Aviation transponder interrogation modes

Interrogation modes

Several different RF communication protocols have been standardized for aviation transponders:

Mode A

When the transponder receives an interrogation request it broadcasts the configured transponder code (or "squawk code"). This is referred to as "Mode 3A" or more commonly, Mode A. A separate type of response called "Ident" can be initiated from the airplane by pressing a button on the transponder control panel.

Mode A with Mode C

A Mode A transponder code response can be augmented by a pressure altitude response, which is then referred to as Mode C operation. 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. The altitude information is passed to the transponder using a modified form of the Gray code called a Gillham Code.

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.

Mode S

Another 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 S transponders are compatible with Mode A and Mode C Secondary Surveillance Radar (SSR) systems. This is the type of transponder that makes the ACAS II (Airborne Collision Avoidance System) and the ADS-B systems function. The format of the messages is documented in ICAO Doc 9688, Manual on Mode S Specific Services.

Mode S features

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. This information includes the call sign of the aircraft and/or the transponder's permanent ICAO 24-bit address in the form of a hex code. One of the hidden features of Mode S transponders is that it is downwards compatible, that means, an aircraft equipped with a Mode S transponder can still be used to send replies to Mode A/C interrogations. This feature can be activated by a specific type of interrogation sequence called inter-mode.

ICAO 24-bit address

Mode S equipment on aircraft are assigned a unique ICAO 24-bit address or (informally) Mode-S "hex code" upon national registration and this address becomes a part of the aircraft's Certificate of Registration. Normally, the address is never changed, however, the transponders are reprogrammable and, occasionally, are moved from one aircraft to another (presumably for operational or cost purposes), either by maintenance or by changing the appropriate entry in the aircraft's Flight management system.

There are 16,777,214 (2²⁴-2) unique ICAO 24-bit addresses (hex codes) available. The ICAO 24-bit address can be represented in three digital formats: hexadecimal, octal, and binary. These addresses are used to provide a unique identity normally allocated to an individual aircraft or registration.

As an example, following is the ICAO 24-bit address assigned to the Shuttle Carrier Aircraft with the registration N905NA:

These are all the same 24-bit address of the Shuttle Carrier Aircraft, represented in different digital formatting (see above).

Issues with Mode S transponders

One major issue with Mode S transponders is that pilots have frequently been entering the wrong flight identity into their Mode S transponders. In this case, the capabilities of ACAS II and Mode S SSR can be degraded.

Extended squitter

In 2009 the ICAO published an "extended" form of Mode S with more messages formats to use with ADS-B; it was further refined in 2012.