Which data does SSR provide to air traffic control that PSR does not?

• A. Identity data and altitude (Mode C) from transponder.
• B. It detects aircraft passively from reflections, without transponders.
• C. It measures weather reflectivity data from atmospheric targets.
• D. It provides advisory weather data to pilots.

Answer: (B)

Secondary surveillance radar works by interrogating a cooperative transponder in the aircraft and receiving its replies. Those transponder responses carry the aircraft’s identity (the transponder code or address) and altitude data (Mode C). This is information SSR can provide to air traffic control that primary radar cannot, because primary radar only detects the aircraft by the reflected signal and has no way to obtain identity or altitude from the aircraft itself. The option describing passive detection without transponders refers to primary radar, not SSR. Weather reflectivity data and pilot weather advisories come from weather radar systems and meteorological services, not SSR.