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→Decoding: removed gratituitous requests for citation of obvious facts. The "Gray Code" page is already cited. |
This is hardly gratuitous, this data is uncited and has been for four years. If the refs are obvious then please add them or else as per WP:V this needs to removed after being unreferenced for so long. |
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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. This is suitable for most light general aviation aircraft. Up to and including 62700 ft requires D4. Up to and including 126700 ft requires D4 and D2. Note that D1 is never used.<ref name="Code List" />
The datum used by altitude encoders is −1200 ft although many will not output anything lower than −1000 ft. Negative [[flight level]]s are included in the code to permit altitude measurement at low levels when the ambient pressure is high.{{Citation needed|date=December 2010}}
Note that the altitude code output by a standard altitude encoder is a pressure altitude. That is to say, it is always with respect to a pressure datum of 1013.2 mBar (hectopascals) or 29.92 inHg. It does not indicate the height above sea level (altitude) or the ground (height). Pressure altitudes are referred to as flight levels and are expressed to the nearest 100 ft. For clarity here is a sample of the Gillham code from 0 ft to 1000 ft; note that 1000 ft is equivalent to a flight level of 10.
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== Decoding ==
The Gillham code is an unusual mix of codes. It is a parallel binary code that uses a Gray code to ensure that there are not multiple bit changes between adjacent altitudes. The bit pattern is split into those bits used to indicate the number of 500 ft increments and those used to indicate the number of 100 ft increments. The split is as follows.{{Citation needed|date=December 2010}}
Bits D1–B4 use a standard Gray code to store the number of 500 ft increments.<ref name="Code Explanation" />
Bits C1–C4 use a non-linear reflected Gray code to store the number of 100 ft increments +1. The values when converted to decimal follow this repeating pattern: 1 2 3 4 7 7 4 3 2 1 1 2 3 4 7 ....{{Citation needed|date=December 2010}}
The C bits must be converted from Gray to standard binary, the 7 changed to a 5 and the reflected order changed when the 500 ft increment is even.{{Citation needed|date=December 2010}}
The number resulting from combining the above gives the flight level above the minimum datum (−1200 ft). This offset must be removed to give a corrected altitude value.{{Citation needed|date=December 2010}}
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 lookup 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>
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