ADS-B and Mode S

Transponder Modes: A, C, and S

Transponders are key to surveillance communication. Mode A transmits a four-digit code identifying the aircraft. Mode C adds pressure altitude reporting, improving ATC's ability to separate traffic. Mode S (Selective) builds on this by providing each aircraft with a unique code and enabling two-way data link communication, supporting functions like TCAS and more detailed aircraft identification.

Mode S Explained

Mode S transponders transmit not just the squawk code and altitude, but also aircraft registration and status. This enables ATC to uniquely identify and track each aircraft, even in dense airspace. Mode S also allows for air-to-ground and ground-to-air data link, supporting advanced services and safety systems like TCAS II.

ADS-B Overview

ADS-B (Automatic Dependent Surveillance–Broadcast) is a separate but complementary system. It uses GPS to determine the aircraft's position, which is then broadcast automatically to ATC and nearby aircraft. This enhances situational awareness, collision avoidance, and traffic management, especially in areas with limited radar coverage.

ADS-B vs Mode S

• Mode S: Responds to ground radar interrogation, transmits unique ID, altitude, and supports data link.
• ADS-B: Continuously broadcasts position, velocity, and other data without interrogation, improving real-time visibility for ATC and other aircraft.

Both systems are crucial for modern surveillance, but ADS-B is more proactive, while Mode S is reactive to radar interrogation. Together, they form the backbone of today's surveillance communication in aviation.