What are the different transponder modes and what information do they provide?

Aircraft transponders respond to ground radar interrogations with identification and altitude data. Modes include: Mode A - transmits 4-digit squawk code for identification; Mode C - adds pressure altitude (in 100 ft increments); Mode S - adds unique 24-bit aircraft address, selective addressing, and data link capability for enhanced data exchange; and ADS-B (Automatic Dependent Surveillance-Broadcast) - continuously broadcasts GPS position, altitude, velocity, and ID without interrogation. ADS-B Out is now mandatory in most controlled airspace. Mode S and ADS-B enable collision avoidance (TCAS) and improved ATC surveillance.