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    Re: GPS shortcomings.
    From: Lu Abel
    Date: 2005 Jun 8, 14:58 -0700

    George's posting and subsequent discussions of GPS outages caused me to
    think more about the problem of reliable navigation and reliability in
    general, and I'm going to alter my "use the $1000 to buy four GPSs and a
    lot of batteries" statement.
    Reliability comes from two sources:  Reliability of individual
    components (for example, we hardly worry about the tires on our cars the
    way our grandfathers might have 90 years ago), plus redundancy.
    Redundancy gives backup in case a system fails (eg, multiple engines on
    airplanes).  But there's two forms of redundancy:  simple component
    redundancy (multiple engines) and alternative ways to get the job done
    (sails for when my engine fails, an engine for when my sails can't
    propel the boat).
    Subsequent postings by both me and others have noted that GPS, while
    extremely reliable, does have some chance of going down.  Worse, it has
    a chance of telling lies (I'd far rather have my receiver say "no
    signal" than give me a L/Lo that was wrong).  I suspect the push to keep
    Loran going in the British Isles, like the US's change from wanting to
    shut Loran off is based on this idea of alternative ways to get the job
    done.  If it's cheap enough, why not keep Loran on the air -- that way
    if GPS fails despite all the reliability built into it (or, I'll
    cynically note, if the US government decides to mess with it, as they
    periodically threaten to do in the name of our various crusades) there's
    an alternative.
    Along that line, I will eat crow on my earlier posting and say that I'd
    take the price of one of the extra GPS receivers and use it instead to
    purchase a plastic sextant (as one wag put it, "the other GPS - a gray
    plastic sextant") one or more cheap solar-powered scientific calculators
    (mine cost $15 a dozen years ago and it claims that if you can read the
    digits it's got enough light to work).  I'd track my wristwatch's
    behavior through my GPS's time display so I had a good handle on its
    timekeeping properties (most digital watches are very accurate
    timepieces these days).  Not sure how I'd handle the Nautical Almanac
    problem -- just buy it (but then there's the perpetual problem long
    distance voyagers have, how to get a new NA every year), or see if
    there's a cheap calculator that can provide it.  I'd still trust my GPSs
    and be very happy I had multiple of them in protected storage on board.
      But I'd probably keep the sextant handy and keep up my celnav skills.
    Again, my primary method of navigating would be using my GPS, and my
    primary backup would be those extra GPSs carefully stored away.  But a
    plastic sextant is a good way of providing an alternative technology.
    Lu Abel
    George Huxtable wrote:
    > I know that references to GPS are discouraged, if not forbidden, on this
    > list. But I wonder if a note about GPS shortcomings would be more
    > acceptable?
    > I've just received a leaflet from the British Lighthouse authorities,
    > pushing for use of the new Loran-C transmitter at Rugby, England.
    > In which is the statement-
    > "GNSS systems themselves also suffer occasionally from undetected failures;
    > for example, a satellite clock error on 1 January 2004 gave rise to errors
    > in measured positions of up to 45 kilometres in Western Europe for a period
    > of some 3 hours, resulting in onboard navigational failures."
    > I hadn't heard about that event, which sounds rather serious. Is it common
    > knowledge? Can anyone offer further details?
    > It wouldn't have affected me. I would have been by the fireside on that day.
    > George.
    > ================================================================
    > contact George Huxtable by email at george@huxtable.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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