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Another method for determining index error would be to compare an
observation with it's back sight provided that the body is above 60
degrees of elevation. Different parts of the arc are used so any arc
corrections need to be applied.

Greg Rudzinski

On Nov 3, 10:47 pm, glap...@PACBELL.NET wrote:
> 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.
>
> Seehttp://www.pisces-press.com/desert/14thMarch.jpgfor 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. Seehttp://www.pisces-press.com/desert/19thMarch.jpg
>
> Geoffrey Kolbe


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