Upon interrogation, Mode S transponders transmit information about the aircraft to the SSR system, to TCAS receivers on board aircraft and to the ADS-B SSR system. and China.) The format of Mode S messages is documented in ICAO Doc 9688, Manual on Mode S Specific Services. Likewise, a Mode S transponder is required to implement 1090ES extended squitter ADS-B Out, but there are other ways to implement ADS-B Out (in the U.S.

A TCAS-equipped aircraft must have a Mode S transponder, but not all Mode S transponders include TCAS. This is the type of transponder that is used for TCAS or ACAS II ( Airborne Collision Avoidance System) functions, and is required to implement the extended squitter broadcast, one means of participating in ADS-B systems. Mode S transponders are compatible with Mode A and Mode C Secondary Surveillance Radar (SSR) systems. See also: Secondary surveillance radar § Mode SĪnother mode called Mode S (Select) is designed to help avoiding overinterrogation of the transponder (having many radars in busy areas) and to allow automatic collision avoidance. Mode A and C responses are used to help air traffic controllers identify a particular aircraft's position and altitude on a radar screen, in order to maintain separation. The altitude information is passed to the transponder using a modified form of the modified Gray code called a Gillham code. Pressure altitude is obtained from an altitude encoder, either a separate self-contained unit mounted in the aircraft or an integral part of the transponder. A separate type of response called 'Ident' can be initiated from the airplane by pressing a button on the transponder control panel.Ī Mode A transponder code response can be augmented by a pressure altitude response, which is then referred to as Mode C operation. This is referred to as 'Mode 3A' or more commonly, Mode A. When the transponder receives an interrogation request, it broadcasts the configured transponder code (or ' squawk code'). Each aircraft is assigned a fixed 24-bit address. Provides multiple information formats to a selective interrogation.

Provides a cryptographically secured version of Mode S and ADS-B GPS position Provides a 3-pulse reply, delay is based on the encrypted challenge Provides the aircraft's pressure altitude and is usually combined with Mode 3/A to provide a combination of a 4-digit octal code and altitude as Mode 3 A/C, often referred to as Mode A and C Mode 3/A is often combined with Mode C to provide altitude information as well. Provides a 4-digit octal identification code for the aircraft, set in the cockpit but assigned by the air traffic controller. Provides 4-digit octal unit code (set on ground for fighters, can be changed in flight by transport aircraft) Provides 2-digit 5-bit mission code (cockpit selectable) Several different RF communication protocols have been standardized for aviation transponders:

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