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    Re: Exercise #14 Multi-Moon LOP's
    From: Jeremy C
    Date: 2009 May 1, 12:45 -0700

    George wrote:
    1. As we've seen before, when looking at scatter, a single result tells us
    almost nothing. It may "hit the spot" just by chance. Consistency is what's
    2. We don't know in detail how that program handled the data it was given.
    Jeremy has told us it was fed with a GPS position as DR, with known course
    and speed, and he "  plugged each moonline ... as individual moon LOP's.".
    Without understanding what went on in detail, let me make a guess. Given an
    individual position line, and a DR position, what could a computer make of
    it? It might well do its best, by finding the most probable spot on that
    LOP, which would be the nearest point on that line to the DR position. In
    which case, it would inevitably end up with a position very close to that
    DR. And then, it would do the same for all the other position lines, and
    somehow average the result. End result, a position close to the DR! I've
    suggested to Jeremy that next time he tries such an exercise, he plugs in
    deliberately-different trial-positions for his DR, to see if the answer
    changes. I wonder, if he had offered the computer just one "moonline", what
    would the result have been then?
    Now, I don't know how that program operates, internally, and I doubt whether
    Jeremy or Frank know either. It seems worthwhile, then, to ask some probing
    questions about the value of such an exercise, before building on to such an
    insecure foundation, as Frank has done. Healthy scepticism is called for,
    here as elsewhere.
    To the first point, I will make this a priority experiment on my next trip which should
    offer excellent oppurtunities to shoot rapid fire fix data on a variety of bodies and at
    a variety of latitude in both the northern and southern hemispheres.  I should also have
    a variety of courses both north/south and east/west. I suspect that the moon will be the
    best body as the azimuth changes so rapidly, but we will see what the data shows once I
    shoot them.
    To the second, since I don't have any new data, I just re-crunched the old numbers.  I
    took the original 11 moon lines and skewed the DR position from the accurate 1900 LT GPS
    fix and made it to the nearest whole latitude and longitude to see how the program worked
    the lines.  It did as I suspected, gave me greater intercepts and different azimuths.  In
    any case, I have attached a PDF sheet of the new work by SkyMate Pro and yes, it did give
     the exact same fix as my original GPS fix based DR.
    As to what happens when I plug in just one moon LOP, well not much, it draws the LOP,
    and doesn't even attempt to give a fix or EP.  When I put the first and last lines into
    play, the program gives me a fix of 14 deg 18.0'N and 143 deg-00.7'E which isn't aweful,
    but certainly shows a far greater error than when all of the lines are calculated.
    I think that George may be barking up the wrong tree in a way.  Far more important than
    the DR position used in this multi-shot sight of a single body over time, is the accuracy of
    the course and speed used to advance/retard the lines.  This will grow far more important as
     the time spread between the first and last shots increasing.
    The attached file shows the original solution on page 1, the solution with the 
    DR changed, and lastly, when the SMG and CMG were changed slightly.
    Navigation List archive: www.fer3.com/arc
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