Transponder mode refers to the various ways an aircraft's transponder communicates with ATC and other aircraft; each mode is the type of information sent. Mode A is the most basic: it transmits only the four-digit squawk code used for identification. Mode C adds pressure-altitude data, while Mode S adds selective-call and data-link functions. Mode 3/A is the civilian-military hybrid for identification, Mode 1 gives a 2-digit mission code, and Mode 5 delivers a cryptographically secured version of Mode S and ADS-B GPS position for military use.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the meaning of “transponder mode” in aviation?
A "transponder mode" refers to the different ways an aircraft's transponder can communicate with ATC and other aircraft. Mode 1 to Mode 5 are military transponder modes. A ‘transponder mode’ is the type of information sent by the transponder, while a transponder squawk code is the four-digit number that carries that information. When pilots speak of Mode A, Mode C, or Mode S, they are describing which data set the transponder will return when the ground-based secondary surveillance radar sends an interrogation. Mode A is the most basic transponder mode and it only transmits the four-digit squawk code so that air traffic control can identify the aircraft on the radar screen but it gives no altitude information. Mode C adds pressure-altitude information encoded in 100-foot increments (30.48 meters) and is therefore required in Class A, B, C airspace and above 10,000 ft MSL (3,048 meters). Together Mode A and Mode C are normally referred to simply as Mode C.
Mode S is short for ‘Mode Select’ and it provides cooperative secondary aircraft surveillance by adding a unique 24-bit ICAO aircraft address, selective interrogation to reduce overinterrogation, and the ability to transmit extended aircraft data like position, heading, airspeed and vertical rate in digital formatted messages and analog video. Because transponder codes and transponder modes are not the same thing, ATC assigns a new squawk code while the crew selects the appropriate mode for the airspace they are entering.
What are the types of transponders in aviation?
Mode 1 to Mode 5 are military transponder modes, while modes A, B, C, D, and S are civilian transponder modes. Mode B and mode D are not in use and only modes A, C, and S are operational.The types of transponders in aviation are detailed below.
- Mode A Transponder: Mode A is the most basic transponder mode. It only transmits the four-digit squawk code and is useful for identifying an aircraft and its position. It is often combined with Mode C.
- Mode C Transponder: Mode C is the most widely used transponder mode. Mode C provides information on the aircraft’s pressure altitude. Combined with mode A, ATC and other aircraft can receive an aircraft’s unique squawk code, position, and altitude. Mode C adds automatic pressure-altitude reporting in 100-foot increments, derived from an altitude encoder preset to standard atmospheric pressure. This data lets ATC maintain vertical separation.
- Mode S Transponder: Mode S stands for “Mode Select” and supports selective interrogation, collision-avoidance logic, and can integrate ADS B Out and TCAS. Some units receive position information from an on-board GPS receiver and broadcast the aircraft's unique ICAO address. The FAA requires Mode C or better transponders in Class A, B, and C airspace and above 10,000 feet MSL (3,048 meters). Within 30 nautical miles of primary airports in Class B airspace, the same Mode C rule applies, an area known as the ‘Mode C’ veil. Military aircraft operate the same three modes and always add encrypted or tactical features. Every transponder is installed in the cockpit dashboard and connects to either a straight metal antenna with a spherical tip or a fin-blade antenna. Switch positions - STBY, ON, ALT, TST - control electrical power and reporting functions. ON operates in Mode 3/A identification only, while ALT enables Mode C altitude reporting.
