Which surveillance system obtains aircraft identity and altitude from transponder replies?

• A. Doppler Radar
• B. ILS
• C. Secondary Surveillance Radar (SSR)
• D. Primary Surveillance Radar (PSR)

Answer: (C)

The ability to obtain aircraft identity and altitude from transponder replies comes from secondary surveillance radar. In this setup, the radar system interrogates the airborne transponder, which responds with a data block that includes an identity code (and, with Mode C, the aircraft’s altitude). This cooperative data lets controllers see who is on the radar and how high they are, in addition to the aircraft’s location.

Primary surveillance radar, by contrast, relies on reflections from the aircraft and provides position only, with no transponder data. Doppler radar measures velocity, not identity or altitude from transponder replies. The ILS is an approach aid for guidance, not a surveillance system.