Gillham code
Binary code
Gillham code is a zero-padded 12-bit binary code using a parallel nine- to eleven-wire interface, the Gillham interface, that is used to transmit uncorrected barometric altitude between an encoding altimeter or analog air data computer and a digital transponder. It is a modified form of a Gray code and is sometimes referred to simply as a "Gray code" in avionics literature.
History
The Gillham interface and code are an outgrowth of the 12-bit IFF Mark X system, which was introduced in the 1950s. The civil transponder interrogation modes A and C were defined in air traffic control (ATC) and secondary surveillance radar (SSR) in 1960.
The code is named after Ronald Lionel Gillham, a signals officer at Air Navigational Services, Ministry of Transport and Civil Aviation, who had been appointed a civil member of the Most Excellent Order of the British Empire (MBE) in the Queen's 1955 Birthday Honours. He was the UK's representative to the International Air Transport Association (IATA) committee developing the specification for the second generation of air traffic control system, known in the UK as "Plan Ahead", and is said to have had the idea of using a modified Gray code. The final code variant was developed in late 1961 for the ICAO Communications Division meeting (VII COM) held in January/February 1962, and described in a 1962 FAA report. The exact timeframe and circumstances of the term Gillham code being coined are unclear, but by 1963 the code was already recognized under this name. By the mid-1960s the code was also known as MOA–Gillham code or ICAO–Gillham code. ARINC 572 specified the code as well in 1968.
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This entry incorporates text from “Gillham code” 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.