Mode S and Mode C are two distinct modes of operation in secondary surveillance radar (SSR) systems used in air traffic control and surveillance.
Mode S, or selective calling, is an advanced version of SSR that allows for more accurate and efficient identification and communication of aircraft compared to older radar systems. Mode S transponders on aircraft are equipped with unique 24-bit addresses, called Mode S addresses or ICAO (International Civil Aviation Organization) 24-bit addresses. These addresses enable selective interrogation and communication between ground radar stations (interrogators) and specific aircraft, reducing the likelihood of interference and improving overall surveillance effectiveness in congested airspace. Mode S also supports two-way data communication between aircraft systems and ground systems, improving air traffic management capabilities.
Mode C, on the other hand, is a mode within SSR that primarily provides altitude information of an aircraft. Mode C transponders transmit the aircraft’s pressure altitude to ground radar stations upon interrogation. This altitude information is crucial for air traffic controllers to maintain safe vertical separation between aircraft, particularly during flight in controlled airspace where accurate altitude reporting is essential for aircraft avoidance purposes. collision and airspace management.
Mode S, in essence, stands for selective calling, emphasizing its ability to selectively communicate with specific aircraft using single mode addresses. Unlike older radar systems where all aircraft in the radar coverage area respond to general interrogations, Mode S allows for more efficient and targeted communication between ground systems and individual aircraft equipped with Mode S transponders. Selective addressing reduces data collisions, improves monitoring accuracy, and supports advanced applications such as Automatic Dependent Surveillance Data Link (ADS-B) and Mode S (Mode S DLS).
A Code C code refers to altitude information transmitted by an aircraft’s Mode C transponder in response to radar interrogations. The Code C code provides the aircraft pressure altitude in 100-foot increments up to a maximum of 29,000 feet. This altitude information is displayed to air traffic controllers, allowing them to monitor and manage aircraft altitude, ensure safe separation distances between aircraft, and coordinate efficient routing within controlled airspace. Mode Altitude Reporting is an essential element of air traffic control operations, contributing to the safe and efficient management of air flow.