Communication, navigation and surveillance (CNS) are the three main functions that form the infrastructure for air traffic management and ensure that air traffic is safe and efficient.
Communication,
i.e. aviation communication refers to communication between two or more aircraft, the exchange of data or verbal information between aircraft and air traffic control and the ground-based communication infrastructure of the ATM network (like the aeronautical fixed service). For continental airspace, VHF (civil) and UHF (military) systems are used whereas for oceanic areas, high-frequency systems and SATCOMs are used.
Navigation,
i.e. air navigation, refers to the process of planning, recording, and controlling the movement of an aircraft from one place to another by providing accurate, reliable and seamless position determination capability.
Surveillance
systems are used by air traffic control to determine the position of aircraft.