Gillham code

Gillham code
Gillham code
Digits	12
Tracks	9..11
Continuity	no
Cyclic	yes
Minimum distance	1
Maximum distance	1
Lexicography	no
	v; t; e;

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.

Gillham code is a zero-padded 12-bit binary code using a parallel nine-^[1] to eleven-wire interface,^[2] 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.^[3]

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 (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.^[4] He was the UK's representative to the International Air Transport Association (IATA) 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.^{[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.^[7]^[8]^[9] 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.^[10]^[11] By the mid-1960s the code was also known as MOA–Gillham code^[12] or ICAO–Gillham code. ARINC 572 specified the code as well in 1968.^[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.

Altitude encoder

An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics.^[16]^[17] 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.^[18]

The encoder has an open-collector output, compatible with 14 V or 28 V electrical systems.^{[citation needed]}

Coding

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.^[3] 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 (9-wire interface^[1]). This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4 (10-wire interface^[2]). Up to and including 126700 ft requires D4 and D2 (11-wire interface^[2]). D1 is never used.^[19]^[20]

Gillham binary code [D124 A124 B124 C124]	Squawk octal code [ABCD]	Height [m]	Height [ft]
000 000 000 001	0040	−365.76	−1200
000 000 000 011	0060	−335.28	−1100
000 000 000 010	0020	−304.8	−1000
000 000 000 110	0030	−274.32	−900
000 000 000 100	0010	−243.84	−800
000 000 001 100	0410	−213.36	−700
000 000 001 110	0430	−182.88	−600
000 000 001 010	0420	−152.4	−500
000 000 001 011	0460	−121.92	−400
000 000 001 001	0440	−91.44	−300
000 000 011 001	0640	−60.96	−200
000 000 011 011	0660	−30.48	−100
000 000 011 010	0620	0	0
000 000 011 110	0630	30.48	100
000 000 011 100	0610	60.96	200
000 000 010 100	0210	91.44	300
000 000 010 110	0230	121.92	400
000 000 010 010	0220	152.4	500
000 000 010 011	0260	182.88	600
000 000 010 001	0240	213.36	700
000 000 110 001	0340	243.84	800
000 000 110 011	0360	274.32	900
000 000 110 010	0320	304.8	1000
000 000 110 110	0330	335.28	1100
000 000 110 100	0310	365.76	1200
000 000 111 100	0710		1300
000 000 111 110	0730		1400
000 000 111 010	0720		1500
000 000 111 011	0760		1600
000 000 111 001	0740		1700
000 000 101 001	0540		1800
000 000 101 011	0560		1900
000 000 101 010	0520		2000
000 000 101 110	0530		2100
000 000 101 100	0510		2200
000 000 100 100	0110		2300
000 000 100 110	0130		2400
000 000 100 010	0120		2500
000 000 100 011	0160		2600
000 000 100 001	0140		2700
…	…	…	…
010 000 000 110	0032		126400
010 000 000 010	0022		126500
010 000 000 011	0062		126600
010 000 000 001	0042		126700

Decoding

Bits D2 (msbit) through B4 (lsbit) encode the pressure altitude in 500 ft increments (above a base altitude of −1000±250 ft) in a standard 8-bit reflected binary code (Gray code).^[19]^[21]^[22]^[23]^[24] The specification stops at code 1000000 (126500±250 ft), 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^[12]^[25]^[26]^[27]^[28] (with the first 5 states resembling O'Brien code type II^[29]^[5]^[23]^[24]^[27]^[28]) to encode the offset from the 500 ft altitude in 100 ft increments.^[3] 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.^[19]^[21] Codes 000, 101 and 111 are not used.^[30]^{: 13(6.17–21)}

The Gillham code can be decoded using various methods. Standard techniques use hardware^[30] or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.^[21]

Notes

^ Anecdotally, Ronald Lionel Gillham had the idea for the modified Gray code while having a family dinner. Reportedly, he died in March 1968.^{[citation needed]}

