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    Re: Role of CN at sea, was Re: Averaging sights ...
    From: George Huxtable
    Date: 2004 Oct 13, 11:14 +0100

    Nels Tomlinson asked-
    >Has anyone ever heard of a vessel loosing all of its GPSs while at
    >sea?  If it hasn't happened yet, I suspect it will.
    We are, reasonably, asked avoid discussing GPS on this list. However, I
    can't resist this opportunity to tell a true tale which, being to the
    discredit of a GPS installation, may be acceptable to Nav-l.
    It's a story that's been fully investigated and publicised over the years,
    so apologies are offered to those that know it already. I'm telling it from
    memory, so if some details are wrong, please correct me.
    It concerns a modern well-found cruise liner which left Bermuda on passage
    to Boston (may have been New York). She had been fitted out, at
    considerable expense, with all the latest navigational gear.
    This system was installed in the early days of GPS, when only an incomplete
    set of GPS satellites was flying around. As a result, it was quite usual to
    find that from certain locations at certain times, less than the minimum 3
    sats could be seen. Such local GPS outages, lasting say 15 to 30 minutes,
    were rather common, and the strategy chosen to overcome the problem was,
    for safety, to arrange a fall-back to using dead reckoning. And so, course
    by gyrocompass and distance from shaft-turns were fed into the navigational
    computer, together with the GPS signal. When the GPS failed to provide a
    position, the last stored position was updated by DR, and a special symbol
    appeared on the display to indicate what was happening. When the local
    outage ended, the system reverted to GPS position, and the special symbol
    disappeared. Seems OK, doesn't it?
    Leaving Bermuda, one of the crew, doing a job on the deck above the bridge,
    caught or tripped on the co-ax lead from the GPS antenna, breaking the
    connection. So, with no GPS signal, the system did just what it was
    designed to do, reverted to DR mode, and displayed the appropriate symbol
    on its screen.
    Unfortunately, nobody noticed that symbol, or understood its implications.
    As I seem to remember, there wasn't even a manual available on the bridge
    to explain it to an inquisitive crew member (had there been one).
    But now, intead of a few minutes of GPS outage, GPS navigation had
    disappeared, never to return. Or not until someone fixed that antenna
    downlead, which nobody then knew was broken.
    So we have a cruise liner departing on an ocean voyage, relying on DR alone
    for her navigation, but without being aware of it.
    She was in a similar situation to a vessel of say 30 years ago, without
    such electronic aids, reliant on celestial navigation but with continually
    overcast skies. That was indeed a common state of affairs, and the
    fall-back, then, was to use DR in just the same way. But then, that
    navigator of 30 years ago knew that he didn't know exactly where he was.
    The dangerous aspect of this passage of the cruise liner was that they
    thought that the indicated lat and long showed exactly where they were to
    within a few hundred feet; but it didn't. Every day, the ocean current set,
    the tides, perhaps a bit of leeway angle, conspired together to displace
    their displayed position from reality, by an unknown but increasing amount.
    That was no problem in the deep ocean, of course, but near the journey's
    end they would be "in soundings", and then needed to follow a known
    channel. They steered for that channel very precisely, according to the
    faulty display, but in reality they were many miles off it. As I vaguely
    remember, an unexplained navigational mark was seen, but nobody bothered to
    investigate, because they thought they had their exact position
    independently of all such aids.
    And so, inevitably, the cruise liner went aground. I don't remember that
    any lives were lost, and I think she was refloated soon after, without
    serious consequences. So the end-result was undramatic, when it could so
    easily have been otherwise.
    A cautionary tale, isn't it? It couldn't happen quite the same way now,
    after new rules and recommendations have emerged from the ponderings of a
    board of inquiry. It shows a failure of imagination, on the part of the
    system designer, to predict the possibility of that chain of events, which
    with hindsight made such an accident almost inevitable, eventually. No
    doubt his design was given the all-clear by some safety committee.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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