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    Re: GPS shortcomings.
    From: Lu Abel
    Date: 2005 Jun 8, 22:05 -0700

    Peter Fogg wrote:
    >>On Behalf Of Lu Abel
    >>We're talking about the sextant being a backup navigation instrument,
    >>not the primary one, so I must ask why you think a Davis Mk 15 or Mk 25
    >>wouldn't be up to the task.
    > Metal is more reliable. Won't warp and is less liable to temperature
    > effects. Nothing new or controversial here.
    Do you speak from experience or just theory about "plastic?"
    My understanding is that Davis's are made out of a form of "engineered"
    plastic that is very strong and has virtually no temperature coefficient
    of expansion.   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.
    Lu Abel

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