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    Improving accuracy of fix by shooting 4 stars
    From: Gary LaPook
    Date: 2007 Nov 03, 23:47 -0700

    A while ago Geoffrey Kolbe posted that one can determine index error
    and get more accurate fixes by shooting 4 stars 90� apart. (See his
    post below)
    
    The ION cd has an article that discusses this also, it is item 158.
    
    Here is a link to the ION table of contents:
    
    http://www.ion.org/publications/toc/celnavTOC.pdf
    
    
    
    
    gl
    
    
    
    
    
    
    
    Alex, the index error of a bubble sextant can be determined from an
    unknown
    position.
    
    By taking sightings on bodies in opposition, the separation of the
    resultant position lines will, on average, be twice the index error.
    
    In my posting #2531, I advocated taking four sightings, a North-South
    pair
    and an East-West pair, on bodies at the Cardinal points. The North-
    South
    pair effectively give you your latitude and the East-West pair your
    longitude. The position lines from the four sightings should form a
    square
    box if index error is the dominant error. If the box is not
    substantially
    square, there is a non-systematic error somewhere which is of the same
    order or greater than the index error.
    
    See http://www.pisces-press.com/desert/14thMarch.jpg for an example of
    what
    I mean. In this example, the box is substantially square, giving us
    confidence that the dominant error is indeed index error. The size of
    the
    box is about 10' on a side, from which we deduce that the uncorrected
    index
    error was 5'. The position lines are all 'away from' the body in
    question,
    so the sextant altitudes are 5' too small. I need to add 5' to my
    altitudes
    to correct the index error.
    
    A subsequent round of sights at a different location a few days later
    show
    the results when this 5' is factored in. See
    http://www.pisces-press.com/desert/19thMarch.jpg
    
    Geoffrey Kolbe
    
    
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