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    Re: In Honor Of Jeremy Two Stars Full Lan's : One Upper And One Lower Culmination
    From: Antoine Cou�tte
    Date: 2009 Sep 20, 23:27 -0700

    Sep 21, 2009
    RE NavList 9823
    Dear Peter,
    Thank you for your replies to both submitted Star LAN's. I can see that you 
    are a very active and quick meber of NavList.
    Just a few explanations :
    I did mention not only Ship's course and and speed ( as read from the Loch and 
    (true) gyrocompass - BUT ALSO sea current speed and direction, which means 
    that in order to get Speed and Course made good, i.e. on the Earth bottom, 
    you need to "vector-add" both vessel's speed/direction and sea current 
    speed/direction. This is important, and especially for the South Atlantic 
    example, when we were experiencing a 2.5 kt current over a quite significant 
    period of time.
    As regards Observation # 7 in the Mediterranean example, if you use Marc Saint 
    Hilaire's method, then you can fully process it, as it is a good and valid 
    observation. On the other hand, if you process Local Apparent Noons through 
    dedicated specific Programs, such as the one used by Andr?s Ruiz (see NavList 
    9791), you need to be fully aware of such software limitations. Most 
    generally they cannot adequately accomodate observations too far away 
    (timewise) from local apparent noon time. And in the Med example, because 
    DENEB culminates so high, and accordingly varies so fast "height-wise", a 
    time second degree algorithm/approximation - as apparently And?s might be 
    using - might not be "powerful" enough to adequately deal with such 
    conditions (culmination so close to zenit) while in most cases (if not all) 
    the Marc Saint Hilaire Method can still be used with rather safely. Its only 
    real limitation is when all azimut are too close (+/- 180?) .... and I 
    appreciate that we are here on "the borderline" in both examples, but if we 
    have nothing else, better use it while being aware of its limitations.
    Specific algorims limitations is one of the topics I wish to address in an 
    upcoming message to NavList dealing with the "pro's" ,and "con's" about 
    LAN's, or "LAM's" as Jeremy has them ... since it may not be sufficiently 
    covered elsewhere, at least to the best of my knowledge (and I apologize to 
    who ever has already adressed it in depth).
    Such document will require some careful preparation from my side, so I need 
    time and will publish it here if this topic seems of some interest to our 
    NavList community.
    Now, and as regards your results - the reply you have been longing/looking for 
    ... - your Med result is fairly close from what I got (then independently 
    crossed check by TRANSIT fixes), but for the South Atalntic results, it seems 
    to me that your Longitude results could be significantly improved. ... unless 
    I might have typed erroneous values, in spite of the great care I took in 
    proof reading this message yesterday. I will take some further time as soon 
    as I can in order to verify the data I published on both examples.
    And by the way, what method did you use to work both LAN's ? If you rework 
    both examples from your published results as DR positions and go with the 
    Azimut/Intercept method (i.e. the so-called Marc Saint Hilaire), do you still 
    get the very same final fixes ?
    Thank you again for your interest. I will do my best to cross check once more 
    the data I published yesterday in this LAN dual example, and get back to you 
    afterwards. You certainly deserve, at the least, that I should not get your 
    time and attention with uncorrectly copied/reproduced typed data.
    Until then, please accept my Very Best Regards
    Good Luck and enjoy ...
    Antoine M. "Kermit" Couette
    NavList message boards: www.fer3.com/arc
    Or post by email to: NavList@fer3.com
    To unsubscribe, email NavList-unsubscribe@fer3.com

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