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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.⁵ The final code variant was developed in late 1961⁶ for the ICAO Communications Division meeting (VII COM) held in January/February 1962,⁷ and described in a 1962 FAA report.⁸⁹¹⁰ 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.¹¹¹² By the mid-1960s the code was also known as MOA–Gillham code¹³ or ICAO–Gillham code. ARINC 572 specified the code as well in 1968.¹⁴¹⁵

Once recommended by the ICAO for automatic height transmission for air traffic control purposes,¹⁶¹⁷ the interface is now discouraged¹⁸ 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.¹⁹²⁰ 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.²¹

The encoder has an open-collector output, compatible with 14 V or 28 V electrical systems.

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 C4.²² 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 interface²³). This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4 (10-wire interface²⁴). Up to and including 126700 ft requires D4 and D2 (11-wire interface²⁵). D1 is never used.²⁶²⁷

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).^2829303132 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^3334353637 (with the first 5 states resembling O'Brien code type II^383940414243) to encode the offset from the 500 ft altitude in 100 ft increments.⁴⁴ 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.⁴⁵⁴⁶ Codes 000, 101 and 111 are not used.⁴⁷: 13(6.17–21) 

The Gillham code can be decoded using various methods. Standard techniques use hardware⁴⁸ or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.⁴⁹

See also

• Air traffic control radar beacon system (ATCRBS)
• Selective Identification Feature (SIF)
• IFF code
• Flight level
• ARINC 429

Notes

Further reading

• Military Handbook: Encoders - Shaft Angle To Digital (PDF). United States Department of Defense. 1991-09-30. MIL-HDBK-231A. Archived (PDF) from the original on 2020-07-25. Retrieved 2020-07-25. (NB. Supersedes MIL-HDBK-231(AS) (1970-07-01).)
• Annex 10 - Volume IV - Surveillance Radar and Collision Avoidance Systems Archived 6 May 2014 at the Wayback Machine; 4th Edition; ICAO; 280 pages; 2007.
• DO-181E Minimum Operational Performance Standards for ATCRBS / Mode S Airborne Equipment; Rev E; RTCA; 2011.
• Ashley, Allan (September 1960). Study of Altitude Reporting via ATC Radar Beacon System. Deer Park, New York: Airborne Instruments Laboratory. Report 5791-23.
  • "Study of Altitude Reporting via ATC Radar Beacon System". Consolidated Abstracts of Technical Reports: General distribution. 1957–1962 (Abstract). 1962. p. #62-45.

References

1. 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. https://petitcessnavoyageur.files.wordpress.com/2016/01/kmh880-kta-870-im-006-10609-0003_3.pdf ↩

2. 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. 978-0-7506-8695-2 ↩

3. Phillips, Darryl (2012) [1998]. "Mode A and Mode C - The straight scoop on how it works". AirSport Avionics. Archived from the original on 2012-06-14. Retrieved 2018-01-14. https://web.archive.org/web/20120614184629/http://www.airsport-corp.com/modec.htm ↩

4. "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] https://www.thegazette.co.uk/London/issue/40497/supplement/3267 ↩

5. 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] /wiki/Wikipedia:Citation_needed ↩

6. 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) /wiki/Institute_of_Radio_Engineers ↩

7. "1983 Pioneer Award". IEEE Transactions on Aerospace and Electronic Systems. AES-19 (4). IEEE: 648–656. July 1983. doi:10.1109/TAES.1983.309363. Archived from the original on 2020-05-16. Retrieved 2020-05-16. […] 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) https://ieeexplore.ieee.org/document/4102842 ↩

8. Airborne Instruments Laboratory, a division of Cutler-Hammer, Inc. (1962-05-19). Final Engineering Report on Evaluation of L-band Secondary Radar. For ANDB under CAA (Report). Deer Park, Long Island, New York, USA: Federal Aviation Administration (FAA), Aviation Research And Development Service. Report 8893-SP-1. /wiki/Cutler-Hammer,_Inc. ↩

9. 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) /wiki/Cutler-Hammer,_Inc. ↩

10. 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 […] https://books.google.com/books?id=v04eERVZ_HMC ↩

