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    Celestial accuracy
    From: Paul Hirose
    Date: 1999 Oct 30, 12:31 AM

    I've been off the air for several days and have lost a number of
    messages in the "celestial accuracy" thread, but I have a few items to
    toss in.  Apologies if any of this is redundant.
    
    
    The best celestial nav I've ever seen was performed by a robot, in the
    form of an astro-inertial navigation system.  Looking through a flat
    porthole in the top of the aircraft, this device located and tracked
    several stars per minute from a catalog of 61 stars.  (I never did see
    a list of the stars.)  This was not an experimental device; it's in
    service today.  It could see the stars even in bright sunlight!
    Performance is still classified as far as I know, but I will say that
    while flying at jet aircraft speed it could beat an expert human
    standing on solid ground with a sharp horizon and a fine German
    sextant.
    
    Dan Allen - the .108' error from your mean of 5 observations was
    probably good luck.  The standard deviation of your intercepts is
    2.40'.  My understanding is that taking the mean of n observations
    will reduce random error by a factor of square root of n.  So I'd
    expect the standard error of your mean to be 2.40'/2.24, or 1.07'.  (I
    realize I'm walking on thin ice, analyzing only 5 observations.)
    
    Did you notice that correcting your refraction to actual conditions
    made the result worse?
    
    
    George Huxtable - a statistical study of dip error has been done.
    From the 1984 Bowditch Vol 1, chapter on sextant altitude corrections:
    
    "An investigation by the Carnegie Institution of Washington showed
    that of 5,000 measurements of dip at sea, no value differed from the
    tabulated value by more than 2.5', except for one difference of 10.6'.
    Extreme values of more than 30' have been reported and even values of
    several DEGREES have been encountered in polar regions."
    
    Unfortunately, I have no other details.  I wonder how one goes about
    measuring dip at sea.  The only method that occurs to me is to set up
    a theodolite on shore, oil rig or other steady platform and measure
    the sea horizon with respect to the spirit bubble.
    

       
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