Transponder

A transponder responds to interrogations from ATC radar by broadcasting a four-digit squawk code and pressure altitude. Mode C transponders reply with altitude only. Mode S transponders go further, adding a unique 24-bit aircraft address so ATC can correlate each radar return to a specific tail number. Mode S is the current standard in most modern training aircraft, though older Mode C units are still legal in many aircraft.

Mode C is what is actually required in the Mode C veil around Class B airports, and in Class C airspace. For the 2020 ADS-B Out mandate, a Mode S transponder is required if using the 1090ES path (the most common approach). However, aircraft flying below FL180 can alternatively meet the ADS-B Out requirement using a UAT (Universal Access Transceiver), which does not require a Mode S transponder. Either way, a transponder with at least Mode C capability remains separately required for the airspace.