Gillham code: Difference between revisions

Revision as of 21:14, 7 December 2010

Revision as of 21:29, 7 December 2010

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Light aircraft electrical systems are typically 12V or 28V. 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.<ref name="Code Explanation"> [http://www.airsport-corp.com/modec.htm Explanation of Transponder Coding]</ref> The Gillham code contains a D1 bit but this is unused in practical applications.

Different classes of altitude encoder do not use all of the available bits. All use the A, B & C bits, increasing altitude limits require more of the D bits. Up to and including 30700ft does not require any of the D bits. This is suitable for most light general aviation aircraft. Up to and including 62700ft requires D4. Up to and including 126700ft requires D4 & D2. Note that D1 is never used.{{Citation<ref needed|datename=December"Code 2010}}List"></ref>

The datum used by altitude encoders is -1200ft although many will not output anything lower than -1000ft. Negative flight levels 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 1013.2mBar (Hectopascals) or 29.92inHg. 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 100ft. For clarity here is a sample of the Gillham code from 0ft to 1000ft, note that 1000ft is equivalent to a [[flight level]] of 10.{{Citation needed|date=December 2010}}

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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 500ft increments and those used to indicate the number of 100ft increments. The split is as follows.{{Citation needed|date=December 2010}}

Bits D1 - B4 use a standard Gray code to store the number of 500ft increments.{{Citation<ref needed|datename=December"Code 2010}}Explanation"></ref>

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 repeating pattern: 1 2 3 4 7 7 4 3 2 1 1 2 3 4 7 ....{{Citation needed|date=December 2010}}