A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Guy Schwartz
Date: 2009 Mar 8, 14:13 -0700
Thank you for the insight.
I owe you a trip on my boat. I know you are from the bay area.
Let me know when you want to go sailing.
I’ll keep the GPS on 8-).
HO249 was originally intended for aircraft navigation. For aircraft navigation, knowing one's position to tenth-mile accuracy is unimportant -- by the time you figure out where you were, you ain't there any more. So HO249 is "coarse" in the results it produces.
A better comparison would be HO229, which is basically a tabulation of Law of Cosines solutions with 0.1' accuracy. To tabulate all possible solutions would require 5400^3 table entries (all possible LHAs, Decs, and DR Lats), an impossibly large number. To reduce the size of these tables by a factor of 60, HO229 requires that you use an Assumed Position that makes your LHA an exact number of degrees. I first learned sight reduction using HO229 and I found no difference between the results produced by 229 using an AP up to 30 miles from my exact position and a Law of Cosines calculation using my exact position.
Since I'm not sure whether your question was directed at the "coarseness" of NASR or the requirement to use an AP that might be miles from your known position -- using an AP within a few dozen miles of your exact position should NOT affect your result. You should get the same LOP. On the other hand, if a sight reduction method is "coarse" (for example, it only calculates in half-minute increments rather than 0.1' increments) then, yes, you might get a LOP off by a few tenths of a mile from that produced by a Law of Cosines answer. Hopefully your sextant skills are sufficient that being off by a tenth or so won't push you outside the 3 mile requirement.
By the way, the US Power Squadrons' celestial courses are now under the stewardship of a guy named Ken Beckman. Ken is a retired Air Force colonel who served as the navigator on Air Force One in the pre-GPS days. He also was the last person to hold the title of Chief Navigator of the US Air Force. I suspect he knows his stuff!
Guy Schwartz wrote:
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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