Gillham code is a digital code using an eleven-wire interface that is used to transmit uncorrected barometric altitude between an encoding altimeter or analog air data computer and a transponder. It is a modified form of a Gray code and is sometimes referred to as a "Gray code" in avionics literature.
The code was named in memory of Ronald Lionel Gillham, the UK's representative to the IATA committee developing the specification for the second generation of Air Traffic Control System, known in the UK as "Plan Ahead", who died suddenly in March 1968. Gillham was said to have had the idea of using a modified Gray code while at a family dinner.
The Gillham code's only application is in the transmission of height information from an altitude encoder to a secondary surveillance radar (SSR) transponder as used in aircraft.
Read more about Gillham Code: Altitude Encoder, Decoding
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