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RE :
http://www.fer3.com/arc/m2.aspx?i=115713
[NavList] Re: 12 Feb 2011 Unorthodox Jupiter Lunar from a moving platform
From: lars.h.bergman---com
Date: 21 Feb 2011 14:30

Dear Antoine,

In attached pdf I describe my methodology (this time with matching Jupiter). The revised result 46d35'N 2d32'W.

Best regards,
Lars

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Dear Lars,

Thank you very much for your detailed document.

Like you, I am a little surprised at this 2' difference in Latitude, while we know that with different methods we can achieve a better result.

I have taken some time to review some of your numbers, starting with your dLAT and dep Table. Eveything seems correct and there is a perfect match in the overall distance you indicate between 1st and last observations (27.5 NM) and the one derived from the Ship's Navigation System which I can replay here on my laptop computer (Vive la Technique ...!).

The most probable explanation to this 2' or so discrepancy comes from the Moon LOP which you move 20 miles North and 2 miles east. The MOON LOP is not really a "straight" line in the vicinity of the observation since the Moon is quite high. There are Tables which permit drawing the "curvature" of such a LOP. Maybe this would explain our significant difference in Latitude.

If you "iterate" once more, I am not sure that you would entirely get rid of this 2' or so discrepancy in Latitude since you would again (no ?) need to move one of the LOP's (maybe the SUN LOP this time) by about same N/S and E/W amounts.

*******

Transporting LOP's over time on a Chart, i.e. working LOP's in "absolute" mode is very difficult, (physical) space consuming and quite tedious if you do not want to lose accuracy.

Likewise, using auxiliary positions far away from actual DR, just to avoid painstaking interpolation from the HO2xx tables, yields inaccuracies most often overlooked.

I have frequently and regularly spotted inaccuracies well over 1 NM here and there (e.g. some recent "Ocean Navigator" Navigation Exercises) just for this reason, and the actual magnitude of the systematic errors resulting from such "Auxiliary positions Method" seems to be sometimes really underestimated.

*******

Since I am no longer familiar with moving LOP's on a big Chart (absolute mode)
I will kindly listen to comments from experienced CelNav People (Frank ? others ? ). Maybe they will spot a more convincing explanation to this "annoying" 2' inaccuracy in Latitude.

*******

Don't take me wrong though ... I am not "hammering" Tabular use methods, and I am keeping familiar with them, ... just in case. Tabular methods can be your last recourse, and for this reason I keep paying a lot of respect to them.

*******

Lars, you have again achieved wonderful results through your classical method with Tables and Logarithms.

To tell the truth, I thought that such a method would be hugely difficult to implement given our changes in Course and/or Speed.

Thank you very much and congratulations again !

Who comes next ? :-)))

Best Friendly Regards to all from

Kermit

Antoine M. "Kermit" Couëtte

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