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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Exercise #14 Multi-Moon LOP's
Date: 2009 May 1, 15:52 -0400

```Hi Jeremy

You are going to have to resend that file, it is somehow corrupted.  At least for me!

Reworking the system with a changed DR simply changed the intercepts and the
azimuths? Sounds just like old Saint-Hilaire is at work again.

Best Regards

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of jcaoy@yahoo.com
Sent: Friday, May 01, 2009 3:45 PM
To: NavList@fer3.com
Subject: [NavList 8126] Re: Exercise #14 Multi-Moon LOP's

George wrote:

1. As we've seen before, when looking at scatter, a single result tells us
almost nothing. It may "hit the spot" just by chance. Consistency is what's
needed.
2. We don't know in detail how that program handled the data it was given.
Jeremy has told us it was fed with a GPS position as DR, with known course and
speed, and he "  plugged each moonline ... as individual moon LOP's.".
Without understanding what went on in detail, let me make a guess. Given an
individual position line, and a DR position, what could a computer make of
it? It might well do its best, by finding the most probable spot on that LOP,
which would be the nearest point on that line to the DR position. In which
case, it would inevitably end up with a position very close to that DR. And
then, it would do the same for all the other position lines, and somehow
average the result. End result, a position close to the DR! I've suggested to
Jeremy that next time he tries such an exercise, he plugs in
deliberately-different trial-positions for his DR, to see if the answer
changes. I wonder, if he had offered the computer just one "moonline", what
would the result have been then?
Now, I don't know how that program operates, internally, and I doubt whether
Jeremy or Frank know either. It seems worthwhile, then, to ask some probing
questions about the value of such an exercise, before building on to such an
insecure foundation, as Frank has done. Healthy scepticism is called for,
here as elsewhere.

-------------------------------------------------------------------------

To the first point, I will make this a priority experiment on my next trip
which should offer excellent oppurtunities to shoot rapid fire fix data on a
variety of bodies and at a variety of latitude in both the northern and
southern hemispheres.  I should also have a variety of courses both
north/south and east/west. I suspect that the moon will be the best body as
the azimuth changes so rapidly, but we will see what the data shows once I
shoot them.

To the second, since I don't have any new data, I just re-crunched the old
numbers.  I took the original 11 moon lines and skewed the DR position from
the accurate 1900 LT GPS fix and made it to the nearest whole latitude and
longitude to see how the program worked the lines.  It did as I suspected,
gave me greater intercepts and different azimuths.  In any case, I have
attached a PDF sheet of the new work by SkyMate Pro and yes, it did give  the
exact same fix as my original GPS fix based DR.

As to what happens when I plug in just one moon LOP, well not much, it draws
the LOP, and doesn't even attempt to give a fix or EP.  When I put the first
and last lines into play, the program gives me a fix of 14 deg 18.0'N and 143
deg-00.7'E which isn't aweful, but certainly shows a far greater error than
when all of the lines are calculated.

I think that George may be barking up the wrong tree in a way.  Far more
important than the DR position used in this multi-shot sight of a single body
over time, is the accuracy of the course and speed used to advance/retard the
lines.  This will grow far more important as  the time spread between the
first and last shots increasing.

The attached file shows the original solution on page 1, the solution with the
DR changed, and lastly, when the SMG and CMG were changed slightly.

Jeremy

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