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Subject: Re: Accuracy of position (sextant error simulation)
From: George Huxtable (george@XXX.XXX)
Date: Thu Oct 21 1999 - 16:00:07 EDT
This is a response too a posting made earlier today by Jim Manzari.
His analysis illustrates the dangers in using statistics to draw
conclusions from unrealistic and incorrect assumptions.
Let's deal first which what, according to Jim, is the "largest single
error, which dominates all others", and can give rise to an altitude error,
in one case, of -34 arc minutes. This depends entirely on what Jim has
arbitrarily chosen as a reasonable scatter for his angle phi, the tilt of
the sextant from the vertical. In that worst case of -34 minutes, my
estimate of the tilt that produced it is about 8 degrees from the vertical
(it depends slightly on what the actual altitude was for that measurement).
No wonder tilt dominates all other errors! What navigator would get a
sighted body to kiss the horizon, and then end up with an altitude
corresponding to the value he would have got with 8 degrees tilt, with
enough error to put the upper limb of the Sun where the lower limb ought to
be? The notion is absurd. I ask Jim to state how he has defined the scatter
of sextant tilt, and on what basis he has chosen that scatter. Is it
realistic?
Another factor Jim has considered is sextant index error. This plays no
part whatsoever in the altitude error as long as the index error has been
checked and allowed-for (the easiest thing in the world to do). I
interleave index checks between each sextant sight, for my plastic sextant,
though it's not necessary to go that far. The index-error contribution to
altitude error is ZERO.
Jim has considered refraction of the light from the body, and correctly
found that at angles above 30 degrees, variations are negligible. But he
has ignored the unpredictable changes in the dip caused by refraction in
the light path from the horizon, an important factor if not THE most
important factor. See my mailings on this topic earlier today.
I could go on, but I think I've said enough.
George Huxtable.
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george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel, or fax, to 01865 820222 or (int.) +44 1865 820222.
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