Gillham code: Difference between revisions

Revision as of 14:25, 9 October 2013

Revision as of 00:08, 20 February 2014

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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&nbsp;ft does not require any of the D bits. This is suitable for most light general aviation aircraft. Up to and including 62700&nbsp;ft requires D4. Up to and including 126700&nbsp;ft requires D4 and D2. Note that D1 is never used.<ref name="Code List" />

The datum used by altitude encoders is −1200&nbsp;ft although many will not output anything lower than −1000&nbsp;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&nbsp;mBar (hectopascals) or 29.92&nbsp;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&nbsp;ft. For clarity here is a sample of the Gillham code from 0&nbsp;ft to 1000&nbsp;ft; note that 1000&nbsp;ft is equivalent to a flight level of 10.