Which of the following is NOT a component of the secondary radar system?

In a secondary radar system, the primary components function together to enhance the identification and tracking of aircraft. The interrogator is responsible for sending out a signal to request information from aircraft transponders, which, in turn, respond with data such as their identity and altitude. The decoder processes these signals and converts them into readable information for air traffic control.

The detector, while not a component of the secondary radar system, serves a different purpose more aligned with primary radar systems. This component is involved in the detection of reflected signals from targets, which is crucial for primary radar operations but not involved in the functioning of a secondary radar system that relies on active responses from onboard transponders.

Thus, the presence of the interrogator, transponder, and decoder in secondary radar makes the detector the component that does not belong, given its specific function in primary radar systems instead.