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A [[digital]] [[code]] using an eleven-wire interface that is used to transmit uncorrected [[barometric]] [[altitude]] between an encoding altimeter or analog [[air data]] unit 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 the late 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. Mr Gillham was said to have had the idea of using a modified Gray Code while at a family dinner.
== Altitude Encoder ==
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Note that the altitude code output by a standard altitude encoder is a pressure altitude. That is to say it is always with respect to a pressure datum of 1013.2mBar (Hectopascals) or 29.92inHg. It does not indicate the height above sea level (altitude) or the ground (height). Pressure altitudes are referred to as Flight Levels and are expressed to the nearest 100ft.
== Decoding the Gillham Code ==
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[[Category:Data transmission]]
[[de:Gillham-Code]]
[[fr:Code Gillham]]
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