What does a secondary radar system provide that a primary radar does not?

• A. Identity and Altitude
• B. Range and Bearing
• C. Velocity information only
• D. Weather data

Answer: (D)

Secondary radar works by interrogating aircraft transponders and receiving a reply that includes information the primary radar can’t provide. The transponder returns an identity code, so controllers know who the aircraft is, and, with altitude reporting (Mode C), how high it is. Primary radar, on the other hand, can only show where a target is by its range and bearing based on reflections; it doesn’t reveal who the target is or its altitude. Weather data comes from weather radar, not from secondary surveillance radar, so it isn’t part of what secondary radar adds. Thus, the feature that secondary radar provides that primary radar does not is the aircraft’s identity and altitude.