NavList:
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
Lu:
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-).
Guy
From: NavList@fer3.com
[mailto:NavList@fer3.com] On Behalf Of Lu Abel
Sent: Sunday, March 08, 2009 1:35 PM
To: NavList@fer3.com
Subject: [NavList 7595] Re: Say it aint so......
Guy:
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!
Lu
Guy Schwartz wrote:
Lu
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.
Guy
From: NavList@fer3.com [mailto:NavList@fer3.com] On
Behalf Of Lu Abel
Sent: Saturday, March 07, 2009 8:37 PM
To: NavList@googlegroups..com
Subject: [NavList 7586] Re: Say it aint so......
Guy:
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?
Lu Abel
Guy Schwartz wrote:
Hello folks:
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?
Thank you,
Guy
"May the SCHWARTZ BE WITH YOU"
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