Primary vs Secondary Radar

Primary radar detects an object by bouncing radio waves off it and measuring the echo; secondary radar asks the aircraft’s transponder to reply with its identity and altitude. Air traffic control uses both.

Primary radar

A rotating antenna sends out pulses and listens for reflections. It does not need any equipment on the aircraft, so it can see aircraft that are not cooperating, and also birds, weather and terrain. It tells controllers where something is, but not what it is.

Secondary radar

The ground station sends an interrogation signal. The aircraft’s transponder replies with a code or its Mode S address, and altitude. This gives controllers identity and height, and replies are stronger and travel farther than weak echoes. Read Mode S transponders.

Why both

Primary radar provides a backup if a transponder fails or is switched off. Secondary radar provides the identity and altitude that make traffic management practical.

Radar and public trackers

Public flight trackers generally do not see radar. They listen to transponder broadcasts (ADS-B) and replies (MLAT). That is why an aircraft with its transponder off may disappear from a tracker even though air traffic control still sees it; see why flights disappear from the map.

General information only, not legal or safety advice. Details change; check with your airline and the relevant authority. See our editorial policy.