A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Guy Schwartz
Date: 2009 Mar 7, 21:42 -0800
When I first learned Celestial Navigation (US sailing) we used HO 249. I then learned law of cosines which greatly improved (decreased distances) the GPS or DR location relative to the fix location.
So I guess I do expect law of cosine to be more accurate than HO 249, NASR. There is no rounding to whole numbers with any intermediate factors using the Law of cosine.
While I studied for the USPS Navigator grade back when Ageton rather than NASR was the accepted "compact table" reduction method, I do not believe the use of an Assumed Position rather than a DR as the starting point for sight reduction should significantly affect the accuracy of a sight reduction. The major items affecting the accuracy of a sight reduction (and remember, this is for calculating Hc and comparing it to Ho) is the accuracy of almanac data and the granularity of the tabular reduction data. For a quick sanity check, consider what would happen if you were doing Law of Cosines with your DR position the same as the assumed L/Lo positions required by NASR. Would you expect one to be more accurate than the other?
Guy Schwartz wrote:
I am studying for the Navigator grade in the US Power Squadron.
Part of my sight folder is a two object sight that need to be plotted using law of cosine and using the Nautical Almanac Sight Reduction methods.
The power squadron has a 3 mile tolerance (GPS location vs. fix position).
My concern is that the 3 mile tolerance is for both Law of cosine and NASR methods.
It seems to me the NASR would deserve a larger tolerance in that there are assumed L and Lo positions as starting points
Am I right?
"May the SCHWARTZ BE WITH YOU"
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