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How ATC radar actually works

Primary echoes, transponder replies and satellite broadcasts — where the dots on the scope come from.

The radar picture a controller works from is not one technology but a fusion of several, each answering a different question: is something there?, who is it?, and how high is it?

Primary radar: the raw echo

Primary surveillance radar (PSR) is the classic spinning antenna: it fires pulses of radio energy and listens for reflections off an aircraft's skin. Its great virtue is independence — the aircraft needs no equipment at all, which is why primary radar remains the backstop for finding aircraft whose electronics have failed. Its weaknesses are just as fundamental: the echo says nothing about identity or altitude, weather and terrain clutter the picture, and small or distant targets fade.

Secondary radar: asking the aircraft

Secondary surveillance radar (SSR) works by conversation instead of reflection. A ground interrogator transmits a coded question; a transponder on the aircraft answers. In Mode A the reply carries the four-digit squawk code the controller assigned ("squawk 4521"); Mode C adds the aircraft's pressure altitude, which is why the data block can show a live level readout. Because the reply is an active transmission rather than a faint echo, SSR sees farther and cleaner than primary radar — but it only sees aircraft that are equipped and switched on.

Mode S and the data block

Mode S ("select") modernised SSR by giving every airframe a unique 24-bit address, letting the ground station interrogate one specific aircraft instead of shouting at all of them. That killed the garbling that plagued busy skies, allowed only 4,096 squawk codes to serve far more aircraft, and opened a data channel that can carry the callsign, selected altitude and more — much of what fills a modern data block arrives via Mode S.

ADS-B: the aircraft broadcasts itself

ADS-B (Automatic Dependent Surveillance–Broadcast) turns the model inside out: the aircraft determines its own position by GNSS and simply broadcasts it — position, altitude, velocity, identity — about twice a second, no interrogation needed. Ground stations (and other aircraft) just listen. It is cheap, precise and works where radar cannot reach, which is why oceanic and remote airspace increasingly runs on ADS-B, now including satellite reception. Its dependence is its caveat: if the aircraft's own navigation is wrong, the broadcast is wrong, which is why controlled airspace still layers ADS-B with SSR and primary radar rather than replacing them outright.

Why the scope looks the way it does

Put it together and the classic scope makes sense: the sweep repaints targets once per antenna revolution (a few seconds — watch a target "jump" rather than glide), the history trail shows the last few paints so direction and speed are visible at a glance, and the data block fuses transponder identity and Mode C altitude with computed ground speed. The ATC Radar Simulator models exactly this: targets update on the sweep, trails build behind them, and each label shows callsign, level and speed the way a real approach scope would. See the glossary for every term on the screen.