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    Re: accuracy of automatic celestial navigation
    From: Dan Allen
    Date: 2002 Dec 6, 22:18 -0800

    On Friday, December 6, 2002, at 04:05 PM, George Huxtable wrote:
    
    > For those of us that sail our small craft out at sea, my opinion is
    > that if we can achieve a precision of 2 min, we are doing pretty well.
    > What do others think?
    
    I would agree.  2 nmi at sea is very decent; on land we should be able
    to get to 1/2 mile or better.
    
    On land I recently took a string of shots using a Tamaya Jupiter
    sextant with a 7X scope.  I shot the sun mid afternoon over a period of
    20 minutes.  (This was on the coast of Oregon at a beachhouse on the
    water.)  I used a Garmin GPS in averaging mode to determine my actual
    position to 15 feet or so.  The resulting intercepts in nmi are
    interesting to analyze.  Because my assumed position is known to within
    15 feet, these can be taken as errors in my sights.  Here they are in
    order:
    
    0.91
    -0.06
    0.59
    -0.05
    0.22
    0.55
    0.22
    -0.28
    -0.53
    -0.57
    -0.77
    -0.03
    -0.19
    -0.11
    -0.43
    -0.62
    0.27
    0.24
    0.44
    0.25
    
    The mean of the intercepts is a mere 0.0025 nmi -- very good -- but the
    individual readings varied over a 1.68 nmi range -- not so good.
    
    When we are "warming up" and we haven't used our sextants in a while I
    find that the first shots are often in greater error than later sights.
      This is in fact the case with this set of shots shown above: the worst
    error came from the very first sight.  Therefore I have found that it
    is wise throw out the the first reading or two, or at least to discount
    them.
    
    Other experiments I have done:
    
    I took five shots in my backyard using the same sextant and an
    artificial horizon and got a 1.27 nmi range of errors and a mean error
    of 0.644 nmi.
    
    I took fourteen shots in my backyard using a Tamaya Venus (not nearly
    as powerful of scope) and an artificial horizon and got a 4.3 nmi range
    of errors but a mean error of -0.45 nmi because of the greater number
    of sights taken.
    
    And for my finale, one afternoon I used six different sextants in my
    backyard and I took a total of 46 sights using an artificial horizon.
    I got an 11.2 nmi range of errors with a mean error of
    -1.23 nmi.
    
     From this the statistical nature is shown to be very useful: although
    any one shot could be in error up to almost a mile, a series of shots
    averaged does indeed improve accuracy to very good levels; STELLA
    appears to do this, and thus an automated system is very appealing.
    
    Dan
    
    
    

       
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