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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: AP terminology
From: Peter Hakel
Date: 2009 Nov 17, 11:35 -0800
From: "Anabasis75@aol.com" <Anabasis75@aol.com>
To: navlist@fer3.com
Sent: Tue, November 17, 2009 9:04:51 AM
Subject: Re: [NavList 10725] Re: AP terminology, WAS: 2-Body Fix -- take three
[parts deleted by PH]
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From: Peter Hakel
Date: 2009 Nov 17, 11:35 -0800
It looks like that one-body fix problem again. John Karl and I gave formal solutions to this a few months ago. I don't know of any historical reference actually giving such a solution. That is certainly because the method is rather impractical, so it is never used and described. Your mention of radar, Jeremy, led me to look into the "Fix by Range and Bearing to One Object" in Bowditch. The azimuth measurement accuracy is stated to be 5 degrees. This makes me believe that this is really only useful across short distances (you mention pilotage waters). Bowditch gives no equations how to do this; I assume it's because for such short ranges you can just plot directly without getting into the celestial triangle.
If you were to do such a one-star fix, Jeremy, how would you do it and where does your method come from? I am still curious about this and appreciate any info on the subject.
Peter Hakel
If you were to do such a one-star fix, Jeremy, how would you do it and where does your method come from? I am still curious about this and appreciate any info on the subject.
Peter Hakel
From: "Anabasis75@aol.com" <Anabasis75@aol.com>
To: navlist@fer3.com
Sent: Tue, November 17, 2009 9:04:51 AM
Subject: Re: [NavList 10725] Re: AP terminology, WAS: 2-Body Fix -- take three
In addition to twilight, another piece of data might be provided by the
navigator if there was sufficient manpower to do this in the case where no DR
can be provided. In this particular case, only ONE star is needed to get
an approximate position. This of course is an altitude AND azimuth.
This is the same as using radar off of a single object for a position in
pilotage waters and offers the same limitation. That is to say that in
both the case of Radar and Celestial, the observation of azimuth/bearing is not
nearly as accurate as altitude/range. however if no DR is available, you
can get a rough position (dare I call it an EP?) with just this data.
If we have two stars and even if a single azimuth determination is made, we can
easily determine which of the two fixes is the correct one to choose.
The trouble with all of this, and the reason it was not popular, is that
you can't easily do this with the mechanical tools available at sea. Sure
a computer can do it all, as I said before, and Andres has demonstrated; but
it's not very easy to do with plotting tools and the charts that we
have. I don't have a compass on my ship that can stretch that far on
any chart so doing circles of equal altitude isn't an option except in very rare
cases in the tropics (less than 2.5 degrees of Zenith distance).
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