What is a Mode in the context of radio signals in air traffic control?

In the context of air traffic control, a Mode refers specifically to a method characterized by a certain pulse spacing of signals transmitted for identification and position reporting. This term is mainly associated with radar and transponder systems, such as in the case of Secondary Surveillance Radar (SSR), where different modes are used for various functionalities, such as Mode A for identity reporting, Mode C for altitude reporting, and Mode S for advanced communications.

Pulse spacing is critical because it defines how the radar system interrogates transponders to receive information back from the aircraft, allowing controllers to effectively track and communicate with aircraft in their airspace. This operational aspect ensures that the radar system can differentiate between multiple aircraft and maintain accurate situational awareness for effective air traffic management.