Gillham code

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Gillham code
Digits12
Tracks9..11[1][2]
Continuityno
Cyclicyes
Minimum distance1
Maximum distance1
Lexicographyno
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[edit]

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. The exact timeframe and circumstances of the term Gillham code being coined are unclear, but by 1962 the code was broadly recognized under this name[5][6] and described in an FAA report.[7][8] By the mid-1960s the code was also known as MOA–Gillham code[9] or ICAO–Gillham code. ARINC 572 specified the code as well in 1968.[10][11]

Once recommended by the ICAO for automatic height transmission for air traffic control purposes,[8][12] it is now discouraged[2] and has been mostly replaced by modern serial communication in newer aircraft.

Altitude encoder[edit]

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.

An altitude encoder takes the form of a small metal box containing a pressure sensor and signal conditioning electronics.[13][14] 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.[15]

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 (open-drain) 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.[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.[16][17]

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[edit]

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).[16][18][19][20][21] 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[9][22] (with the first 5 states resembling O'Brien code type II[23][20][21]) 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.[16][18] Codes 000, 101 and 111 are not used.[24]:13(6.17–21)

The Gillham code can be decoded using various methods. Standard techniques use hardware[24] or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.[18]

See also[edit]

References[edit]

  1. ^ 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.
  2. ^ 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.
  3. ^ a b c 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.
  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. […]
  5. ^ "(Unknown)". Computer Design (cd). Computer Design Publishing Corporation. 1–2: 45. 1962. 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. […]
  6. ^ "(Unknown)". Control Engineering. Technical Publishing Company. 10: 110. 1963. Retrieved 2018-01-16. […] Designed to be compatible with American and European traffic systems, a beacon encoder available from Norden Div., United Aircraft Corp., Nonvalk, Conn., puts out a modified Gray code known as the Gillham code. […]
  7. ^ (Unknown) (Report). Federal Aviation Administration (FAA). May 1962.
  8. ^ a b United Service and Royal Aero Club (Great Britain) (1964). "(Unknown)". Flight International. Illiffe Transport Publications. 85 (2): 593. […] Altitude encoding: 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 […]
  9. ^ a b Wheeler, Edwin L. (1969-12-30) [1968-04-05]. "US Patent: Analog to digital encoder". Conrac Corp. Patent US3487460A. Archived from the original on 2018-01-21. 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 […]
  10. ^ Mark 2 Subsonic Air Data System. Annapolis, Maryland, USA: Aeronautical Radio, Incorporated (ARINC). 1968-02-15. p. 55. ARINC 572.
  11. ^ Mark 2 Air Traffic Control Transponder. Aeronautical Radio, Incorporated (ARINC). ARINC 572-1.
  12. ^ Wightman, Eric Jeffrey (2017) [1972]. "Chapter 6. Displacement measurement". Instrumentation in Process Control (Revised ed.). Butterworth-Heinemann. p. 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. […]
  13. ^ "Ameriking AK-350 Altitude Encoder". Ameri-king. 2004. Archived from the original on 2016-06-25. Retrieved 2018-01-14.
  14. ^ "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.
  15. ^ "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.
  16. ^ a b c Phillips, Darryl (2012-07-26) [1998]. "Altitude - MODEC ASCII". AirSport Avionics. Archived from the original on 2012-07-26.
  17. ^ D.F.S., Marc (2000-11-27). "Single Gillham code". ForPilots. Archived from the original on 2018-01-17. Retrieved 2018-01-17.
  18. ^ a b c 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.
  19. ^ Gray, Frank (1953-03-17), Pulse code communication (NB. U.S. Patent 2,632,058 filed November 1947.)
  20. ^ a b Steinbuch, Karl W., ed. (1962). Written at Karlsruhe, Germany. Taschenbuch der Nachrichtenverarbeitung (in German) (1 ed.). Berlin / Göttingen / New York: Springer-Verlag OHG. pp. 71–74. LCCN 62-14511.
  21. ^ a b 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). 2 (3 ed.). Berlin, Germany: Springer Verlag. pp. 98–100. ISBN 3-540-06241-6. LCCN 73-80607.
  22. ^ Spaulding, Carl P. (1965-01-12) [1954-03-09]. "US Patent: Digital coding and translating system". Datex Corp. Patent US3165731A. Archived from the original on 2018-01-21. Retrieved 2018-01-21.
  23. ^ O'Brien, Joseph A. (May 1956). "Cyclic Decimal Codes for Analogue to Digital Converters". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 75 (2): 120–122. doi:10.1109/TCE.1956.6372498. ISSN 0097-2452.
  24. ^ a b [1], Langheinrich, Hans, "United States Patent US3805041 - Circuit for converting one code into another code", assigned to VDO Tachometer Werke Adolf Schindling GmbH 

Further reading[edit]

Industry specifications