11. "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. […] https://books.google.com/books?id=npg_AQAAIAAJ ↩

12. "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] https://books.google.com/books?id=DPpIAQAAIAAJ ↩

13. 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 […] https://patentimages.storage.googleapis.com/f0/c0/60/9c3231f7e8ed44/US3487460.pdf ↩

14. Mark 2 Subsonic Air Data System. Annapolis, Maryland, USA: Aeronautical Radio, Incorporated (ARINC). 1968-02-15. p. 55. ARINC 572. /wiki/Aeronautical_Radio,_Incorporated ↩

15. Mark 2 Air Traffic Control Transponder. Aeronautical Radio, Incorporated (ARINC). ARINC 572-1. /wiki/Aeronautical_Radio,_Incorporated ↩

16. 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 […] https://books.google.com/books?id=v04eERVZ_HMC ↩

17. 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. […] 978-1-48316335-2 ↩

18. 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. 978-0-7506-8695-2 ↩

19. "Ameriking AK-350 Altitude Encoder". Ameri-king. 2004. Archived from the original on 2016-06-25. Retrieved 2018-01-14. https://web.archive.org/web/20160625175313/http://www.ameri-king.com/altitude_encoder.html ↩

20. "Model E-04 406/121.5 MHz ELT". Products. ACK Technologies, Inc. 2002. Archived from the original on 2018-01-16. Retrieved 2018-01-14. http://www.ackavionics.com/products.htm ↩

21. "Altitude Encoder Model 8800-T Operating Manual" (PDF). Shadin Avionics. 2016. OP8800-TC Rev. F. Archived (PDF) from the original on 2018-01-16. Retrieved 2018-01-14. https://www.shadin.com/wp-content/uploads/2020/06/OP8800TC.pdf ↩

22. Phillips, Darryl (2012) [1998]. "Mode A and Mode C - The straight scoop on how it works". AirSport Avionics. Archived from the original on 2012-06-14. Retrieved 2018-01-14. https://web.archive.org/web/20120614184629/http://www.airsport-corp.com/modec.htm ↩

23. 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. https://petitcessnavoyageur.files.wordpress.com/2016/01/kmh880-kta-870-im-006-10609-0003_3.pdf ↩

24. 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. 978-0-7506-8695-2 ↩

25. 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. 978-0-7506-8695-2 ↩

26. Phillips, Darryl (2012-07-26) [1998]. "Altitude - MODEC ASCII". AirSport Avionics. Archived from the original on 2012-07-26. https://web.archive.org/web/20120726003224/http://www.airsport-corp.com/modecascii.txt ↩

27. D. F. S., Marc (2000-11-27). "Single Gillham code". ForPilots. Archived from the original on 2018-01-17. Retrieved 2018-01-17. https://web.archive.org/web/20180117174656/http://www.forpilots.com/archive/rec.aviation.owning/5/msg5377.htm ↩

28. Phillips, Darryl (2012-07-26) [1998]. "Altitude - MODEC ASCII". AirSport Avionics. Archived from the original on 2012-07-26. https://web.archive.org/web/20120726003224/http://www.airsport-corp.com/modecascii.txt ↩

29. Stewart, K. (2010-12-03). "Aviation Gray Code: Gillham Code Explained". Custom Computer Services (CCS). Archived from the original on 2018-01-16. Retrieved 2018-01-14. http://www.ccsinfo.com/forum/viewtopic.php?p=140960#140960 ↩

30. Gray, Frank (1953-03-17) [1947-11-13]. Pulse Code Communication (PDF). New York, USA: Bell Telephone Laboratories, Incorporated. U.S. patent 2,632,058. Serial No. 785697. Archived (PDF) from the original on 2020-08-05. Retrieved 2020-08-05. (13 pages) /wiki/Frank_Gray_(researcher) ↩

31. Steinbuch, Karl W., ed. (1962). Taschenbuch der Nachrichtenverarbeitung (in German) (1 ed.). Karlsruhe, Germany: Springer-Verlag OHG. pp. 71–74. LCCN 62-14511. /wiki/Karl_W._Steinbuch ↩

