Mode S, or selective calling, is primarily used in aviation for improved aircraft identification, tracking and communication systems in air traffic control (ATC) systems. It allows selective interrogation of individual aircraft equipped with Mode S transponders using unique 24-bit addresses, known as Mode S addresses or ICAO (International Civil Aviation Organization) addresses. Mode S enables more effective monitoring and management of air traffic by reducing data collisions, improving situational awareness, and supporting advanced features such as Automatic Dependent Surveillance (ADS-B) data link and Mode S of Mode S (MODE S DLS).
The main advantage of Mode S is its ability to provide more precise and selective communication compared to older radar systems like Mode A and Mode C. Mode S transponders can transmit detailed aircraft identification, position, altitude and other data in response to targeted queries from ground radar stations. This selective address reduces interference, improves surveillance accuracy, supports more dynamic air traffic management, and improves safety in busy airspace environments.
Comparing Mode S and Mode C, both serve distinct purposes in SSR systems. Mode C primarily provides aircraft altitude information to air traffic controllers. It transmits the aircraft’s pressure altitude in response to radar interrogations, allowing controllers to maintain safe vertical separation between aircraft. Mode S, while capable of providing altitude information (via the Mode C feature), offers additional benefits such as selective addressing, two-way data communication, and expanded surveillance capabilities via features such as ADS -B. Therefore, Mode S is considered more advanced and versatile than Mode C in terms of features and capabilities.
Mode S is required in airspace where advanced surveillance capabilities and selective addressing are required for effective air traffic management. It is mandated in certain controlled airspace regions, particularly where radar coverage and effective communication are essential to maintaining safe and orderly air traffic operations. Aircraft operating in required Mode S airspace must be equipped with Mode S transponders meeting international standards to ensure interoperability and seamless integration with ATC systems.
AC mode refers to a combined mode where an aircraft’s transponder is capable of operating in Mode A (which provides a four-digit octal code for identification) and Mode C (which provides altitude information ). AC mode transponders transmit both identification and altitude information when interrogated by ground radar stations. In contrast, Mode S introduces advanced capabilities such as selective addressing, extended squitter for automatic aircraft data broadcasting, and two-way data link communication. Mode S improves surveillance efficiency, reduces data collisions and supports modern air traffic management requirements compared to the more basic functionality of Mode AC.