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    Re: Compact Data for Navigation and Astronomy
    From: JC Sutherland
    Date: 1999 Sep 09, 1:59 PM

    Hello Andres,
    No, I am not using the program in The 'Compact Data for Navigation and
    Astronomy'.
    However, I would recommend this book as a very good alternative to the
    'Nautical Almanac'. It is cheaper and the data covers five years. Also
    the program and data on the CD that comes with it when loaded into a PC
    will provide a complete Astro sight reduction package. It will evaluate
    a position fix, obtained from combinations of sights, using any of the
    navigational bodies and give a statistically calculated Ellipse of
    Probability for the complete fix. Unfortunately the program on the disk
    cannot be used by a calculator and for this purpose it is necessary to
    use the data printed in the book instead. The book gives all the
    formulae required and the data is easy to use.
    
    
     A yacht navigator would not wish to rely on a single sextant
    observation of the Sun or other body at sea as this would give no
    indication of its reliability.
    By taking several sights through the sextant one after the other within
    a few minutes and by plotting the trend on graph paper, the linearity of
    the group can be assessed. Any blunders can be rejected and the most
    suitable sight selected for subsequent reduction to a P/L. This method
    has been in use for many years but without some calculator help the
    sights cannot be analysed statistically.
    
    Using the Ti-82, this plotting can be done on the calculator screen, the
    raw data compared with a plotted Linear(or a Quadratic)'Least Squares
    Fit' equation evaluated by the Ti-82 and a statistical 'Figure of Merit'
    calculated. See Note below.
    Once this procedure has been run on the Ti-82, using a sea horizon and
    data corrected for Index Error and Watch Error, the selected least
    squares fit equation can then be used in my sight reduction programs.
    these will first  calculate the 'Most Probable Altitude' for the 'Mean
    Time of Sight' derived from this input data. This combination of TIME
    and ALTITUDE is used to calculate an INTERCEPT and AZIMUTH using the
    'Compact Data' to evaluate GHA and DEC etc. Finally the 'figure of
    merit' of the sight group used directly to indicate the Probable Error
    of the plotted P/L.
    
    I have written some sight reduction programs specifically for the
    Ti-82,and based on the 'Compact data' above. If there is sufficient
    interest I will send a sample of them to the group for an opinion as
    soon as I have dotted the T's and crossed the I's.
    
    FOOTNOTE,
    For a training exercise, since it is not necessary for the sights to be
    'Reduced' to a real P/L in order to find the 'Figure of Merit', IT IS
    NOT NECESSARY FOR A REAL HORIZON TO BE USED to gain experience in using
    the marine sextant. Timed sights can be taken ashore using a distant
    roof or hilltop, for example, as a dummy horizon. Provided that this
    dummy horizon is horizontal and distant and is vertically below the Sun
    or Moon at the time of the sight.
    Nor is it necessary to apply any corrections (except perhaps Index Error
    if a plastic sextant is used).  The Figure of Merit calculated by the
    Ti-82 would be the measure of the observers skill. The aim of the
    exercise would be to gain practice by attempting to reduce its value
    while at the same time reducing the span of time needed to acquire a set
    of observations.
    

       
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