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    Re: Making an artificial horizon, and leveling thereof
    From: George Huxtable
    Date: 2011 Jan 22, 11:55 -0000

    Alan wrote-
    "On displacement of calculated fix from GPS coordinates, many under 3 NM, a
    few around 1 NM, with a couple under 1 NM. Of course, there were some "wild
    hairs" which I attribute to "operator error", yours truly being the
    I doubt whether his results are affected at all by the two possible reasons
    he suggested; twist of the shield assembly and distance of the observer
    from it. But if he is seeing discrepancies of around 3 miles, something
    seems wrong. That would involve an error in sextant reading of around 6',
    because that reading is halved when using a reflection. Indeed, use of an
    artificial horizon should be a procedure of great accuracy. Not only is
    there that factor of 2 in the sextant reading, but also the dip of the
    horizon, which is usually the biggest unknown in altitude navigation, is no
    longer involved.
    Some study of those worse results, rather than his ones that show the least
    discrepancy, may be productive. I suggest that Alan posts to the list an
    example or two of such reflected Sun altitudes, those that show
    discrepancies of a few arc-minutes (= miles), providing the following
    Date and time (to the second), sextant reading, index error, which limb
    used, and observer's position, according to GPS or map. If temperatures and
    pressures were recorded, so much the better, but unless those values were
    extreme, or the Sun was low, they will have little effect.
    List members may then be able to help in tracking down the origin of any
    contact George Huxtable, at  george{at}hux.me.uk
     or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    You also asked , "And why do you need to stand back to see the two views?"
    I really don't know, just that it seemed that there was an optimum distance
    from which I could see both the reflected and sextant suns, this seeming to
    be back from the AH, rather than closer to it.
    As to alignment, I'm just going on the basis of the instruction sheet that
    came with the thing, which stated that the ah should be oritned so that
    there was no shadow at either side.
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