NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Single-body fix method
From: Christian Scheele
Date: 2009 Aug 5, 21:09 +0200
From: Christian Scheele
Date: 2009 Aug 5, 21:09 +0200
Can anybody point me to an open-access source dealing with Byrd's and Weems' attempts to make use of the "single-body fix method"? I believe Weems undertook these tests to aid Byrd in his air navigation in polar regions. The "single body fix method" has been the subject of one, perhaps several threads on this site, which I have only been paying attention to recently. James N. Wilson has also written an article about it which is on the ION CD, but I am not familiar with the work as I do not have a professional interest in celestial navigation or any of the natural sciences. I understand that the method's weakness are the requirement of knowledge of the body's azimuth and the limitations it places on the fast-moving observer. I saw a digestible -speaking in very personal terms - description of it complete with a derivation of the required formulae in the online ION Newsletter in an article about half way down the page by Joe Portney entitled "Portney's Corner: The Lost Sub Quick Fix". Here is the adddress: http://www.ion.org/newsletter/v11n1.html A follow-up reader's letter under the heading "Pondering Portney's Ponderables" in the subsequent online issue claimed/reported on errors that appeared in an embedded diagram and in the derivation of the formulae in the earlier article. The web address at which this second article can be found is: http://www.ion.org/newsletter/v11n2.html The first article merely alludes to the Byrd and Weems trials involving this procedure, involving sights of the sun taken through "the open hatch of a seaplane." I am still pondering the method. To this end I am reading a chapter in Charles Cottter's "The Elements of Navigation" on the subject of "rates of change" (of celestial bodies' altitudes), although this particular description is limited to meridian observations and is only indirectly related to the subject of the "single-body fix method", Cotter's objective here being the determination of the sun's maximum altitude. At present I am unsure as to why Cotter puts observed changes in altitude of a celestial body, a function of the combination of movement of the celestial body and the observer's own movement, into a formula that is a "partial integral" (my own term), covering the rate of change in one minute of time in one linear equation, rather than a "true" integral function, but maybe that's because we are talking about a slow-moving observer (on a ship) and it is therefore good enough to use an approximation. In any event, I imagine that practical application of the method (if, indeed, it is practical at all), where the period of observation of a body's rate of change may be a minute or a half a minute (for as much as I know), must by necessity involve the "partial", rather than the "real" derivative, but I think this is a different consideration. To those readers who are familiar with this cited chapter, is the method which Cotter explains an approximation as I suspect, or is it complete and to be used without reservation as suggested? Christian Scheele --~--~---------~--~----~------------~-------~--~----~ NavList message boards: www.fer3.com/arc Or post by email to: NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---