Gillham code: Difference between revisions

Revision as of 13:06, 18 May 2020

Revision as of 13:13, 18 May 2020

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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="Darryl_1998_ASCII"/><ref name="Stewart_2010"/><ref name="Gray_1953"/><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="Datex_1965"/><ref name="Dokter_1973"/> (with the first 5 states resembling [[O'Brien code type II]]<ref name="O'Brien_1956Brien_1955"/><ref name="Ashley_1961"/><ref name="Steinbuch_1962"/><ref name="Steinbuch-Weber_1974"/><ref name="Dokter_1973"/>) to encode the offset from the 500&nbsp;ft altitude in 100&nbsp;ft increments.<ref name="Darryl_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="Darryl_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 -->

The Gillham code can be decoded using various methods. Standard techniques use hardware<ref name="Decoder Patent"/> or software solutions. The latter often uses a lookup table but an algorithmic approach can be taken.<ref name="Stewart_2010"/>

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<ref name="Steinbuch_1962">{{cite book |title=Taschenbuch der Nachrichtenverarbeitung |language=German |editor-first=Karl W. |editor-last=Steinbuch |editor-link=Karl W. Steinbuch |date=1962 |edition=1 |publisher=[[Springer-Verlag OHG]] |___location=Karlsruhe, Germany |publication-place=Berlin / Göttingen / New York |lccn=62-14511 |pages=71–74}}</ref>

<ref name="Steinbuch-Weber_1974">{{cite book |title=Taschenbuch der Informatik – Band II – Struktur und Programmierung von EDV-Systemen |language=German |editor-first1=Karl W. |editor-last1=Steinbuch |editor-link1=Karl W. Steinbuch |editor-first2=Wolfgang |editor-last2=Weber |editor-first3=Traute |editor-last3=Heinemann |date=1974 |orig-year=1967 |edition=3 |volume=2 |work=Taschenbuch der Nachrichtenverarbeitung |publisher=[[Springer Verlag]] |___location=Berlin, Germany |isbn=3-540-06241-6 |lccn=73-80607 |pages=98–100}}</ref>

<ref name="O'Brien_1956Brien_1955">{{cite journal |author-first=Joseph A. |author-last=O'Brien |title=Cyclic Decimal Codes for Analogue to Digital Converters |journal=[[Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics]] |___location=Bell Telephone Laboratories, Whippany, New Jersey, USA |volume=75 |issue=2 |date=May 1956 |orig-year=1955-11-15, 1955-06-23 |issn=0097-2452 |doi=10.1109/TCE.1956.6372498 |id=Paper 56-21 |pages=120–122 |url=https://pdfslide.net/documents/cyclic-decimal-codes-for-analogue-to-digital-converters.html |access-date=2020-05-18 |url-status=live}} (3 pages) (NB. This paper was prepared for presentation at the AIEE Winter General Meeting, New York, USA, 1955-01-30 to 1955-02-03.)</ref>

<ref name="FAA_1962">{{cite book |title=Final Engineering Report on Evaluation of L-band Secondary Radar. For ANDB under CAA. |author=((Airborne Instruments Laboratory, a division of [[Cutler-Hammer, Inc.]])) |publisher=[[Federal Aviation Administration]] (FAA), Aviation Research And Development Service |type=Report |id=Report 8893-SP-1 |date=1962-05-19 |___location=Deer Park, Long Island, New York, USA |url= |access-date= |url-status=live |archive-url= |archive-date=}}</ref>

<ref name="FAA_1962_T6">{{cite book |title=Height Code Tables For Use With Air Traffic Control Radar Beacon System |author=((Airborne Instruments Laboratory, a division of [[Cutler-Hammer, Inc.]])) |publisher=[[Federal Aviation Administration]] (FAA), Aviation Research And Development Service |type=Report |id=Report 8893-SP-1. Contract FAA/BRD-329. Task 6 |date=May 1962 |___location=Deer Park, Long Island, New York, USA |url=https://apps.dtic.mil/dtic/tr/fulltext/u2/615818.pdf |access-date=2020-05-17 |url-status=live |archive-url=https://web.archive.org/web/20200517142356/https://apps.dtic.mil/dtic/tr/fulltext/u2/615818.pdf |archive-date=2020-05-17}} (43 pages)</ref>