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In a Nutshell - What is a Transponder?
Débora de Araújo • Mar 10, 2023
In a Nutshell - What is a Transponder?
A transponder is an electronic device installed on an aircraft that automatically transmits information about the aircraft's identity, location and altitude to air traffic control (ATC) radar systems. It receives a signal from an ATC radar and subsequently sends a reply signal that includes the aircraft's identification code (also known as a squawk code) and other information, such as altitude.
Transponders operate on a specific frequency, known as the Secondary Surveillance Radar (SSR) frequency, which is different from the primary radar used by ATC to detect the aircraft's position. SSR relies on the aircraft to reply to a signal sent by ground radar stations; this way, ATC can track the aircraft's position, speed and altitude more accurately than with primary radar alone.
This device has different modes and codes that provide information to ATC, such as the aircraft's altitude and flight status (e.g., whether it is in climb, descent, or level flight). The pilot controls the transponder, which can be set to different modes and codes as required by ATC or according to the flight phase.
It is important to note that transponders are mandatory for all aircraft flying in controlled airspace, and pilots must turn on their transponders and set them to the correct code as instructed by ATC.
Emergency codes
Code Use
7500 Aircraft hijacking (ICAO)
7600 Radio failure (lost communications) (ICAO)
7700 Emergency (ICAO)
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