Definition
A ground-based secondary surveillance radar component that interrogates aircraft transponders using Mode S (Mode Select) protocol, enabling selective, addressable interrogation of individual aircraft and the exchange of data such as identity, altitude, and aircraft-derived information for use by air traffic control and TCAS.
Plain English
A piece of radar equipment on the ground that talks to a specific aircraft's transponder one at a time, asking for its identity and altitude, instead of pinging every aircraft in the area at once.
Context Anchor
Seen in discussions of cockpit traffic displays and safety systems that depend on ground-based radar coverage.
Derivation
Mode S' stands for 'Mode Select.' The 'S' signals that the sensor can select and address one aircraft at a time using its unique 24-bit address, rather than calling out to all aircraft in range as older Mode A and Mode C systems do.
Why Pilots Care
Mode S sensors enable precise traffic awareness and are required for operations in certain airspace and for ADS-B Out compliance.
Grounding Statement
If your airplane is within range of a Mode S Sensor, the ground system can help build a traffic picture for the cockpit and for air traffic control.
Intuition Check
Do not read “sensor” here as a small device inside your airplane. In this context, a Mode S Sensor is usually a ground radar facility that detects and tracks aircraft through transponder replies.
Example Sentence 1
The Mode S sensor at the approach control facility interrogated each aircraft individually, reducing radio frequency congestion in the busy terminal area.
Example Sentence 2
TCAS used information from the Mode S sensor to display nearby traffic and issue a resolution advisory.