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}}[[File:CessnaARC-RT-359ATransponder04.jpg|thumb|upright=1.14|A Cessna ARC RT-359A [[Transponder (aviation)|transponder]] (the beige box) in the instrument panel of an [[Grumman American AA-1|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-<ref name="Honeywell_2002"/> to eleven-wire [[interface (computing)|interface]],<ref name="Tooley-Wyatt_2009"/> the '''Gillham interface''', that is used to transmit uncorrected [[Barometer|barometric]] [[altitude]] between an encoding [[altimeter]] or analog [[air data computer]] and a [[Digital data|digital]] [[Transponder (aviation)|transponder]]. It is a modified form of a [[Gray code]] and is sometimes referred to simply as a "Gray code" in [[avionics]] literature.<ref name="
== History ==
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An altitude encoder takes the form of a small metal box containing a [[pressure sensor]] and signal conditioning electronics.<ref name="Ameriking_2004"/><ref name="ACK Encoder"/> 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.<ref name="Shadin_2016"/>
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]]) [[transistor]]s 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.<ref name="
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<ref name="Honeywell_2002"/>). This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4 (10-wire interface<ref name="Tooley-Wyatt_2009"/>). Up to and including 126700 ft requires D4 and D2 (11-wire interface<ref name="Tooley-Wyatt_2009"/>). D1 is never used.<ref name="
{| class="wikitable" style="margin-left:4em; text-align:right"
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== 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).<ref name="
Bits C1, C2 and C4 use a mirrored 5-state 3-bit Gray BCD code of a [[Giannini Datex code]] type<ref name="Wheeler_1969"/><ref name="Datex_1965"/><ref name="Dokter_1973"/><ref name="Dokter_1975"/> (with the first 5 states resembling [[O'Brien code type II]]<ref name="O'Brien_1955"/><ref name="Ashley_1961"/><ref name="Steinbuch_1962"/><ref name="Steinbuch-Weber_1974"/><ref name="Dokter_1973"/><ref name="Dokter_1975"/>) to encode the offset from the 500 ft altitude in 100 ft increments.<ref name="
The Gillham code can be decoded using various methods. Standard techniques use hardware<ref name="Decoder Patent"/> or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.<ref name="Stewart_2010"/>
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{{Reflist|refs=
<ref name="Wightman_2017">{{cite book |author-first=Eric Jeffrey |author-last=Wightman |title=Instrumentation in Process Control |date=2017 |orig-year=1972 |edition=Revised |publisher=[[Butterworth-Heinemann]] |isbn=978-1-48316335-2<!-- 1-48316335-0 --> |chapter=Chapter 6. Displacement measurement |pages=122–123 [123] |chapter-url=https://books.google.com/books?id=8WEhBQAAQBAJ&pg=PA123 |url=https://books.google.com/books?id=8WEhBQAAQBAJ |quote=[…] 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. […]}}</ref>
<ref name="
<ref name="Ameriking_2004">{{cite web |title=Ameriking AK-350 Altitude Encoder |publisher=Ameri-king |url=http://www.ameri-king.com/altitude_encoder.html |access-date=2018-01-14 |year=2004 |url-status=dead |archive-url=https://web.archive.org/web/20160625175313/http://www.ameri-king.com/altitude_encoder.html |archive-date=2016-06-25}}</ref>
<ref name="ACK Encoder">{{cite web |title=Model E-04 406/121.5 MHz ELT |work=Products |publisher=ACK Technologies, Inc. |date=2002 |url=http://www.ackavionics.com/products.htm |access-date=2018-01-14 |url-status=live |archive-url=https://web.archive.org/web/20180116184013/http://www.ackavionics.com/products.htm |archive-date=2018-01-16}}</ref>
<ref name="Shadin_2016">{{cite web |title=Altitude Encoder Model 8800-T Operating Manual |date=2016 |id=OP8800-TC Rev. F |publisher=Shadin Avionics |url=https://www.shadin.com/documents/manuals/OP8800TC.pdf |access-date=2018-01-14 |url-status=live |archive-url=https://web.archive.org/web/20180116190458/https://www.shadin.com/documents/manuals/OP8800TC.pdf |archive-date=2018-01-16}}</ref>
<ref name="
<ref name="Decoder Patent">{{cite patent |title=United States Patent US3805041 - Circuit for converting one code into another code |inventor-first=Hans |inventor-last=Langheinrich |assignee=[[VDO Tachometer Werke Adolf Schindling GmbH]] |fdate=1971-10-27 |adate=1974-04-16 |id=Application 192830 patent 3805041 |url=http://www.freepatentsonline.com/3805041.pdf |access-date=2018-01-14 |url-status=live |archive-url=https://web.archive.org/web/20200518095507/http://www.freepatentsonline.com/3805041.pdf |archive-date=2020-05-18}} (7 pages)</ref>
<ref name="Stewart_2010">{{cite web |title=Aviation Gray Code: Gillham Code Explained |date=2010-12-03 |author-first=K. |author-last=Stewart |publisher=Custom Computer Services (CCS) |url=http://www.ccsinfo.com/forum/viewtopic.php?p=140960#140960 |access-date=2018-01-14 |url-status=live |archive-url=https://web.archive.org/web/20180116184525/http://www.ccsinfo.com/forum/viewtopic.php?p=140960 |archive-date=2018-01-16}}</ref>
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