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    Re: Problems with AstronavPC
    From: George Huxtable
    Date: 2004 Feb 16, 22:42 +0000

    Aubrey O'Callaghan said, referring to section 11, "Position from intercept
    and azimuth", in the Nautical Almanac"-
    >I wondered if this was the same algorithm as in the Almanac, it was
    >something I was going to check tonight, but now it's confirmed
    >That being said, I remember plugging this formula into a spreadsheet (on a
    >Psion 3A - I was at anchorage by some Caribbean island at the time)
    >I then placed my initial position nowhere near where I was. After about 3
    >iterations it converged to my actual position. I was quite surprised as I
    >had thought that one's initial guess should be approximately where one is
    >(at least within a few degrees). I even tried different hemispheres as
    >starting position.
    There's a bit of extra information about that matter, given in section 7.4
    of the AstronavPC booklet, but not in the Almanac, as follows-
    "It is possible, but not advisable to start the iterations with a position
    that is in a different hemisphere. Provided L(I) is kept in the range
    -180deg to +180deg and B(I) in the range -90deg to +90deg the solution in
    most cases will begin to converge after a few iterations."
    That seems pretty all-embracing, doesn't it? But it is a bit odd, because
    L(I) and B(I) are the improved values, not the first values which are L(F)
    and B(F). I think the statement above could only make sense if it was
    intended to apply to L(F) and B(F).
    By the way, I hope it's obvious that where I have written L(F) is where the
    original document had L with a subscript F, and so on.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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