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Alan wrote-

"On displacement of calculated fix from GPS coordinates, many under 3 NM, a
few around 1 NM, with a couple under 1 NM. Of course, there were some "wild
hairs" which I attribute to "operator error", yours truly being the
operator."

I doubt whether his results are affected at all by the two possible reasons
he suggested; twist of the shield assembly and distance of the observer
from it. But if he is seeing discrepancies of around 3 miles, something
seems wrong. That would involve an error in sextant reading of around 6',
because that reading is halved when using a reflection. Indeed, use of an
artificial horizon should be a procedure of great accuracy. Not only is
there that factor of 2 in the sextant reading, but also the dip of the
horizon, which is usually the biggest unknown in altitude navigation, is no
longer involved.

Some study of those worse results, rather than his ones that show the least
discrepancy, may be productive. I suggest that Alan posts to the list an
example or two of such reflected Sun altitudes, those that show
discrepancies of a few arc-minutes (= miles), providing the following
information-

Date and time (to the second), sextant reading, index error, which limb
used, and observer's position, according to GPS or map. If temperatures and
pressures were recorded, so much the better, but unless those values were
extreme, or the Sun was low, they will have little effect.

List members may then be able to help in tracking down the origin of any
discrepancies.

George.

contact George Huxtable, at  george{at}hux.me.uk
 or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.





You also asked , "And why do you need to stand back to see the two views?"
I really don't know, just that it seemed that there was an optimum distance
from which I could see both the reflected and sextant suns, this seeming to
be back from the AH, rather than closer to it.

As to alignment, I'm just going on the basis of the instruction sheet that
came with the thing, which stated that the ah should be oritned so that
there was no shadow at either side.

Best.

Alan
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