Which IFF mode is widely used for civilian ATC identification?

• A. Mode 3A
• B. Mode 1
• C. Mode 2
• D. Mode S

Answer: (B)

In civilian air traffic identification, the transponder responds with a four-digit code that ATC assigns, allowing controllers to identify and separate aircraft on radar. This is Mode 3A (often called Mode A), the mode widely used for civilian ATC identification. The four-digit squawk code is entered by the pilot or assigned by ATC and transmitted in response to radar interrogations, providing a unique identity for each flight and helping controllers correlate radar returns with a specific aircraft.

Other modes serve different purposes: Mode 1 is mainly for military mission codes, Mode 2 is used for military identity, and Mode S provides more advanced data link and selective addressing for enhanced surveillance. While Mode S is increasingly common in modern airspace, Mode 3A remains the foundational civilian identity mode for ATC identification.