Transponder Modes

A transponder is a radar-response device aboard an aircraft. It replies to radar interrogations from the ground and returns information. Modes define which information is transmitted.

Source: ICAO Annex 10 Volume IV Surveillance Radar and Collision Avoidance Systems; EUROCONTROL Specifications.

Mode A (identification)

Pure 4-digit code transmission:

Code: 4 digits, each 0-7 (octal) → 4096 possible codes.
Transmits only: the set code (squawk code).
No altitude transmitted.

Historic: oldest mode, since 1960s.

Mode C (altitude reporting)

Mode A + altitude information:

Additionally sends the pressure altitude (based on 1013.25 hPa).
Altitude is automatically derived from the encoding altimeter (Mode C altitude reporting encoder).
Aviation-critical: ATC sees 3D position instead of just 2D.
Mandatory in many airspaces since the 1980s (e.g. USA above 12,500 ft).

Mode S (selective)

Most modern mode:

Selective addressing: each transponder has a unique 24-bit address (ICAO address).
Downlink of aircraft data: speed, heading, vertical rate, magnetic heading, etc.
TCAS support: Mode S is the basis of the Traffic Collision Avoidance System.
Data rate higher than Mode A/C.

Mode S sub-levels:

Mode S Elementary Surveillance (ELS): 24-bit address + flight ID + altitude.
Mode S Enhanced Surveillance (EHS): additionally aircraft parameters (magnetic heading, IAS, Mach, vertical rate, etc.).

ADS-B (Automatic Dependent Surveillance — Broadcast)

Most modern form:

Aircraft broadcasts its position (GPS-based), altitude, speed continuously.
Ground stations listen.
Receivers also in other aircraft (ADS-B In) → traffic awareness.
Mode S 1090 MHz Extended Squitter is the most common ADS-B frequency.

EU requirement: ADS-B Out for many aircraft mandatory since 7 December 2020 (EU 2017/386).

Cockpit transponder operation

Stand-by (SBY)

Transponder on but not transmitting.
Used during engine start and taxi (at some fields).

Mode ON / ALT (altitude)

Mode A: code only.
Mode C / ALT: code + altitude.

IDENT button

Press → transponder sends a brief "identification" → ATC sees the aircraft flash on radar.
Use: when ATC says "squawk ident" or "confirm ident".

Aviation-critical codes

Four international codes (see next lesson "Standard codes"):

7700: distress.
7600: radio failure.
7500: hijack.
2000: IFR conspicuity (standard for IFR without assignment).
7000: VFR conspicuity (standard in Europe, ICAO).

Transponder test

Pre-flight check: transponder in STBY or SET, verify:

Lights illuminate correctly.
Code display shows 4 digits.
Altitude (Mode C) shown in cockpit (e.g. on collision warner).

When to change transponder?

Pre-flight

Code = VFR default (e.g. 7000 in Europe).
Mode = ON or ALT.

In flight

When ATC assigns: switch to assigned code.
VFR leaving controlled airspace → code back to VFR default.
Emergency: 7700.

Mandatory transponder use

Airspaces with transponder requirement

ICAO Annex 11 + national regulations:

Class A, B, C: all traffic Mode C required.
Class D, E: often only above certain altitudes (e.g. Class E > 5000 ft in DE/AT).
Class G: usually no requirement.

EU Reg 1207/2011

Mode S / ADS-B Out mandatory for many commercial and larger GA aircraft.
Smaller GA: per national rules.

Code memory

Modern radios often store the last 4-8 codes:

Pilot presses recall to return to previous codes.
Faster switching.

Cross-reference

Subject 060 Lesson "GNSS": ADS-B Out integration.
Subject 010 Lesson "Airspaces": transponder requirement by class.