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    Re: GPS shortcomings.
    From: Frank Reed CT
    Date: 2005 Jun 11, 03:08 EDT

    Lu Abel, you wrote:
    " I'll agree "metal is  more reliable," but by how much?  I
    have co-taught USPS's celestial  courses for many years and about 90% of
    our students have used Davis  sextants.  In years of checking sights I've
    never run across one that  was off because the sextant was off -- any
    errors have always been traced to  student error.  My own Davis Mark 15
    is almost two decades old and has  never given me a problem."
    So how do they compare? Can you do some tests?  Try this: get out your Davis
    plastic sextant and any decent metal sextant. Take  a couple of dozen sun
    sights alternating between the two instruments and compare  the results. When I
    have done this, I generally find a scatter of about two to  three minutes of arc
    around correct, calculated values with a Davis Mark 15  plastic sextant and
    about 0.5 minutes of arc with a metal sextant. If  few  minutes of arc error
    doesn't bother you, then a plastic sextant is just fine. I  own one plastic
    sextant and three metal sextants (though two of these are  temporary investments).
    42.0N 87.7W, or 41.4N  72.1W.

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