An on-board device that answers secondary radar interrogations with an identity code, its altitude and, in Mode S, a unique 24-bit address.
What it is
Primary radar only sees echoes; secondary surveillance radar (SSR) interrogates aircraft, and their transponders reply with data. In Mode A the reply is the four-digit code assigned by ATC; Mode C adds pressure altitude; and Mode S, the modern standard, replies selectively, identifies itself with a unique 24-bit address and can carry much more information. Interrogations arrive on 1030 MHz and replies go out on 1090 MHz.
That address, known as the hex code, is allocated by the state of registry. It stays with the aircraft when it changes airline and usually changes when the aircraft moves to another country's register; in some countries, such as the United States, it also changes with a new registration.
The transponder also underpins the TCAS collision avoidance system, which interrogates nearby aircraft, and ADS-B broadcasts via Mode S extended squitter. Crews switch it on before entering the runway and keep it active for the whole flight.
On Vuelopedia
An aircraft's record on Vuelopedia shows its ICAO 24-bit address (‘Hex / ICAO 24-bit’).
Related terms
Cite
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This page was last updated on October 9, 2026
APA
Vuelopedia. (2026, October 9). Transponder. Retrieved October 17, 2026, from https://vuelopedia.org/en/glossary/transponder
MLA
“Transponder.” Vuelopedia, Fundación Poderia, 9 Oct. 2026, vuelopedia.org/en/glossary/transponder. Accessed 17 Oct. 2026.
Chicago
Vuelopedia. “Transponder.” Fundación Poderia. Last modified October 9, 2026. https://vuelopedia.org/en/glossary/transponder.
Harvard
Vuelopedia (2026) Transponder. Available at: https://vuelopedia.org/en/glossary/transponder (Accessed: 17 October 2026).
BibTeX
@misc{vuelopedia_en_glossary_transponder,
author = {{Vuelopedia}},
title = {{Transponder}},
publisher = {Fundación Poderia},
year = {2026},
date = {2026-10-09},
url = {https://vuelopedia.org/en/glossary/transponder},
howpublished = {\url{https://vuelopedia.org/en/glossary/transponder}},
urldate = {2026-10-17},
note = {Accessed 2026-10-17}
}Wikipedia
{{cite web |url=https://vuelopedia.org/en/glossary/transponder |title=Transponder |website=Vuelopedia |publisher=Fundación Poderia |date=2026-10-09 |access-date=2026-10-17 |language=en}}