Primary Surveillance Radar (PSR)
PSR is classical radar — pulse out, echo back, plot range and bearing. It sees any aircraft whether the aircraft cooperates or not. The downside: PSR has no idea who the aircraft is or how high it's flying. The famous green sweep on a controller's screen mostly comes from PSR.
Secondary Surveillance Radar (SSR)
SSR is the civilian descendant of military IFF. The radar transmits an interrogation pulse. The aircraft's transponder replies with a four-digit squawk code and (in Mode C) its altitude. The controller now knows the aircraft is American Airlines 1187 at flight level 350 — without having to read it off the radar return, which only shows a dot.
ADS-B — the next replacement
Automatic Dependent Surveillance-Broadcast lets each aircraft transmit its own GPS-derived position, altitude, velocity and identity twice per second. Ground stations and other aircraft receive it directly — no radar required. ADS-B has been mandatory in most controlled airspace since 2020 and gives the controller a much richer picture than radar alone, especially for low-altitude flight where radar coverage is poor.
Why the radar still spins
ADS-B depends on the aircraft choosing to broadcast accurate data. Primary radar doesn't. As long as some aircraft are old, lying about position, or hostile, the rotating dish at every major airport stays a non-negotiable backup. Modern ATC uses all three — PSR, SSR, ADS-B — fused into a single track per aircraft.