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    Re: Line of Position Fix
    From: Brad Morris
    Date: 2012 Oct 18, 18:52 -0400

    Gentlemen

    I have no wish to hijack Debra's straight forward questions into a discussion of the quality of the celestial fix as it pertains to the cocked hat.

    But...

    Paragraph 2609 does not talk about your fix being inside or outside the hat.  It doesn't speak to the accuracy of that fix, that's only available to us in the GPS age.  The purpose of the multi-body, multi-observation fix is to obtain a sense of its consistency.  If your LOPs seem to cross about or around a common location, then indeed, that's consistent within itself.  Two observations, one of each body, will be less consistent, reproducible and most likely prone to error.

    Best Regards
    Brad

    On Oct 18, 2012 6:42 PM, "Apache Runner" <apacherunner---.com> wrote:
    Not that I have anything useful to add on the cocked hat issue, but my students did triangulation with compasses as an assignment last week.   We had our fair share of funkiness with three LOP's giving three different locations, even when they factored in what they supposed their compass uncertainty was, there were often three non-overlapping regions of uncertainty (or certainty?).   

    We used backpacker-type compasses, and I swear it is next to impossible to do better than +/- 10 degrees with those blamed things.

    On Thu, Oct 18, 2012 at 5:37 PM, Lu Abel <luabel{at}ymail.com> wrote:
    I agree that even on land LOPs are likely to be off by a mile or even more and so fixes have that uncertainty; it's just the nature of celestial navigation.

    But I have to question the claim that getting three LOPs will give a better fix.  As someone pointed out in a discussion on this list a few years ago, there's only a one in eight chance of a fix even being inside the cocked hat and there's no certitude at all that the fix will be in the center of the hat.



    From: Brad Morris <bradley.r.morris---.com>
    To: NavList@fer3.com
    Sent: Thursday, October 18, 2012 12:45 PM
    Subject: [NavList] Re: Line of Position Fix

    Hi Debra
    The best two times of the day are the civil twilights.  Those are just before dawn and just after sunset.  The best bodies to use are the 57 navigational stars.
    Just as a note, you will want 3 bodies in your fix.  It is unlikely that 2 LOPs will cross at your position, in general, and 3 will produced a 'cocked hat'.
    Best Regards
    Brad Morris
    On Oct 18, 2012 3:23 PM, "Debra Hillman" <wombatroo---.com> wrote:
    Could somebody help me with my problem . I am learning on my own at present from books on celestial navigation.I need to get a fix with two lines of position but not sure which celestial body other than the sun and the best times of the day to achieve the fix.

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