Definition
A radar system that locates and identifies aircraft by interrogating an onboard transponder rather than by reflecting radio energy off the airframe. The ground station transmits a coded interrogation pulse; the aircraft's transponder receives it and replies with a coded signal containing the assigned squawk code and, when Mode C or Mode S is active, pressure altitude and other data. The controller's display then shows the aircraft's position, identity, and altitude as a data block alongside the radar return.
Plain English
A radar that doesn't just see aircraft — it asks them who they are. The ground station sends a question to the aircraft's transponder, and the transponder sends back an answer with the aircraft's code and altitude.
Context Anchor
Seen in instrument flying and radar navigation when describing how air traffic control identifies aircraft and receives altitude information from them.
Derivation
Called secondary because it works alongside primary radar. Primary radar bounces a signal off the aircraft itself to detect it. Secondary radar adds a second, cooperative system: it asks the aircraft a question and relies on the transponder to answer. The two together give controllers both detection and identification.
Why Pilots Care
Supplies controllers with aircraft identity and altitude, supporting safe separation in controlled airspace.
Grounding Statement
An SSR ground station sends out a question, and the aircraft’s equipment sends back an answer that helps air traffic control know which aircraft it is.
Intuition Check
Secondary does not mean unimportant here. It means this radar depends on a second step: the aircraft must reply to the ground radar signal.
Example Sentence 1
After the controller assigned a new squawk code, the aircraft's data block updated on the SSR display almost immediately.
Example Sentence 2
SSR replies allowed the radar display to show which target belonged to which airplane in busy airspace.