Gillham code


Gillham code is a zero-padded 12-bit binary code using a parallel nine- to eleven-wire interface, the Gillham interface, that is used to transmit uncorrected barometric altitude between an encoding altimeter or analog air data computer and a digital transponder. It is a modified form of a Gray code and is sometimes referred to simply as a "Gray code" in avionics literature.

History

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 aircraft electrical 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.
Gillham binary code Squawk octal code Height Height
000 000 000 0010040−365.76−1200
000 000 000 0110060−335.28−1100
000 000 000 0100020−304.8−1000
000 000 000 1100030−274.32−900
000 000 000 1000010−243.84−800
000 000 001 1000410−213.36−700
000 000 001 1100430−182.88−600
000 000 001 0100420−152.4−500
000 000 001 0110460−121.92−400
000 000 001 0010440−91.44−300
000 000 011 0010640−60.96−200
000 000 011 0110660−30.48−100
000 000 011 010062000
000 000 011 110063030.48100
000 000 011 100061060.96200
000 000 010 100021091.44300
000 000 010 1100230121.92400
000 000 010 0100220152.4500
000 000 010 0110260182.88600
000 000 010 0010240213.36700
000 000 110 0010340243.84800
000 000 110 0110360274.32900
000 000 110 0100320304.81000
000 000 110 1100330335.281100
000 000 110 1000310365.761200
000 000 111 10007101300
000 000 111 11007301400
000 000 111 01007201500
000 000 111 01107601600
000 000 111 00107401700
000 000 101 00105401800
000 000 101 01105601900
000 000 101 01005202000
000 000 101 11005302100
000 000 101 10005102200
000 000 100 10001102300
000 000 100 11001302400
000 000 100 01001202500
000 000 100 01101602600
000 000 100 00101402700
010 000 000 1100032126400
010 000 000 0100022126500
010 000 000 0110062126600
010 000 000 0010042126700

Decoding

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.