Definition
A radar system that detects aircraft by transmitting an interrogation signal which triggers a reply from an aircraft's transponder. The reply contains coded information such as the aircraft's identity (squawk code) and, for Mode C or Mode S equipped aircraft, pressure altitude. Unlike primary radar, which relies on reflected energy from the aircraft's skin, secondary surveillance radar depends on an active electronic reply from equipment on board the aircraft.
Plain English
A radar that doesn't just bounce a signal off the airplane. It sends a question, and a small piece of equipment on the airplane (the transponder) answers back with the airplane's ID and altitude. That reply is what shows up on the controller's screen.
Context Anchor
You encounter this term when learning how air traffic control identifies aircraft on radar and how transponder codes and altitude reporting appear to controllers.
Derivation
Secondary because it works alongside primary radar as a second, cooperative system. Surveillance from Latin roots meaning 'to watch over.' The name reflects that this radar watches aircraft by interrogating them rather than by simply detecting them.
Why Pilots Care
It gives controllers precise aircraft identity and altitude, enabling safer separation and reducing voice communication workload.
Analogy
Like a radar gun that not only spots a car but also reads a device inside it for the license plate and speed.
Intuition Check
“Secondary” does not mean optional or unimportant here. It means the radar system gets its information from an aircraft reply, not just from a radar echo.
Example Sentence 1
Secondary surveillance radar gives the controller the aircraft's call sign and altitude as soon as the transponder replies to the interrogation.
Example Sentence 2
Secondary surveillance radar allows ATC to track many aircraft accurately even in busy terminal areas.