The Gillham interface and code are an outgrowth of the 12-bit IFF Mark X system, which was introduced in the 1950s. The civil transponder interrogation modes A and C were defined in air traffic control and secondary surveillance radar 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 in the Queen's 1955 Birthday Honours. He was the UK's representative to the International Air Transport Association committee developing the specification for the second generation of air traffic control system, known in the UK as "Plan Ahead", and is said to have had the idea of using a modified Gray code. The final code variant was developed in late 1961 for the ICAO Communications Division meeting held in January/February 1962, and described in a 1962 FAA report. The exact timeframe and circumstances of the term Gillham code being coined are unclear, but by 1963 the code was already recognized under this name. By the mid-1960s the code was also known as MOA–Gillham code or ICAO–Gillham code. ARINC 572 specified the code as well in 1968. Once recommended by the ICAO for automatic height transmission for air traffic control purposes, it is now discouraged and has been mostly replaced by modern serial communication in newer aircraft.
Altitude encoder
An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics. The pressure sensor is often heated, which requires a warm-up time during which height information is either unavailable or inaccurate. Older style units can have a warm-up time of up to 10 minutes; more modern units warm up in less than 2 minutes. Some of the very latest encoders incorporate unheated 'instant on' type sensors. During the warm-up of older style units the height information may gradually increase until it settles at its final value. 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. Light aircraftelectrical systems are typically 14 V or 28 V. To allow seamless integration with either, the encoder uses a number of open-collector transistors to interface to the transponder. 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. As a twelfth bit, the Gillham code contains a D1 bit but this is unused and consequently set to zero in practical applications. Different classes of altitude encoder do not use all of the available bits. All use the A, B and C bits; increasing altitude limits require more of the D bits. Up to and including 30700 ft does not require any of the D bits. This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4. Up to and including 126700 ft requires D4 and D2. D1 is never used.
Bits D2 through B4 encode the pressure altitude in 500 ft increments in a standard 8-bit reflected binary code. The specification stops at code 1000000, above which D1 would be needed as a most significant bit. Bits C1, C2 and C4 use a mirrored 5-state 3-bit Gray BCD code of a Giannini Datex code type to encode the offset from the 500 ft altitude in 100 ft increments. Specifically, if the parity of the 500 ft code is even then codes 001, 011, 010, 110 and 100 encode −200, −100, 0, +100 and +200 ft relative to the 500 ft altitude. If the parity is odd, the assignments are reversed. Codes 000, 101 and 111 are not used. The Gillham code can be decoded using various methods. Standard techniques use hardware or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.
