transponder modes (A and C) replied to every interrogation aimed in their direction. Mode S was designed so that ground radar could 'select' and address one specific aircraft, which reduced radio congestion and opened the door to data link services.
Why Pilots Care
Supports efficient traffic management and reduces unnecessary transponder replies in busy airspace.
Analogy
It is like calling one person’s phone instead of making an announcement over a loudspeaker. Mode S lets the system reach a particular aircraft by its own address.
Intuition Check
Do not read the “S” as just a random model letter. In Mode S, it points to selective communication with a specific aircraft.
Example Sentence 1
Because the aircraft is equipped with a Mode S transponder, it can receive Traffic Information Service alerts from the ground radar.
Example Sentence 2
Mode S replies provided the controller with precise altitude and call sign data during the handoff.