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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
Brad


-----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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