Transponder (aeronautics)
airborne radio transponder used to transmit specific aircraft information in response to interrogation
A transponder (short for transmitter-responder and sometimes abbreviated to XPDR, XPNDR, TPDR or TP) is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other.
Air traffic control (ATC) units use the term "squawk" when they are assigning an aircraft a transponder code, e.g., "Squawk 7421". "Squawk" thus can be said to mean "select transponder code" and "squawking xxxx" to mean "I have selected transponder code xxxx".
The transponder receives interrogation from the secondary surveillance radar on 1030 MHz and replies on 1090 MHz.
Secondary surveillance radar
Secondary surveillance radar (SSR) is referred to as "secondary", to distinguish it from the "primary radar" that works by reflecting a radio signal off the skin of the aircraft. Primary radar determines range and bearing to a target with reasonably high fidelity, but it cannot determine target elevation (altitude) reliably except at close range. SSR uses an active transponder (beacon) to transmit a response to an interrogation by a secondary radar. This response most often includes the aircraft's pressure altitude and a 4-digit octal identifier.
“Transponder (aeronautics)” enters the record as airborne radio transponder used to transmit specific aircraft information in response to interrogation. Crown Archives preserves that source wording while asking what Transponder, aeronautics and airborne can confirm, complicate or overturn.
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Vocabulary and entity names are the principal evidence signals here, because they determine the precision of every later search. The source revision retrieved here is dated Jul 4, 2026. The linked authority identifier is Q1332920. None of the 0 selected statements returned an explicit reference. The first chronological checks are 1030 and 1090.
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This entry incorporates text from “Transponder (aeronautics)” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.