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    Re: Relative plotting vs Geographical plotting
    From: George Huxtable
    Date: 2002 Jan 12, 1:38 AM

    Peter Smith wrote-
    >George Huxtable [mailto:george@HUXTABLE.U-NET.COM] said:
    >> I don't think Peter Smith's solution can possibly be correct: commonsense
    >> tells me so. We have two ships travelling in nearly the same direction,
    >> the second ship is slowly overtaking the first. In that case, the speed of
    >> the overtaking ship must be greater than that of the ship being overtaken.
    >Hmmm. I'm not sure from the above who's the "second ship" and who's
    >the "first ship". The target is ahead of us and to Port (bearing 322d).
    >The bearing is constant, so we are converging. Since the target is
    >ahead and the range is decreasing, we must be overtaking the target.
    >> ...
    >> If you add the vectors 12 knots at 150 and 1.45 knots at 142, using trig
    >> drawing, you end up with 13.44 knots at 149.1.
    >But that gives the target a greater speed than our own ship, even
    >though we are overtaking it.
    >Moreover, the target is on our Port bow (bearing 322d). For our
    >courses to converge (constant bearing), the target's course must
    >be a little to starboard (i.e., numerically GREATER) of ours. Thus,
    >if we're making 150d, he should be making a little greater then 150d.
    >If he's making 149.1d, we would be diverging.
    >Can you read an Excel spreadsheet? I have both the geographic and
    >relative vectors problems worked out trigonometrically. I'll send
    >it to you and see what you think.
    > -- Peter
    George Huxtable replies-
    Aha! The penny drops... Now I see why we disagree. I didn't read Pierre
    Boucher's problem fully. Here is what he said-
    >Own course and speed:
    >150d   12.0 kt
    >On the radar screen (relative bearing mode):
    >at 0448 a "spot" at 7.0 nm  322d   on the screen
    >at 0554 a "spot" at 5.4 nm  322d   on the screen
    >Request :
    >Other ship's course
    >Other ship's speed
    Pierre states quite clearly "relative bearing mode". For some reason, I
    took it as North-up mode instead. It makes all the difference, of course. I
    was quite wrong. Sorry about that.
    When I repeat the calculation, summing vectors, I now get exactly the same
    result as Peter Smith, 10.9 knots at 154.7 degrees.
    Thanks for putting me right, Peter.
    George Huxtable.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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