Mode S

Mode S (Mode Select) is an advanced Secondary Surveillance Radar (SSR) transponder mode that provides unique aircraft identification and a wide range of enhanced surveillance data. Unlike older transponder modes (Mode A for identity, Mode C for altitude), Mode S assigns each aircraft a unique 24-bit ICAO address, allowing individual interrogation and eliminating garbling problems in dense traffic.

Mode S transponders provide:

• Aircraft identification: Flight number or registration transmitted automatically
• Pressure altitude: In 25-foot increments (vs. 100-foot for Mode C)
• Enhanced surveillance data: Selected altitude, indicated airspeed, heading, roll angle, and other parameters (via BDS registers)
• ADS-B capability: Mode S transponders with Extended Squitter form the basis for ADS-B Out transmission

Mode S is mandatory in most controlled airspace worldwide. The European mandate (Mode S EHS — Enhanced Surveillance) requires Mode S transponders to respond to additional data queries, providing controllers with downlinked aircraft parameters for improved situational awareness.