16 Lectures on Radar Applications · 02: Air Traffic Control Radar — Primary, Secondary, and Surface Surveillance

Mon, 26 Oct 2026 00:00:00 +0000

An airliner cruising at 35,000 feet never becomes a “missing dot” on a controller’s screen — behind it stands a layered system of radars and surveillance. Aviation is the most mature civilian application in this 16-part Radar Applications series, and also the most finely divided: each class of radar owns one slice of airspace and one kind of problem.

Primary and Secondary Radar: Independent Detection vs. Cooperative Reply

The first layer of ground-based ATC radar is primary surveillance radar (PSR): it transmits pulses into the sky and detects aircraft by the echoes reflected off the airframe — it sees the target whether or not the aircraft cooperates, and that independence is the bedrock of flight safety. The second layer is secondary surveillance radar (SSR): the ground station transmits an interrogation, and the aircraft’s transponder replies with a coded message carrying its flight identity and barometric altitude. SSR reaches farther and carries richer information, but it depends on the aircraft having its transponder switched on. This complementary relationship — and where ADS-B fits into it — is the starting point of modern ATC system design. Secondary radars almost universally use monopulse angle measurement, extracting precise bearing from a single interrogation.

Aviation Safety on Counter UAV Radar — Low-Altitude Surveillance Radar

16 Lectures on Radar Applications · 02: Air Traffic Control Radar — Primary, Secondary, and Surface Surveillance

Primary and Secondary Radar: Independent Detection vs. Cooperative Reply