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    Re: AP terminology
    From: Peter Hakel
    Date: 2009 Nov 17, 11:35 -0800
    It looks like that one-body fix problem again.  John Karl and I gave formal solutions to this a few months ago.  I don't know of any historical reference actually giving such a solution.  That is certainly because the method is rather impractical, so it is never used and described.  Your mention of radar, Jeremy, led me to look into the "Fix by Range and Bearing to One Object" in Bowditch.  The azimuth measurement accuracy is stated to be 5 degrees.  This makes me believe that this is really only useful across short distances (you mention pilotage waters).  Bowditch gives no equations how to do this; I assume it's because for such short ranges you can just plot directly without getting into the celestial triangle.

    If you were to do such a one-star fix, Jeremy, how would you do it and where does your method come from?  I am still curious about this and appreciate any info on the subject.


    Peter Hakel



    From: "Anabasis75---.com" <Anabasis75---.com>
    To: navlist@fer3.com
    Sent: Tue, November 17, 2009 9:04:51 AM
    Subject: Re: [NavList 10725] Re: AP terminology, WAS: 2-Body Fix -- take three

     
    In addition to twilight, another piece of data might be provided by the navigator if there was sufficient manpower to do this in the case where no DR can be provided.  In this particular case, only ONE star is needed to get an approximate position.  This of course is an altitude AND azimuth.  This is the same as using radar off of a single object for a position in pilotage waters and offers the same limitation.  That is to say that in both the case of Radar and Celestial, the observation of azimuth/bearing is not nearly as accurate as altitude/range.  however if no DR is available, you can get a rough position (dare I call it an EP?) with just this data.  If we have two stars and even if a single azimuth determination is made, we can easily determine which of the two fixes is the correct one to choose.
     
    The trouble with all of this, and the reason it was not popular, is that you can't easily do this with the mechanical tools available at sea.  Sure a computer can do it all, as I said before, and Andres has demonstrated; but it's not very easy to do with plotting tools and the charts that we have.   I don't have a compass on my ship that can stretch that far on any chart so doing circles of equal altitude isn't an option except in very rare cases in the tropics (less than 2.5 degrees of Zenith distance).
     
    [parts deleted by PH]

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