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{{Unreferenced|date=December 2010}}
A [[digital]] [[code]] using an eleven-wire interface that is used to transmit uncorrected [[barometric]] [[altitude]] between an encoding altimeter or analog [[air data computer]] and a [[transponder]]. It is a modified form of a [[Gray Code]] and is sometimes referred to as a "Gray Code" in avionics literature.<ref name="Code List"> [http://www.airsport-corp.com/modecascii.txt List of altitudes and Gillham codes]</ref>
The code was named in memory of the late Ronald Lionel Gillham, the UK's representative to the IATA committee developing the specification for the second generation of Air Traffic Control System, known in the UK as "Plan Ahead", who died suddenly in March 1968. Mr Gillham was said to have had the idea of using a modified Gray Code while at a family dinner.
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== Altitude Encoder ==
The encoder uses bits designated D1 D2 D4 A1 A2 A4 B1 B2 B4 C1 C2 C4.<ref name="Code Explanation"> [http://www.airsport-corp.com/modec.htm Explanation of Transponder Coding]</ref>
Different classes of altitude encoder do not use all of the available bits. All use the A, B & C bits, increasing limits require more of the D bits.
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Up to and including 126700ft requires D4 & D2.
Note that D1 is never used.
The datum used by altitude encoders is -1200ft although many will not output anything lower than -1000ft.
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Bits C1 - C4 use a non-linear reflected Gray code to store the number of 100ft increments +1. The values when converted to decimal follow this pattern: 1 2 3 4 7 7 4 3 2 1 1 2 3 4 7 ....
The C bits must be converted from Gray to binary, the 7 changed to a 5 and the reflected order changed when the 500ft increment is even.
The number resulting from combining the above gives the flight level above the minimum datum.
The Gillham code can be decoded using various methods. Standard techniques use hardware <ref name="Decoder Patent"> [http://www.freepatentsonline.com/3805041.pdf Circuit to decode Gillham code]</ref> or software solutions. The latter often uses a look up table but an algorithmic approach can be taken.<ref name="Algorithm C code"> [http://www.ccsinfo.com/forum/viewtopic.php?p=77544 Aviation Gray Code, software decoding function in C language]</ref>
==References==
{{Reflist}}
<references />
{{Aviation lists}}
[[Category:Data transmission]]
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