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» Online Classroom   »   » Public Discussion of Cel Nav   » Interpreting LOPs on plotting sheet

   
Author Topic: Interpreting LOPs on plotting sheet
Roger


 - posted May 08, 2009 10:41 AM      Profile for Roger           Edit/Delete Post 
How do I reliably interpret LOPs and choose a fix on the plotting sheet? I refer to plots with more than just two LOPs. I know what the "cocked hat" is but at times there seems to be more than one possibility. I am working through Leonard Gray's book in which problems generate six LOPs. Thanks Roger.
David Burch


 - posted May 08, 2009 12:42 PM      Profile for David Burch           Edit/Delete Post 
The best solution by far is to take the sights judiciously and then the analysis is easy. Namely take 3 stars as near as possible to 120° apart that are roughly the same height in the sky. the relative spacing is the most important, and really all that matters if you stay in the height range of 15° to 85°.

Then the fix is the geometric center of the triangle. If you have a constant error in each sight, it will effectively cancel.

Taking more than 3 sights is generally a mistake, in that it is wasting twilight time when you could be adding more sights to the 3 best. typically you would want at least 4 of each of the 3 for the best fix. then use our "fit slope" method to choose the best of the 4.

You will often see discussion of navigators taking 6, 10 or dozens of stars with an attempt at better fix, but this is false math... simply wrong.

==============

if you take 3 stars say 60° apart the result will look just like that of 3 at 120 apart, but for the 3 at 60 the proper fix is not necessarily inside the triangle. It is a much more complex issue, depending on the nature of the errors themselves. it is best to just avoid this, and if you are forced into it, keep in mind your errors will be larger.

From: Starpath, Seattle, WA
Roger


 - posted May 08, 2009 02:34 PM      Profile for Roger           Edit/Delete Post 
Great response. It is just what I needed. Thanks Roger.


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