Secondary Surveillance Radar (SSR)

Secondary Surveillance Radar (SSR) is a radar system that interrogates aircraft transponders to obtain identification, position, and altitude information. Unlike PSR (which detects aircraft by skin reflection), SSR requires an active transponder in the aircraft and provides much richer data.

SSR Information

When an SSR interrogation triggers the aircraft transponder: Mode A returns the 4-digit squawk code; Mode C returns squawk code plus altitude; Mode S returns the aircraft’s 24-bit ICAO address, altitude, and supports two-way datalink. The display shows a target with an alphanumeric label containing callsign, altitude, and groundspeed.

Related Terms

See also: Transponder, PSR, RADAR.