sensors Transponder directory
Mode-S transponder codes
Each aircraft broadcasts a permanent 24-bit hex address assigned by its country of registration. Browse the most-broadcast transponders in the last 30 days.
| # | Mode-S | Registration | Type | Broadcasts | Last seen |
|---|
What is a Mode-S transponder code?
A Mode-S transponder code, also known as an ICAO 24-bit address or hex code, is a permanent identifier broadcast by every aircraft equipped with an ADS-B or Mode-S transponder. The code is assigned by the country where the aircraft is first registered, from blocks defined in ICAO Annex 10 Volume III.
Unlike a callsign (which changes per flight) or a registration (which can change if an aircraft is sold to an operator in another country), the Mode-S code stays with the airframe for the duration of its registration in a given country. Flight trackers use it as the canonical identifier for position reports, callsign history, and ownership correlation.