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 IARTA 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.