What technology allows secondary radar to identify aircraft?

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The correct answer is transponder technology, which is fundamental for secondary radar systems to identify aircraft. In secondary radar, aircraft are equipped with transponders that actively respond to radar signals sent by ground-based radar systems. When the radar emits a signal, the transponder on the aircraft receives this signal and transmits back a coded signal, which contains vital information such as the aircraft's identity and altitude. This interaction allows air traffic control to not only detect the presence of an aircraft but also gain additional data necessary for tracking and managing air traffic effectively.

The other options do not contribute to the identification capability of secondary radar. Infrared sensors are primarily used for detecting heat signatures and are not a part of radar technology. Radio waves are essential for radar in general, including both primary and secondary systems, but they do not identify aircraft without the assistance of transponder technology. Optical cameras are used for visual monitoring and can provide images of aircraft, but they do not play a role in the radar identification process. Thus, transponder technology is specifically what enables secondary radar to identify aircraft correctly.