When the AN/TPX-42A Secondary Surveillance Radar (SSR) transmits two pulses out, what should it receive back?

When the AN/TPX-42A Secondary Surveillance Radar (SSR) transmits two pulses, it is designed to receive back a coded reply from an aircraft transponder. This functionality is critical for the operation of SSR as it allows the radar system to identify and track aircraft accurately.

The coded replies contain valuable information about the aircraft, such as its identity and altitude. This information is transmitted back in response to the SSR's interrogation pulses, enabling air traffic control systems to maintain situational awareness of all aircraft within a given airspace.

The other choices do not align with the operational purpose of the SSR. A single interrogation pulse refers to the transmission method rather than the expected reply, challenge video pulses relate to other aspects of radar functionality, and receiving the same two pulses would not provide the SSR with the necessary information to identify or track the aircraft. Hence, the correct answer emphasizes the essential interaction between the SSR and the aircraft transponder, which is vital for effective air traffic surveillance.