A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Andrés Ruiz
Date: 2007 Oct 18, 12:12 +0200
I am totally agree with George.
John, the Nautical Almanac Sight Reduction algorithm for n LoPs, (see http://www.geocities.com/andresruizgonzalez/celestial/sr.html) used the St Hilaire process
You can play for two LoPs with the excel spreadsheet or the CelestialFix.exe software available at: http://www.geocities.com/andresruizgonzalez/celestial/nRA.html, and see how the process go on with one, two , or morte iterations or initial position for this process; reference, assumed or DR
.... at some length, that for any
arbitrary position that you specify, (call it the RP) and a defined GP and
altitude of a body, the St Hilaire process will generate another position
(call it the SHP), on the corresponding position circle around the body, as
close as you can get to the RP, and will do so exactly. And I agree. So
That is only a part of the St Hilaire process. The next step is to draw a
tangent to the position circle through that point, knowing the azimuth
direction. And the next step is to do the whole thing again for another
body, and find the position where those tangents intersect.Only then is the
process completed. And it's those parts of the operation that are affected
by any errors in choosing the RP, divergent from the true position.
and John goes on to say-
| 2. A List member has stated that the accuracy of the St. Hilaire
| result depends on how good the initial estimated position is.
I wonder if that was me? Whether or not it was, I think that it's true.
| 3. And therefore the St. Hilaire method is really an iterative method.
And I think that is true also. And indeed, so does St Hilaire. See his
comments on page B2- 368, in which he writes "It seems to us necessary to
determine, at least roughly, the approximations that one obtains by the
calculations and thus to know if it is necessary to make a second
calculation to obtain a more correct position". And on page B2-375 he works
an example in which the error from the true position after one calculation
is 16 miles, which after a second iteration has reduced to 0.2 miles. I
admit that I've made no attempt to follow these sections in detail, however.
| We've just seen that all of this is wrong.
I've seen nothing of the kind. I remain unconvinced, as yet. What am I
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