32. Steinbuch, Karl W.; Weber, Wolfgang; Heinemann, Traute, eds. (1974) [1967]. Taschenbuch der Informatik – Band II – Struktur und Programmierung von EDV-Systemen. Taschenbuch der Nachrichtenverarbeitung (in German). Vol. 2 (3 ed.). Berlin, Germany: Springer Verlag. pp. 98–100. ISBN 3-540-06241-6. LCCN 73-80607. 3-540-06241-6 ↩

33. 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 […] https://patentimages.storage.googleapis.com/f0/c0/60/9c3231f7e8ed44/US3487460.pdf ↩

34. Spaulding, Carl P. (1965-01-12) [1954-03-09]. "Digital coding and translating system" (PDF). Monrovia, California, USA: Datex Corporation. U.S. patent 3165731A. Serial No. 415058. Archived (PDF) from the original on 2020-08-05. Retrieved 2018-01-21. (28 pages) https://patentimages.storage.googleapis.com/7f/1d/09/6a9b1fa3e67cb8/US3165731.pdf ↩

35. Spaulding, Carl P. (1965-07-12). How to Use Shaft Encoders. Monrovia, California, USA: Datex Corporation. (85 pages) ↩

36. Dokter, Folkert; Steinhauer, Jürgen (1973-06-18). "2.4. Coding numbers in the binary system". Digital Electronics. Philips Technical Library (PTL) / Macmillan Education (Reprint of 1st English ed.). Eindhoven, Netherlands: The Macmillan Press Ltd. / N. V. Philips' Gloeilampenfabrieken. pp. 32, 39, 50–53. doi:10.1007/978-1-349-01417-0. ISBN 978-1-349-01419-4. SBN 333-13360-9. Retrieved 2020-05-11. p. 53: […] The Datex code […] uses the O'Brien code II within each decade, and reflected decimal numbers for the decimal transitions. For further processing, code conversion to the natural decimal notation is necessary. Since the O'Brien II code forms a 9s complement, this does not give rise to particular difficulties: whenever the code word for the tens represents an odd number, the code words for the decimal units are given as the 9s complements by inversion of the fourth binary digit. […][permanent dead link] (270 pages) (NB. This is based on a translation of volume I of the two-volume German edition.) 978-1-349-01419-4333-13360-9 ↩

37. Dokter, Folkert; Steinhauer, Jürgen (1975) [1969]. "2.4.4.6. Einschrittige Kodes". Digitale Elektronik in der Meßtechnik und Datenverarbeitung: Theoretische Grundlagen und Schaltungstechnik. Philips Fachbücher (in German). Vol. I (improved and extended 5th ed.). Hamburg, Germany: Deutsche Philips GmbH. p. 60. ISBN 3-87145-272-6. (xii+327+3 pages) (NB. The German edition of volume I was published in 1969, 1971, two editions in 1972, and 1975. Volume II was published in 1970, 1972, 1973, and 1975.) 3-87145-272-6 ↩

38. O'Brien, Joseph A. (May 1956) [1956-11-15, 23 June 1956]. "Cyclic Decimal Codes for Analogue to Digital Converters". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 75 (2). Bell Telephone Laboratories, Whippany, New Jersey, USA: 120–122. doi:10.1109/TCE.1956.6372498. ISSN 0097-2452. S2CID 51657314. Paper 56-21. Retrieved 2020-05-18. (3 pages) (NB. This paper was prepared for presentation at the AIEE Winter General Meeting, New York, USA, 1955-01-30 to 1955-02-03.) https://pdfslide.net/documents/cyclic-decimal-codes-for-analogue-to-digital-converters.html ↩

39. 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) /wiki/Institute_of_Radio_Engineers ↩

40. Steinbuch, Karl W., ed. (1962). Taschenbuch der Nachrichtenverarbeitung (in German) (1 ed.). Karlsruhe, Germany: Springer-Verlag OHG. pp. 71–74. LCCN 62-14511. /wiki/Karl_W._Steinbuch ↩

