Aircraft Transponders: What They Are and How They Work.

  • Post author:
  • Post category:Technical
  • Post last modified:August 1, 2021

I may earn a small commission from links on this page, but I only recommend quality products I trust.

An aircraft’s transponder (short for “transmitter-responder”) is an electronic device on aircraft that transmits a four digit code (called a “squawk code”) which allows the aircraft to be identified by Air Traffic Control.

In the cockpit pilots enter the four digit code assigned to them by ATC into the transponder (also referred to as “TXPDR” or “XPDR”) which identifies the aircraft on the Air Traffic Control radar screen.

How transponders work

There are two basic type of Air Traffic Control (ATC) radar: primary and secondary radar.

Primary radar will show a return on the controllers screen when the radar signal is reflected back to the radar i.e. by an aircraft. Primary radar does not provide any other information to the controller apart from the distance and bearing of the aircraft.

Secondary Surveillance Radar (SSR) transmits an “interrogation” (a request for information) on 1030 MHz that is received by the transponder which in turn replies on 1090 MHz. This reply includes the aircrafts 4-digit squawk code which allows the aircraft callsign or registration to be displayed and can also include the aircraft’s pressure altitude.

How do squawk codes work?

Normally air traffic control will assign a squawk code with the first clearance the flight receives (either on the ground or in the air).

If the flight will be a VFR flight then the pilot will either set a squawk of 1200 in the U.S. or 7000 in most of the rest of the world.

Squawk codes issued to aircraft are unique to that aircraft and will show the aircraft’s flight number or registration on the controllers screen. 

Air Traffic Control “primary radars” show the bearing and distance from the radar to the aircraft. Secondary Surveillance Radar (SSR) will show controllers additional information derived from the squawk code.

Book and aircraft.

The 5 Best Aviation Audiobooks (Chosen by a Pilot) A list of the best aviation audiobooks chosen by a current airline pilot. From fascinating stories at the dawn of aviation to modern day aircraft accidents.

Why do squawk codes range from 0 to 7?

Historically, transponders were designed to used “octal numbers” i.e. from 0 to 7. Squawk codes are 4-digit octal numbers from 0 to 7, resulting in 4096 possible codes.

More information about the advantages of octal numbers in computer programming can be found here.

What happens when ATC tells an aircraft to “squawk ident”

Its common to hear air traffic controller telling a pilot to “squawk ident”. On all transponders (mode A, C and S) there is an ident button and telling a pilot “squawk ident” is an instruction for the pilot to press that button.

When ATC radar picks up the “ident” signal it causes that aircraft to stand out, or “blossom”, on their screens. They use this as another means to positively identify an aircraft – i.e. to make sure they are giving instructions to the correct aircraft.

Air traffic control - controller in control tower

What is the difference between Mode A and Mode C transponders?

Modern transponders can operate in certain “modes” – this may be because of their limit (some light aircraft may have transponders limited to Mode A or C), or because of pilot selection (e.g. following a malfunction).

Generally there are 3 types of transponder modes: A, C and S.

Mode A Transponder

Mode A (“mode alpha”) is the most basic mode. Operating a transponder in Mode A will only show the selected squawk code on the controllers screen.

Mode C Transponder

Mode C (“mode charlie”) combines the basic details of Mode A with pressure altitude giving ATC controllers a read-out of an aircrafts altitude on their screen.

Mode S Transponder

Mode S (“mode Sierra”) is the most advanced mode of the three, and the most common on modern commercial aircraft.

The added benefit of Mode S is that it allows ACAS/TCAS and ADS-B (Automatic Dependent Surveillance-Broadcast).

Book and aircraft.

The 5 Best Aviation Audiobooks (Chosen by a Pilot) A list of the best aviation audiobooks chosen by a current airline pilot. From fascinating stories at the dawn of aviation to modern day aircraft accidents.

