How does primary radar differ from secondary radar?

• A. Primary radar detects echoes from any object; secondary radar relies on aircraft transponder replies.
• B. Primary radar uses transponder replies; secondary does not.
• C. Both systems require transponders.
• D. Primary is weather radar; secondary is navigation.

Answer: (A)

Primary radar works by sending out a pulse and listening for echoes that bounce off any object in its path. It doesn’t rely on anything aboard the object, so it can detect things like aircraft without transponders, weather, birds, or other clutter. Secondary radar, on the other hand, uses a transponder on the aircraft; when the radar interrogates the aircraft, the transponder replies with encoded data such as the aircraft’s identity and altitude. That difference is why the correct statement is that primary radar detects echoes from any object, while secondary radar relies on aircraft transponder replies. The other options mix up which system uses transponders, or confuse radar types with weather or navigation roles.