Mode S Transponders Explained

A Mode S transponder is the radio device in an aircraft that answers radar and transmits identification data. “S” stands for “select”: each aircraft has a unique address, so controllers can interrogate one aircraft at a time.

The older modes

Older transponders (Modes A and C) reply with a four-digit squawk code and the altitude. They answer any radar that asks, which makes it hard to separate replies in busy skies. See squawk codes.

What Mode S adds

• A unique 24-bit address for each aircraft (ICAO 24-bit address).
• Selective interrogation, so radar can ask one aircraft at a time.
• Data links that carry more information, such as altitude, speed and selected altitude.
• The ability to broadcast ADS-B messages (called extended squitter).

Why trackers care

Receivers listening on 1090 MHz hear ADS-B broadcasts and also Mode S replies. If receivers hear replies from the same aircraft, MLAT can calculate its position even if it is not broadcasting GPS data.

Collision avoidance

Mode S transponders also support TCAS, the onboard system that warns of nearby aircraft.

General information only, not legal or safety advice. Details change; check with your airline and the relevant authority. See our editorial policy.