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    Re: Accuracy of sextant observations at sea
    From: Jeremy C
    Date: 2010 Sep 23, 14:56 EDT
    Hi Gary,
     
    I'll quote myself here [NavList 9887]:
     
    On Sep 23, 2009 3:40 pm, Anabasi...---com wrote:
    > I shot 70 star fixes on my trip from Japan to the USA and here are a few  
    > interesting statistics.
    >
    > 1) average fix error from GPS: 0.566 nm  (low of 0.0 and high of 2.7  nm
    > error)
    >
    > 2) 88.6% of the fixes were under 1.0 nm fix error.
    >
    > 3) 60.0% of the fixes were under 0.5 nm fix error
    >
    > 4) two fixes had errors under 0.05 nm (under 100 meters).
    >
    > My next project is to determine the average intercept based on the GPS fix  
    > as the DR but that is over 700 data points so might be awhile.
    >
    > Jeremy

     
    As you can see, these are position errors, not LOP errors.  I also ran standard Dev for my fixes and it was 0.5813 nm.  I will add that my worst fix was 2.7 nm off under awful conditions.
     
    The trouble I have with LOP statistics is that there are so many variables at play as render them only interesting but not very useful.  With a good sky and horizon I can pinwheel every time (intercepts well under 1 nm), if the horizon and/or sky is terrible, I can have errors of several miles.  Also, can someone expect the aforementioned statistics in a bouncing sailboat as opposed to a large ship? Probably not. 
     
    I just ran the statistics for the individual LOPs, 551 in all.   These were shot in a wide variety of conditions including the haze of the East China Sea, near Cape Hope, and in the tropics.  There is a wide variation of error from day to day and twilight to twilight depending on the environment. 
     
    I only ran them on the computer reduced sights as these use GPS fixes for the AP as opposed to my tabular reductions. (planets, the moon, and stars)  I didn't use the computer for most of my sunlines, but there are a few included in there.
     
    Standard deviation for all 551 sights was .800225.
     
    I did shoot 1 moonlight fix.  The position error was about 2.5 nm which would be good enough in a pinch, but well below my standard.
     
    Jeremy
       
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