Collision Avoidance: ACAS and TCAS

ACAS (Airborne Collision and Avoidance System) and TCAS (Traffic Collision and Avoidance System) are systems onboard aircraft that receive transponder signals from nearby aircraft through a series of antennas on the aircraft.

This allows the system to calculate the bearing and distance of other nearby aircraft. If ACAS/TCAS projects the aircraft will fly too close to another traffic it will sound an alarm in the cockpit and provide avoiding action instructions to the pilots.

Learn more about ACAS and TCAS here.


ADS-B, or Automatic Dependent Surveillance-Broadcast, is a system where an aircraft derives its position by satellite navigation and broadcasts its position to ground stations. ADS-B can be used in place of normal Air Traffic Control radar surveillance in places where there are no radars. Additionally, it also allows other aircraft to receive its position in order to provide separation.

Automatic – no pilot input.
Dependent – it depends on data from the aircraft’s navigation systems.
Surveillance – it transmits the aircraft’s position.
Broadcast – it periodically broadcasts the aircraft’s position to ground stations and other aircraft.

Learn about the differences between ADS-B and ADS-C here.

When are transponders used?

Usually we request our departure clearance via datalink about 30 minutes prior to expected push back time. With the clearance ATC will assign us a squawk code and its at this point that we enter it into the transponder.

Normally we leave the transponder on standby until we’re ready to push. Once we have the doors closed and the pushback truck waiting we select the transponder on. The transponder ground mode will allow the ATC ground controller to see us on his screen (this is achieved as part of the SMR – Surface Movement Radar).

More on surface movement radar here.

Airbus A330 transponder txpdr

Transponder panel

The transponder panel is normally located on the center console and contains a number of selectors and buttons. “ATC” on the left side signifies the transponder section, while “TCAS” on the right indicates the TCAS mode selectors, along with the “IDENT” button.

Transponder Mode Selector


Standby (STBY) mode – both transponders are powered but not transmitting.


On the ground the selected transponder operates in Mode S only (i.e. selective interrogation).

Transponder XPDR Selector


Selects either transponder 1 or transponder 2. This is used in the event of a failure of one transponder.



Altitude Reporting (ALT RPTG) is either selected on or off. If selected on the transponder transmits the aircrafts altitude based on standard barometric data.

ATC Fail Light

This illuminates in the event of a failure of the selected transponder.

IDENT Switch

If instructed by ATC the crew will press the IDENT switch which transmits the aircraft identification signal. This may also be used in the event of radio failure to indicate the crew are receiving ATC transmissions but cannot reply.

TCAS Traffic Selector

The Traffic Selector, as the name implies, allows us to filter out certain traffic depending on our requirements.


Proximate and other intruders are displayed only when a TA or RA is present and they are within 2700 ft of the aircraft.


All traffic within 2700 ft is displayed.


Above: Traffic 9900 ft above to 2700 ft below, is displayed.


Below: Traffic 9900 ft below is shown, as well as traffic uptown 2700 ft above. This is the normal selection in the cruise where we’re at high level – having a knowledge of aircraft below is important in the event of an emergency descent, for example.

TCAS mode selector

On the right of the transponder is the Traffic Collision Avoidance System (TCAS) mode selector. The TCAS system receives the broadcasts from other aircraft and if it predicts aircraft will come too close it will issue avoiding action instructions.


Traffic Alert/Resolution Advisory (TA/RA) is the normal position and we select this mode prior to lineup on the runway. During taxi out, and after landing, we select the mode to standby (STBY).

The TA/RA mode will display all Traffic Alerts and Resolution advisories provided 1. ALT RPTG (altitude reporting) is selected on and 2. the ATC transponder mode is selected to either “AUTO” or “ON”.


TA (Traffic Alert) mode is used in the case of aircraft degraded performance. (e.g. in the event of an engine failure, or inability to retract the landing gear). TA’s, proximate traffic and other intruders are displayed. All RA’s are converted to TA’s.


The TCAS part of the transponder is in “standby”. The transponder element still operates normally, but the TCAS is unavailable.

