Gillham code: Difference between revisions

Revision as of 11:16, 5 August 2020

Revision as of 11:17, 5 August 2020

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== Decoding ==

Bits D2 (msbit) through B4 (lsbit) encode the pressure altitude in 500&nbsp;ft increments (above a base altitude of −1000±250&nbsp;ft) in a standard 8-bit [[reflected binary code]] (Gray code).<ref name="Phillips_1998_ASCII"/><ref name="Stewart_2010"/><ref name="Gray_1953Gray_1947"/><ref name="Steinbuch_1962"/><ref name="Steinbuch-Weber_1974"/> The specification stops at code 1000000 (126500±250&nbsp;ft), above which D1 would be needed as a most significant bit.

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="Spaulding_1954"/><ref name="Spaulding_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&nbsp;ft altitude in 100&nbsp;ft increments.<ref name="Phillips_1998_MODEC"/> Specifically, if the parity of the 500&nbsp;ft code is even then codes 001, 011, 010, 110 and 100 encode &minus;200, &minus;100, 0, +100 and +200&nbsp;ft relative to the 500&nbsp;ft altitude. If the parity is odd, the assignments are reversed.<ref name="Phillips_1998_ASCII"/><ref name="Stewart_2010"/> Codes 000, 101 and 111 are not used.<ref name="Decoder Patent"/>{{Rp|13(6.17–21)}}<!-- p. 13, Column 6, lines 17 through 21 -->