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    Re: GNSS is not immortal
    From: Pete Solon Palmer
    Date: 2018 Sep 26, 17:03 -0400
    <<  Why only ocean going vessels?  Because coastal sailors will use their eyes and/or their compass to steer back to land.  >>

    I think CN is useful for coastal sailors too.  If you are approaching land, and you don't know how far away it is, or where your land fall may be, then you are technically lost.

    I have sailed along more than one featureless coast where I
    HAD to take a celestial sight to find out where on that coast I was.

    Regards,
    Pete


    -----Original Message-----
    From: Brad Morris <NoReply_Morris@fer3.com>
    To: globenav <globenav---.com>
    Sent: Wed, Sep 26, 2018 12:33 pm
    Subject: [NavList] Re: GNSS is not immortal

    Hello Bob

    You wrote:  failure to appreciate the nature of an effective Faraday cage....no matter how many handheld units one has aboard as backups -- a GPS only approach to navigation leaves you with something very like a single-point-of-failure for a mission-critical capability required for making a landfall.

    In my view, this is an argument for CN.  Term it the "Hail Mary" use of CN.  After the mariner has a lightning strike, and has discovered that their backup depth of N GPS handhelds are all sequentially bad due to improper stowage, they can finally resort to CN.  Hopefully, our careless mariner has the skills, equipment, almanac and tables (under the assumption that the calculator was similarly not protected) available.  

    That's a fairly shallow pool of intended users.  The pool is restricted to: ocean going recreational vessels, several days out to sea, struck by lightning, failed to have a proper faraday cage around each and every backup GPS, all were fried and the navigator has all of the necessary tools to perform CN.  

    Why only recreational vessels?  Because commercial vessels by their very size and construction will do a far better job of being a natural faraday cage for the GPS unit.  

    Why only ocean going vessels?  Because coastal sailors will use their eyes and/or their compass to steer back to land.  

    All that said, the argument for the Hail Mary use of CN can be sustained.  What if that shallow pool of users includes you?

    Brad













    On Tue, Sep 25, 2018, 4:05 PM Bob Goethe <NoReply_Goethe@fer3.com> wrote:

    >>Please do so identify those scenarios (for GPS failure).

    One scenario is that the recreational sailor has a failure to appreciate the nature of an effective Faraday cage. 

    The proper way to protect a handheld GPS unit from a lightning-generated electromagnetic pulse is to surround it with an insulator (e.g. inside a sealed ziplock bag or plastic case) and then surround it with the conductor (e.g. wrap in aluminum foil). 

    This particular sailor at https://forums.sailboatowners.com/index.php?threads/faraday-cage.146943/ got it wrong:  he had electronics-conductor-insulator rather than electronics-insulator-conductor.

    I had a GPS wrapped in tin foil, inside a Pelican Case stored in a Ziplock bag with desicant and it still got fried. 

    This failure to understand the nature of an effective Faraday cage - which seems to pop up repeatedly on blue water sailing forums - has definitely led to instances where all the electronics connected to the boat's electrical system got fried by a lightning bolt, as did all the hand-held GPS units.  And because of their initial misunderstanding, they come out of their experience of a lightning strike no wiser than before it happened.

    I have an acquaintance who sailed single-handed around the world in her 60s.  She did not carry a sextant, but rather had 5 hand-held GPS units to back up her main GPS.  But because, as I discovered, she did not understand how to protect against EMP, if she HAD experienced a lightning strike, she might well have found that she had a single event that caused a catastrophic, simultaneous failure of every GPS on board.

    The best back up to a GPS may indeed be another GPS...but only if you have a correct understanding of EMP and Faraday cages.

    It is impossible to know how many of the 40 to 60 sailboats a year that disappear without a trace do so after a catastrophic loss of GPS navigational ability.  It could range from "quite a few" to "none".

    But one can at least imagine a scenario where -- no matter how many handheld units one has aboard as backups -- a GPS only approach to navigation leaves you with something very like a single-point-of-failure for a mission-critical capability required for making a landfall.

    Bob

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