Transponder (Mode S)
Radio & navigation
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A transponder replies to radar interrogations with the aircraft's code, pressure altitude and, in Mode S, its identity. It makes you visible to ATC and to other aircraft's collision-avoidance systems.
- German term
- Transponder (Mode S)
Mode S transponders are standard in European GA. Paired with ADS-B Out, the aircraft also broadcasts its position for other receivers.
In practice
Switch to ALT before the runway, not after take-off, so you are visible from the first second of flight.
Related terms
Official sources
Content reviewed: 2026-11
For general information only. Always use official sources, your aircraft's flight manual and current national rules for flight decisions.