What does a transponder actually do in aviation?

07.05.2025

And why is it so important for demand-driven night marking (BNK)?

A transponder is an electronic device that automatically reacts to an activation signal and sends back another signal. The term is made up of the English words "transmitter" and "responder". The whole thing works like this:

A system sends a "request signal" (e.g. radar pulse or radio)
The transponder receives this signal
It automatically responds with a defined piece of information

Transponders are known from everyday life, for example in car keys or from pets. When they are "chipped", a small transponder chip is implanted under their skin. At the veterinarian, this can then be read out and assigned to the owners.

It's similar in aviation: here, the transponder automatically sends signals with information such as flight number, position, altitude and speed to receivers. In Germany, every aircraft must have a transponder installed and switched on.

The light:guard system for demand-controlled night marking detects 1090 MHz transponder signals of the type:

Mode A/C
Mode S
ADS-B

The different modes contain different data. Mode A/C indicates the (temporary) ID and altitude of the flying object, Mode S enables permanent identification and ADS-B also provides information about the trajectory.

The BNK system receives these transponder signals in real time and only switches on the obstacle lighting in the wind farm when an aircraft is actually approaching. As a result, the sky usually remains dark at night – without compromising on flight safety.

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