What is a hex code?

What is a hex code?

A hex code, formally known as an ICAO 24-bit address, is a unique identifier assigned to an aircraft's Mode S transponder upon national registration. Once assigned, this address becomes a permanent part of the aircraft's Certificate of Registration.

Mode S transponders are onboard equipment that allow air traffic control and flight tracking systems to identify and communicate with individual aircraft. Because every hex code is unique to a specific aircraft, it serves as a reliable way to identify and track that aircraft regardless of changes to its registration, operator, or owner.

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