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    Re: Accuracy of position (sextant error simulation)
    From: George Huxtable
    Date: 1999 Oct 21, 2:00 PM

    This is a response too a posting made earlier today by Jim Manzari.
    His analysis illustrates the dangers in using statistics to draw
    conclusions from unrealistic and incorrect assumptions.
    Let's deal first which what, according to Jim, is the "largest single
    error, which dominates all others", and can give rise to an altitude error,
    in one case, of -34 arc minutes. This depends entirely on what Jim has
    arbitrarily chosen as a reasonable scatter for his angle phi, the tilt of
    the sextant from the vertical. In that worst case of -34 minutes, my
    estimate of the tilt that produced it is about 8 degrees from the vertical
    (it depends slightly on what the actual altitude was for that measurement).
    No wonder tilt dominates all other errors! What navigator would get a
    sighted body to kiss the horizon, and then end up with an altitude
    corresponding to the value he would have got with 8 degrees tilt, with
    enough error to put the upper limb of the Sun where the lower limb ought to
    be? The notion is absurd. I ask Jim to state how he has defined the scatter
    of sextant tilt, and on what basis he has chosen that scatter. Is it
    Another factor Jim has considered is sextant index error. This plays no
    part whatsoever in the altitude error as long as the index error has been
    checked and allowed-for (the easiest thing in the world to do). I
    interleave index checks between each sextant sight, for my plastic sextant,
    though it's not necessary to go that far. The index-error contribution to
    altitude error is ZERO.
    Jim has considered refraction of the light from the body, and correctly
    found that at angles above 30 degrees, variations are negligible. But he
    has ignored the unpredictable changes in the dip caused by refraction in
    the light path from the horizon, an important factor if not THE most
    important factor. See my mailings on this topic earlier today.
    I could go on, but I think I've said enough.
    George Huxtable.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.

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