References

^ ^a ^b ^c Honeywell System Installation Manual - Bendix/King KMH 880/KTA 870 Multi-Hazard Awareness Traffic Advisory System (PDF) (Revision 3 ed.). Honeywell International Inc. August 2002 [2001]. Manual number 006-10609-0003. Archived (PDF) from the original on 2018-01-18. Retrieved 2018-01-18.
^ ^a ^b ^c ^d ^e Tooley, Mike; Wyatt, David (2009). "3.5.1 Gillham interface and Gillham code". Aircraft Electrical and Electronic Systems - Principles, Operation and Maintenance (1 ed.). Butterworth-Heinemann (Elsevier Ltd.). p. 69. ISBN 978-0-7506-8695-2.
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^ "No. 40497". The London Gazette (Supplement). 1955-06-03. pp. 3267, 3272, 3274. […] CENTRAL CHANCERY OF THE ORDERS OF KNIGHTHOOD. […] St. James's Palace, S.W.1. […] 9th June, 1955. […] The QUEEN has been graciously pleased, on the occasion of the Celebration of Her Majesty's Birthday, to give orders for the following promotions in, and appointments to, the Most Excellent Order of the British Empire:— […] To be Ordinary Members of the Civil Division of the said Most Excellent Order:— […] Ronald Lionel GILLHAM, Esq., Signals Officer, Air Navigational Services, Ministry of Transport and Civil Aviation. […] [1][2][3]
^ ^a ^b Ashley, Allan (December 1961). "Code Configuration for Automatic Altitude Reporting via ATCRBS". IRE Transactions on Aerospace and Navigational Electronics. ANE-8 (4). Melville, New York, USA: Institute of Radio Engineers: 144–148. doi:10.1109/TANE3.1961.4201819. eISSN 2331-0812. ISSN 0096-1647. S2CID 51647765. (5 pages)
^ "1983 Pioneer Award". IEEE Transactions on Aerospace and Electronic Systems. AES-19 (4). IEEE: 648–656. July 1983. doi:10.1109/TAES.1983.309363. […] The Pioneer Award Committee of the IEEE Aerospace and Electronic Systems Society has named […] Allan Ashley […] Joseph E. Her[r]mann […] James S. Perry […] as recipients of the 1983 Pioneer Award in recognition of the highly significant contributions made by them. "FOR ADVANCING THE STATE OF THE ART OF VOICE AND DATA RADIO COMMUNICATIONS AND ELECTRONICS" The Award was presented at NAECON on May 18, 1983. […] Being aware of developments within the United States and shortly before the ICAO VII COM [in January 1962], the U.K. delegates proposed a compromise code to the United States which quantized altitude in 500 ft steps for a range of 64000 ft by employing a conventional Gray code with a 2.9 µs pulse spacing in the return message, and in a compatible manner subdivided further by 100 ft increments with a 1.45 µs pulse spacing in the return message […] A quick look at the U.K. proposal concluded that the United States could live with the U.K. compromise although greater circuit complexity resulted for coding and decoding. It is to the credit of the U.S. delegation to the ICAO VII COM, and as a result of the advice of Ashley, Herrmann, Perry, and others, that the acceptance of the compatible U.K. proposal was seen as offering a means of obtaining timely agreement on 100 ft increment reportings o that future air traffic control systems could be developed with automatic three dimensional data acquisition. A potential impasse in ICAO was averted, leaving nations free to choose between 100 ft and 500 ft increments of altitude reporting. […] (9 pages)
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^ Airborne Instruments Laboratory, a division of Cutler-Hammer, Inc. (May 1962). Height Code Tables For Use With Air Traffic Control Radar Beacon System (PDF) (Report). Deer Park, Long Island, New York, USA: Federal Aviation Administration (FAA), Aviation Research And Development Service. Report 8893-SP-1. Contract FAA/BRD-329. Task 6. Archived from the original (PDF) on 2020-05-17. Retrieved 2020-05-17. (43 pages)
^ ^a ^b United Service and Royal Aero Club (Great Britain) (1964-04-09). "Altitude encoding". Flight International. 85 (2874). Illiffe Transport Publications: 593. ISSN 0015-3710. […] A new […] encoder with an output in Gillham code, as recommended for altitude encoding by ICAO and described in an FAA report of May 1962, has been introduced […]
^ "Beacon Encoder". Computer Design. 2 (9). Massachusetts, USA: Computer Design Publishing Corporation: 45. September 1963. ISSN 0010-4566. OCLC 802774218. Circle No. 169. Retrieved 2018-01-16. […] Output code of a new Beacon encoder is known as the Gillham code, a modified Gray code designed to be compatible with both American and European traffic systems. […]
^ "New Products". Control Engineering (CtE). 10. Technical Publishing Company: 110. January–December 1963. ISSN 0010-8049. (344) or (345). Retrieved 2018-01-16. […] Designed to be compatible with American and European traffic systems, a beacon encoder available from Norden Div., United Aircraft Corp., Norwalk, Conn., puts out a modified Gray code known as the Gillham code. […] [4]
^ ^a ^b Wheeler, Edwin L. (1969-12-30) [1968-04-05]. Analog to digital encoder (PDF). New York, USA: Conrac Corporation. U.S. patent 3487460A. Serial No. 719026 (397812). Archived (PDF) from the original on 2020-08-05. Retrieved 2018-01-21. […] The MOA-GILLHAM code is essentially the combination of the Gray code discussed thereinabove and the well known Datex code; the Datex code is disclosed in U.S. Patent 3,165,731. The arrangement is such that the Datex code defines the bits for the units count of the encoder and the Gray code defines the bits for each of the higher order decades, the tens, hundreds, etc […]
^ Mark 2 Subsonic Air Data System. Annapolis, Maryland, USA: Aeronautical Radio, Incorporated (ARINC). 1968-02-15. p. 55. ARINC 572.
^ Mark 2 Air Traffic Control Transponder. Aeronautical Radio, Incorporated (ARINC). ARINC 572-1.
^ Wightman, Eric Jeffrey (2017) [1972]. "Chapter 6. Displacement measurement". Instrumentation in Process Control (Revised ed.). Butterworth-Heinemann. pp. 122–123 [123]. ISBN 978-1-48316335-2. […] Other forms of code are also well known. Among these are the Royal Radar Establishment code; The Excess Three decimal code; Gillham code which is recommended by ICAO for automatic height transmission for air traffic control purposes; the Petherick code, and the Leslie and Russell code of the National Engineering Laboratory. Each has its particular merits and they are offered as options by various encoder manufacturers. A discussion of their respective merits is outside the scope of this book. […]
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