Definition
An air traffic control radar service provided in the terminal area (the airspace around an airport) using secondary surveillance radar. Secondary radar works by interrogating the aircraft's transponder, which replies with a coded signal that gives the controller the aircraft's identity and altitude, in addition to its position.
Plain English
A radar service used near airports that tracks aircraft by talking to their transponders, so the controller sees who you are, where you are, and how high you are.
Context Anchor
Seen in FAA acronym and NOTAM-contraction lists, especially when describing whether a radar service near an airport is available, limited, or out of service.
Derivation
"Terminal" comes from the Latin terminus, meaning end or boundary — in aviation, it refers to the airspace at each end of a flight, around an airport. "Secondary" radar is so called because it came after primary radar: instead of bouncing a signal off the aircraft's skin, it asks the aircraft's transponder to reply with information.
Why Pilots Care
Gives controllers reliable aircraft data so they can keep planes safely separated during arrivals and departures.
Intuition Check
“Terminal” does not mean the airport building here; it means the airport-area airspace. “Secondary” does not mean less important; it means the radar depends on a reply signal from the aircraft.
Example Sentence 1
The approach controller was providing terminal secondary radar service, so the pilot's altitude readout appeared directly on the controller's scope.
Example Sentence 2
Pilots in the terminal area received traffic advisories supported by TSEC.