Which system uses both a ground-based interrogator and an aircraft-based transponder?

The system that uses both a ground-based interrogator and an aircraft-based transponder is secondary radar. In secondary radar, the ground station (interrogator) sends out a signal that prompts the aircraft's transponder to reply with information about the aircraft's identity, altitude, and other data. This interaction is essential for modern air traffic control, as it allows for precise tracking and identification of aircraft, especially in busy airspaces.

While primary radar relies solely on reflected signals from the aircraft to determine its position and does not involve any transponder communication, ADS-B operates differently by broadcasting the aircraft's position via a satellite system, though it does not depend on ground-based interrogators for responses. The transponder system, while integral to secondary radar operations, does not operate independently without the ground-based interrogator involved in the radar process.