Gillham code

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.

A Cessna ARC RT-359A transponder (the beige box) in the instrument panel of an American Aviation AA-1 Yankee light aircraft. The transponder gets its altitude information from an encoding altimeter mounted behind the instrument panel that communicates via the Gillham code.
A typical altitude encoder, the ACK Technologies A-30. Note the 15-way D-type connector to send the Gillham code to the transponder and the port on the top of the case that connects to the aircraft's static pressure system.