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.
[nb 1] The final code variant was developed in late 1961[5] for the ICAO Communications Division meeting (VII COM) held in January/February 1962,[6] and described in a 1962 FAA report.
[13][14] Once recommended by the ICAO for automatic height transmission for air traffic control purposes,[9][15] the interface is now discouraged[2] and has been mostly replaced by modern serial communication in newer aircraft.
An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics.
This is not normally a problem as the power would typically be applied before the aircraft enters the runway and so it would be transmitting correct height information soon after take-off.
[citation needed] The height information is represented as 11 binary digits in a parallel form using 11 separate lines designated D2 D4 A1 A2 A4 B1 B2 B4 C1 C2 C4.