Altitude reporting mode S transponder responds to interrogations from which sources?

• A. Ground station only
• B. Ground station and satellite
• C. Airborne and ground station
• D. Airborne only

Answer: (C)

Mode S transponders are designed to reply to two-way interrogations, not to broadcast on their own. The altitude information they provide is included in responses to requests from both ground-based radar interrogators (the ATC surveillance network) and airborne interrogators on other aircraft. Satellites aren’t part of the typical Mode S interrogation system, so they aren’t sources of these altitude requests. That’s why the correct idea is that the transponder responds to interrogations from airborne and ground stations.