Avgeek Alert: An Aircraft Transponder Explainer


All modern aircraft are comprised of various sophisticated systems which work together to ensure safety and efficiency. And a plane’s transponder is a critical component without which it couldn’t get from Point A to Point B.

This word is a hybrid of “transmitter responder”, which communicates with the secondary radar system (SSR) of which it’s part. The SSR emits interrogation signals to the transponder that facilitates its identification on the screens of air traffic controllers and the aircraft’s device responds to it. The transponder provides the controller a series of data that helps him to ensure that the flight takes place in optimal conditions. The reason is that its two objectives are to help its own identification in air traffic control as well as to assist a system called ACAS (Airborne Collision Avoidance System).

The information provided by the transponder to the control tower is used for what’s known as air traffic services (ATS), consisting of:

  • Air traffic control (ATC)
  • Flight Information Service (FIS)
  • Alerting service (ALRS)

How Do Transponders Work?

As we mentioned above, a plane’s transponder communicates with the SSR antenna, and it sends pulsating interrogation signals on a certain frequency (1030 MHz); the aircraft responds on a different frequency (1090 MHz) and with a four-digit code. This code has to be transformed into information that goes to the radar screen, so it’s decoded by the radar decodes and transmitted to the screen via a processor.

Type of Transponders & Their Functions

In the world of telecommunications in general there are two types of transponders: passive and active. According to the information they provide, they fall into three types (the most used in civil aviation):

A (Alpha) – This transponder sends only the identifier. The antenna asks the aircraft for its identification through two pulses (P1 and P3), separated by eight microseconds, and it responds with the aforementioned four-digit identification code that the cabin has but that the cabin has previously assigned. controller. If the radar asks the aircraft what it is, it will send the identification code, which in turn passes through the processor to decode it and check which flight plan it is assigned to. Thus the flight number and four-digit code are unequivocally linked. This information is reflected on the radar screen, which will show the aircraft’s insignia and its position in the airspace.

C (Charlie) – This type, in addition to the identification, also reports the pressure altitude. In this case, instead of asking a single question (who is there?), it asks the aircraft for its altitude, a value that will also be reflected on the radar screen.

S (Sierra) –The most complete and also the most common transponder supplies a greater exchange of data, more information and selective interrogation. It uses a 24-bit code, unique for each aircraft that, with few exceptions, will be maintained throughout its useful life. The code for S transponders is hexadecimal (a combination of six letters and numbers instead of the previous four digits). It sends a data link to the radar with all the flight parametres and has many advantages over other transponders. One of them, that the radar can be directed only at it  instead of all the aircraft that are in its control space. In this case, the radar screen will show, under the flight number that identifies the aircraft, other data such as those related to its altitude, speed, atmospheric conditions, etc.

What Happens if There’s a Conflict?

What we mean by “conflict” is when there are two aircraft flying at the same time, at the same altitude and in opposite directions. This is where S transponders show their greatest strength. The ACAS system we mentioned above warns of the conflict, and through the transponders of both aircraft (they must be S – which, remember, are the most common), they agree, communicating by data link, in which of them descends and which ascends. Such conflicts can also be resolved with other types of transponders, but it is not as direct as with S.


Photo | Ollie Desforges