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    Re: Ratio of variances
    From: Greg Rudzinski
    Date: 2018 Nov 3, 03:40 -0700

    Bill,

    I do averaging of index error using the horizon , Sun , star, or planet every time the sextant comes out of the box. Averaging of timed observations using a properly adjusted metal sextant is I think extra work if aboard a ship or on a small craft in good weather. If using a plastic sextant then averaging of sights would be advised. Plastic is not as stable or consistent as metal when exposed to temp changes or hand squeezing. Accuracy is improved with all sextants if three bodies are observed with 120 degrees of azimuth separation or four bodies with 90 degrees of azimuth separation. Systematic errors are equal and opposing. If altitudes are roughly the same then arc error is also equal and opposing. The size of the plotted square or triangle is of no consequence. I like sight reduction methods that use the DR/GPS position as an assumed position so that gross mistakes can be caught. Frequent gross mistakes can then be identified and guarded against on subsequent observations. 

    Greg Rudzinski

    From: Bill Lionheart
    Date: 2018 Nov 2, 07:40 -0700

    Can NavList members give a practical example where you might take sights for three or four bodies (average of several sights for each) and suspect that one of them is say half (or some other fraction) as accurate as the others?

    If you shot the moon, and two stars for example which do you think would be most accurate? Or maybe the horizon in one direction was a bit less distinct?

    I am mainly thinking of examples to give in a talk or article where the readers might not be so familiar with Celestial Navigation. I simply dont have the practical experience, but I am guessing if you have done fixes every day on a voyage, especially checking against GPS, you'd get a pretty good idea of the standard deviation?

       
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