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    Re: Exercise #14 Multi-Moon LOP's
    From: George Huxtable
    Date: 2009 May 2, 15:41 +0100

    Thanks to Jeremy for removing my worries about the operation of his
    SkyMatePro navigation program, in the circumstances in which it was being
    used.
    
    He has kindly re-analysed the same data, taking a different DR as the
    starting position, and found that the program homes in on exactly the same
    deduced position as before, just as it should. So he has clearly shown that
    my suspicions about the way it was analysing that data were quite
    groundless. It was doing its job properly, in rather difficult
    circumstances.
    
    And given a single observation, with a DR, and nothing else, it refuses to
    deduce a new position. Again, just as it should. Can't be faulted.
    
    So that disposes of the doubts I put forward, in posting [8085], under
    comment 2. Which leaves comment 1, as follows-
    
    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
    needed.
    
    ==================================
    Jeremy helpfully offered this-
    
    "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."
    
    Good. Yes please, Jeremy; the more, the better. Nothing special about the
    Moon, though, to make its azimuth change much faster than other bodies that
    get as high in the sky; feel free to choose anything. Frank's proposed
    procedure hasn't (yet) had limits attached, specifying when it applies, and
    when it doesn't.
    
    To be fair to Jeremy, he has claimed nothing more for this observation (data
    listed in [5416]), than his comment, in [6066], that- "As we can see, it is
    fairly accurate, certainly accurate enough for a deep sea position." It is
    Frank's use of this result, in [8049], as the (only) numerical example
    quoted in support of his proposed procedure, that endows it with extra
    significance. It isn't Jeremy's claims that need testing, but Frank's, once
    they have been stated quantitatively.
    
    =================================
    
    Jeremy ended-
    
    "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. "
    
    Well, I was indeed barking up the wrong tree in suspecting his analysis
    software. The DR position should have no effect on the result, and it was
    pleasing to learn that was indeed the case.
    
    It's certainly true that allowing for course and speed is always important,
    but that doesn't become a bigger (relative) problem when observations are
    made that are extended in time. As you increase the time interval, the shift
    of the vessel, over that time, increases in proportion, and so also does the
    shift in the position of the body that's being observed. As both change in a
    similar way, the speed allowance gets no more important over long intervals,
    and isn't minimised by choosing short ones. No barking, in that case.
    
    George.
    
    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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