41. Steinbuch, Karl W.; Weber, Wolfgang; Heinemann, Traute, eds. (1974) [1967]. Taschenbuch der Informatik – Band II – Struktur und Programmierung von EDV-Systemen. Taschenbuch der Nachrichtenverarbeitung (in German). Vol. 2 (3 ed.). Berlin, Germany: Springer Verlag. pp. 98–100. ISBN 3-540-06241-6. LCCN 73-80607. 3-540-06241-6 ↩

42. Dokter, Folkert; Steinhauer, Jürgen (1973-06-18). "2.4. Coding numbers in the binary system". Digital Electronics. Philips Technical Library (PTL) / Macmillan Education (Reprint of 1st English ed.). Eindhoven, Netherlands: The Macmillan Press Ltd. / N. V. Philips' Gloeilampenfabrieken. pp. 32, 39, 50–53. doi:10.1007/978-1-349-01417-0. ISBN 978-1-349-01419-4. SBN 333-13360-9. Retrieved 2020-05-11. p. 53: […] The Datex code […] uses the O'Brien code II within each decade, and reflected decimal numbers for the decimal transitions. For further processing, code conversion to the natural decimal notation is necessary. Since the O'Brien II code forms a 9s complement, this does not give rise to particular difficulties: whenever the code word for the tens represents an odd number, the code words for the decimal units are given as the 9s complements by inversion of the fourth binary digit. […][permanent dead link] (270 pages) (NB. This is based on a translation of volume I of the two-volume German edition.) 978-1-349-01419-4333-13360-9 ↩

43. Dokter, Folkert; Steinhauer, Jürgen (1975) [1969]. "2.4.4.6. Einschrittige Kodes". Digitale Elektronik in der Meßtechnik und Datenverarbeitung: Theoretische Grundlagen und Schaltungstechnik. Philips Fachbücher (in German). Vol. I (improved and extended 5th ed.). Hamburg, Germany: Deutsche Philips GmbH. p. 60. ISBN 3-87145-272-6. (xii+327+3 pages) (NB. The German edition of volume I was published in 1969, 1971, two editions in 1972, and 1975. Volume II was published in 1970, 1972, 1973, and 1975.) 3-87145-272-6 ↩

44. Phillips, Darryl (2012) [1998]. "Mode A and Mode C - The straight scoop on how it works". AirSport Avionics. Archived from the original on 2012-06-14. Retrieved 2018-01-14. https://web.archive.org/web/20120614184629/http://www.airsport-corp.com/modec.htm ↩

45. Phillips, Darryl (2012-07-26) [1998]. "Altitude - MODEC ASCII". AirSport Avionics. Archived from the original on 2012-07-26. https://web.archive.org/web/20120726003224/http://www.airsport-corp.com/modecascii.txt ↩

46. Stewart, K. (2010-12-03). "Aviation Gray Code: Gillham Code Explained". Custom Computer Services (CCS). Archived from the original on 2018-01-16. Retrieved 2018-01-14. http://www.ccsinfo.com/forum/viewtopic.php?p=140960#140960 ↩

47. Langheinrich, Hans (1974-04-16) [1971-10-27]. Circuit for converting one code into another code (PDF). Frankfurt, Germany: VDO Tachometer Werke Adolf Schindling GmbH. U.S. patent 3,805,041. Application 192830. Archived (PDF) from the original on 2020-08-05. Retrieved 2018-01-14. (7 pages) https://patentimages.storage.googleapis.com/0a/3b/16/a511d87808faae/US3805041.pdf ↩

48. Langheinrich, Hans (1974-04-16) [1971-10-27]. Circuit for converting one code into another code (PDF). Frankfurt, Germany: VDO Tachometer Werke Adolf Schindling GmbH. U.S. patent 3,805,041. Application 192830. Archived (PDF) from the original on 2020-08-05. Retrieved 2018-01-14. (7 pages) https://patentimages.storage.googleapis.com/0a/3b/16/a511d87808faae/US3805041.pdf ↩

49. Stewart, K. (2010-12-03). "Aviation Gray Code: Gillham Code Explained". Custom Computer Services (CCS). Archived from the original on 2018-01-16. Retrieved 2018-01-14. http://www.ccsinfo.com/forum/viewtopic.php?p=140960#140960 ↩