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
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
The code is named after Ronald Lionel Gillham, a signals officer at Air Navigational Services,
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
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.[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]
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
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]
^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
^
Honeywell International Inc. August 2002 [2001]. Manual number 006-10609-0003. Archived
(PDF) from the original on 2018-01-18. Retrieved 2018-01-18.
^"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]
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)
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.
(PDF) on 2020-05-17. Retrieved 2020-05-17. (43 pages)
^
FAA
report of May 1962, has been introduced […]
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. […]
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]
^
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 […]
. 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. […]
^Spaulding, Carl P. (1965-07-12). How to Use Shaft Encoders. Monrovia, California, USA: Datex Corporation. (85 pages)
^
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. […] (270 pages) (NB. This is based on a translation of volume I of the two-volume German edition.)
. (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.)
. 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.)
Ashley, Allan (September 1960). Study of Altitude Reporting via ATC Radar Beacon System. Deer Park, New York: Airborne Instruments Laboratory. Report 5791-23.
