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    Re: H.O. 249, Vol. 1 as a guide for star selection for direct position computation?
    From: Greg Rudzinski
    Date: 2017 Oct 3, 12:16 -0700


    The CN navigator needs knowledge of azimuths to choose optimal stars with the best case being three bodies each differing by 120° in azimuth. The purpose of this is to cancel out systematic errors. The next best choice would be two bodies seperated by 90° of azimuth. This will not cancel out systematic errors but will make the error ellipse a perfect circle. The best tool for selecting stars  would be a star finder. Using a star finder requires knowledge of approximate latitude to select the right template. What you can do is choose three stars that are visible to the navigator but not optimal to calculate an approximate position then use USNO,Pub 249 or a star finder to select stars.

    Greg Rudzinski

    From: Tony Oz
    Date: 2017 Oct 3, 02:12 -0700

    Dear Greg,

    I am not sure (yet?) that a star pair optimal for St.Hilair intercept method (i.e. the one suggested by vol.I H.O.249) is also optimal for the direct position computation. Looking at the picture (attached, courtesy John Karl) I'm inclined to believe that a star pair optimal for the direct position computation should represent a triangle (star1-Pole-star2) such that observer occurs in the very middle of that triangle. Whether this condition coincides with a near-90° difference of the stars' azimuths - I do not know. That's why I ask the initial question.



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