Emergency squawk codes

There are some squawk codes that are reserved for certain situations, usually emergencies. Setting these squawk codes will trigger an alarm in the ATC center monitoring the flight and will also alert nearby sectors of the emergency.

  • 7500 – hijack. Widely publicised online – I won’t get into too many details about this one, but suffice to say keep an eye out for the fighter jets!
  • 7600 – radio communication failure. This signals to Air Traffic Control that a pilot is experiencing radio communication failure. More on lost comms procedures here.
  • 7700 – general emergency, or MAYDAY. This alerts the ATC unit in use (and other nearby units) that the aircraft is experiencing an emergency and may not be able to comply with ATC instructions.

Emergency code pneumonic: “75 taken alive, 76 in a fix, 77 on the way to heaven”.

Reserved squawk codes

Some specific transponder codes are reserved for special use:

SquawkReason for Reservation
1000A squawk of 1000 may be used where ATC can interrogate the Mode S transponder of the aircraft to receive the aircraft callsign/registration.
1200VFR Squawk – United States.
2000Used when entering airspace without an assigned code – e.g. crossing the ocean we squawk 2000 before reaching landfall.
7000VFR Squawk – Rest of World.
7500Unlawful interference/hijack.
7600Radio communications failure.
7777Used in the U.S. for military aircraft engaged in intercept operations without ATC clearance*
Reserved Squawk Codes

*Note, in the event of an interception the intercepted aircraft squawks 7700.

Learn about what happens when an aircraft squawks 7700.

Frequently Asked Questions about Aircraft Transponders

What is the difference between Primary and Secondary radar?

Primary radar relies on the principle of reflectivity and will show a return on the radar screen but without any information.

Secondary Surveillance Radar (SSR) returns include the aircraft’s squawk code which enables the aircraft to be identified and allows the aircraft’s callsign or registration to be displayed alongside the radar return.

What does “Squawk VFR” mean?

“Squawk VFR” is an instruction by Air Traffic Control when leaving controlled airspace to set the transponder code to the relevant VFR (Visual Flight Rules) code.

In the U.S. the VFR squawk code is 1200, whereas in the rest of the world 7000 is generally used.

This indicates to Air Traffic Control that the aircraft is proceeding via Visual Flight Rules and they are responsible for their own navigation and terrain clearance.

What does it mean when ATC says “Squawk Ident”?

The instruction by Air Traffic Control to “squawk ident” is an instruction for the pilot to press the “ident” button on his/her transponder which will illuminate the aircraft on the controllers screen.

Instructing an aircraft to “squawk ident” is another way for Air Traffic Control to positively identify the aircraft in question.

Additionally, in the event of a “lost comms” situation where an aircraft loses radio contact, ATC may request the aircraft to “squawk ident” which would signal that although the aircraft can’t reply on the radio, they are receiving the transmission from Air Traffic Control.

What is a squawk code?

A squawk code, or transponder code, is a 4-digit code entered into the aircraft’s transponder which identifies that aircraft on the Air Traffic Control radar.

What does Mode Charlie/Mode C mean in relation to transponders?

Transponders operating in mode C include the aircraft’s pressure altitude in addition to the aircraft’s transponder code (squawk code).

What are the emergency transponder codes?

7500 unlawful interference(hijack), 7600 radio failure and 7700 emergency/MAYDAY. “75 taken alive, 76 in a fix, 77 on the way to heaven”.

When is a squawk of 2000 used?

Squawking 2000 is used where the Air Traffic Control radars can interrogate the Mode S transponder of the aircraft to receive the aircraft callsign or registration.

What is TCAS?

Traffic Collision Avoidance System (TCAS) is a computer system that plots other aircraft and if it predicts aircraft will fly too close it can issue avoiding instructions to the crew.

What is TCAS “proximate traffic”?

Proximate traffic on TCAS is traffic within 6NM and 1200′ of the aircraft and its symbol is generally displayed as a solid diamond.