When can secondary radar be used as the sole display source?

Secondary radar can be used as the sole display source in Class A airspace because the specific operational requirements and traffic density in this type of airspace allow for reliance on secondary radar systems. Class A airspace is generally defined as the airspace where air traffic control (ATC) services are provided to all aircraft, and all pilots must operate under instrument flight rules (IFR).

In this airspace, secondary radar, which utilizes transponder signals from aircraft to determine their position, speed, and altitude, is sufficient for ATC to maintain separation and provide services. The consistent use of transponders in this airspace ensures that secondary radar can accurately and reliably maintain situational awareness for air traffic controllers, making it an effective primary means of surveillance.

In contrast, other options might suggest scenarios or conditions where reliance on secondary radar would be less appropriate or reliable as a singular source, such as emergencies or failures of primary radar, where a dual system for safety would be ideal.