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Sep 21, 2009

RE NavList 9823

Dear Peter,


Thank you for your replies to both submitted Star LAN's. I can see that you 
are a very active and quick meber of NavList.

Just a few explanations :

*******

IMPORTANT TO ALL PLAYERS IN THIS EXERCISE (Re : NavList 9823)

I did mention not only Ship's course and and speed ( as read from the Loch and 
(true) gyrocompass - BUT ALSO sea current speed and direction, which means 
that in order to get Speed and Course made good, i.e. on the Earth bottom, 
you need to "vector-add" both vessel's speed/direction and sea current 
speed/direction. This is important, and especially for the South Atlantic 
example, when we were experiencing a 2.5 kt current over a quite significant 
period of time.

*******

As regards Observation # 7 in the Mediterranean example, if you use Marc Saint 
Hilaire's method, then you can fully process it, as it is a good and valid 
observation. On the other hand, if you process Local Apparent Noons through 
dedicated specific Programs, such as the one used by Andr?s Ruiz (see NavList 
9791), you need to be fully aware of such software limitations. Most 
generally they cannot adequately accomodate observations too far away 
(timewise) from local apparent noon time. And in the Med example, because 
DENEB culminates so high, and accordingly varies so fast "height-wise", a 
time second degree algorithm/approximation - as apparently And?s might be 
using - might not be "powerful" enough to adequately deal with such 
conditions (culmination so close to zenit) while in most cases (if not all) 
the Marc Saint Hilaire Method can still be used with rather safely. Its only 
real limitation is when all azimut are too close (+/- 180?) .... and I 
appreciate that we are here on "the borderline" in both examples, but if we 
have nothing else, better use it while being aware of its limitations.

Specific algorims limitations is one of the topics I wish to address in an 
upcoming message to NavList dealing with the "pro's" ,and "con's" about 
LAN's, or "LAM's" as Jeremy has them ... since it may not be sufficiently 
covered elsewhere, at least to the best of my knowledge (and I apologize to 
who ever has already adressed it in depth).

Such document will require some careful preparation from my side, so I need 
time and will publish it here if this topic seems of some interest to our 
NavList community.

Now, and as regards your results - the reply you have been longing/looking for 
... - your Med result is fairly close from what I got (then independently 
crossed check by TRANSIT fixes), but for the South Atalntic results, it seems 
to me that your Longitude results could be significantly improved. ... unless 
I might have typed erroneous values, in spite of the great care I took in 
proof reading this message yesterday. I will take some further time as soon 
as I can in order to verify the data I published on both examples.

And by the way, what method did you use to work both LAN's ? If you rework 
both examples from your published results as DR positions and go with the 
Azimut/Intercept method (i.e. the so-called Marc Saint Hilaire), do you still 
get the very same final fixes ?

Thank you again for your interest. I will do my best to cross check once more 
the data I published yesterday in this LAN dual example, and get back to you 
afterwards. You certainly deserve, at the least, that I should not get your 
time and attention with uncorrectly copied/reproduced typed data.


Until then, please accept my Very Best Regards

Good Luck and enjoy ...


Antoine M. "Kermit" Couette


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