Definition
An aircraft fitted with a Secondary Surveillance Radar transponder — a transmitter that replies to interrogations from a ground-based radar with a coded signal containing the aircraft's identity and altitude. This allows air traffic control radar to display the aircraft as a discrete target with attached data, rather than relying solely on a primary radar return from the aircraft's physical reflection.
Plain English
An aircraft carrying a transponder that talks back to ATC's radar. When the radar pings it, the aircraft's transponder answers with a code identifying who it is and how high it's flying, so the controller sees a clear labeled target on the screen.
Context Anchor
Seen in PRM radar and approach-control discussions, where controllers need fast, reliable aircraft tracking near closely spaced runways.
Derivation
Secondary Surveillance Radar is called 'secondary' because it works alongside primary radar. Primary radar bounces a signal off the aircraft's skin to detect it. Secondary radar instead asks the aircraft a question and listens for the aircraft's coded reply — a second, cooperative system. An 'SSR-equipped' aircraft is one that can participate in this two-way exchange.
Why Pilots Care
Enables positive identification, altitude reporting, and safe radar separation during instrument approaches.
Intuition Check
“Secondary” does not mean less important here. It means the radar system gets an active reply from the aircraft, rather than only detecting a reflected radar return.
Example Sentence 1
PRM approaches require SSR-equipped aircraft so that controllers can track each one precisely on the monitor displays.
Example Sentence 2
PRM approaches require an SSR-equipped aircraft so the monitoring controller can track its position precisely.