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From: Trevor J. Kenchington (no email)
Date: Thu Oct 07 2004 - 12:55:12 EDT
Alexandre,
I don't have anything like Doug's depth of experience but I did start
working offshore on government research ships, as part of the scientific
party, before GPS appeared. (In 1987, we had what we we were told was
the first civilian GPS unit in Australia -- $50,000 worth!) Never, in a
total of perhaps 6 months at sea spread over 25 or 30 years, have I seen
anyone take a sextant observation (though the captain of each ship has
always had a sextant on hand).
Outside of Decca or (later) LORAN-C range, we always used SatNav. I
don't know whether that was more or less precise than celestial but it
certainly was a whole lot easier and the unit maintained
continuously-updated DR positions between satellite passes, which would
have needed at least a pair of navigators committed to the task if the
electronics hadn't done the work. (Research ships don't do a lot of
steaming in straight lines, so maintaining a DR plot wouldn't be easy.)
The one time that the SatNav failed, somewhere in the Tasman Sea, we set
course to the northward and, after several hours at 10 knots, made
landfall on Cape Howe by radar at 50 or 60 miles range. No need to break
out a sextant for that trip.
Decca and LORAN-C were, of course, far more precise than celestial.
Omega (a low-frequency, global alternative to LORAN) was likely less
precise, though more convenient. It never seemed to catch on, presumably
because it was overtaken by SatNav.
Trevor Kenchington
You wrote:
> Dear Doug,
> Thank you for your explanation on how Cel nav is practiced
> on modern merchant vessels.
> I suppose they took it much more seriously in the pre-GPS era
> that is 20 years ago.
> A Russian manual (for merchant mates) of early 1970 recommended
> 5 observations per day if conditions permit.
> And averaging 3-5 altitudes for each observation.
>
> As I understand, before GPS, Cel Nav was the most precise
> available method of determining position in the open sea,
> superior to radionavigation.
--
Trevor J. Kenchington PhD
Gadus Associates, Office(902) 889-9250
R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251
Nova Scotia B0J 2L0, CANADA Home (902) 889-3555
Science Serving the Fisheries
http://home.istar.ca/~gadus
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