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    Re: Fix Maximum Probability Positions
    From: Gary LaPook
    Date: 2013 Mar 21, 10:37 -0700
    John, another way to describe your method is that you throw out the first LOP and keep only the first EP. Then you start a DR plot from the EP and then establish a second EP where the DR plot hits the second LOP. I have a problem with that. Traditionally you only start a new DR plot from a fix, not from a position of lesser, and unknown, accuracy. Although the first EP may be the most accurate estimate of your position at that time there is no reason to believe that it has the accuracy of a fix, and any inaccuracy or uncertainty in the first EP is carried forward with the DR plot, and degrading further due to normal uncertainty in DR, 10% of the distance covered is the accepted uncertainty in flight navigation though it is  probably less for surface navigation. So I'll stick with the traditional method of working a running fix.

    gl

    --- On Wed, 3/20/13, John Karl <jhkarl---.net> wrote:

    From: John Karl <jhkarl---.net>
    Subject: [NavList] Re: Fix Maximum Probability Positions
    To: garylapook---.net
    Date: Wednesday, March 20, 2013, 6:59 PM


    David,

    Let me clarify: First, there is no intercept involved, we're talking about any LOPs, not necessarily ones from St. Hilaire sight reductions. Second, it's not a special case. The position on LOP1 is determined both from that LOP (from whatever source) and along it from the other positional information (which I pointed out is always available, if nothing else from previous DR work). It's called an EP (estimated position) because it's partially determined by less accurate data, such as DR.

    Randall,

    I'm afraid you've missed the estimation concept. I used no math, only the criterion that we use all available information, make no unwarranted assumptions, and make no contradictions.

    In the plots there is a contradiction between DR2 and the traditional fix located at RFIX. Otherwise they'd be at the same location. The estimation question is how best to deal with this contradictory information. Now if your DR across oceans is better than LOPs, you can ignore this discussion and leave your GPS ashore.

    Regards,
    John
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