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    Re: Averaging sights on commercial vessels
    From: Trevor Kenchington
    Date: 2004 Oct 7, 16:55 +0000

    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{at}iStar.ca
    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

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