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    Re: GPS Accuracy Now.
    From: William Trayfors
    Date: 2000 May 04, 12:18 PM

    It's indeed a wonderful thing that DoD has been instructed to "turn off"
    selective availability, and that civilians now will have access to the same
    GPS signals the military has enjoyed.  This action is long overdue.
    However, one predictable consequence of enhanced GPS system "accuracy" is
    that some of our fellow mariners, and maybe even an aviator or two, are
    gonna have GPS-assisted groundings!  This will happen because word will get
    around that GPS is now an extremely accurate positioning system and some
    people will use it blindly. There are several reasons to think twice about
    how GPS is used:
    1.  Even if GPS were 100% accurate 100% of the time, there are a lot of
    charts in common use which are referenced to a different datum than WGS-84;
    differences can be      LARGE, e.g., some charts of the Virgin Islands have
    islands "misplaced" by 1/4 mile or more.  I have personal knowledge of a
    custom 70' sloop which was lost on Neckar Island after successfully
    completing a round-the-world voyage with a professional crew; this yacht
    ran aground just 1/4 mile north of the indicated GPS position and, guess
    what....the datum error at that location is 1/4 mile North/South!
    2.  GPS isn't available 100% of the time. If system availability and
    accuracy are, say, 99% reliable that means that during the year it will not
    be available or will not be accurate during some 5,256 minutes or 87.6 hours!!
    3.  There are numerous other sources of GPS error -- both technical and
    user related -- which can render GPS readings misleading for the mariner.
    For most practical navigation uses, I believe the turning off of SA
    will/should have little impact (except, perhaps, GPS-indicated speed will
    be more accurate).   I will continue to believe that GPS readings should be
    treated as just one source of positioning information to be compared with
    all other available sources at all times.  Yes, GPS is the best thing since
    sliced bread.  But, unless you use it intelligently it can cut your finger off!
    S/V Born Free
    Tortola, BVI
    At 07:20 AM 5/3/00 -0400, you wrote:
    >Philip, I agree with your thought that one mile accuracy is adequate for
    >off-shore passagemaking, but I can only imagine that from your comment of
    >accuracy of 30 meters or less being unnecessary, that you don't fish.
    >Will Martin
    >At 12:57 AM 5/3/00 +0200, you wrote:
    >>For maritime users in small craft a GPS accuracy of 100 metres is more than
    >>sufficient.   Provided you can find the fairway or safe water buoy at the
    >>approaches to ones destination or the entrance to the harbour, it is far
    >>to navigate by sight (i.e. pilotage) rather than by 'head-down' reference
    >to the
    >>Out at sea an accuracy of one mile is quite sufficient.
    >>There are occasions when a greater accuracy would be useful such as an anchor
    >>position in a crowded anchorage.    There is no real need to know the boat's
    >>position to an accuracy of 30 metres or less.
    >>Philip Ouvry
    Bill Trayfors 
    The Washington Decision Support Group, Inc.
    Specialists in Advanced Information & Communications Technologies
    2401 South Lynn Street, Arlington, VA 22202
    Office (703) 838-8784   Tech Support (703) 573-WDSG   FAX (703) 838-0019

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