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    Re: What precision is required in cel nav?
    From: David Pike
    Date: 2015 Aug 2, 01:54 -0700


    You wrote on the usefulness of using cel-nav to provide a very rough monitor of GNSS.  Just to play the Devil’s advocate for a moment, you plot your cell-nav line or fix, and it’s some way from your GNSS position.  1.  How big a difference would you accept?  2.   If greater, which would you double check first?  3.  If still unexplained, which fix would you use, or would you MPP them?  4.  Is there a third deciding option, Triplex versus Duplex if you like?  5.  Might a second or third ‘hand held’ GNSS help, possibly with one selected to basic GPS-only (not WAAS or EGNOS) and one selected to GLONASS-only (if that’s possible), aid you in making a decision?  

    I like the twin hand-held GNSS idea.  I navigated TIKI around the Southern Baltic with two Garmin GPS 12s (in case I dropped one).  On one occasion, I came across a buoy a considerable distance from its position on my probably OOD chart, and it was reassuring (if not 100% certain) to have two GPS positions reading exactly the same.  In Linkoping Marina off L. Roxen, I found my two GPS positions set to WGS84 datum about 1km inland from my visual position on the local large scale walking map I’d just bought. Unfortunately, my new walking map had no recognisable (it was in Swedish) datum printed upon it, but it provided proof that places really can have more than one lat and long. 

    On the large ships as lighthouses theme, many years ago, I was navigating a Varsity across the North Sea when the GEE aerial became heavily iced up making the signal unusable for a while, and radio compass tuning was never my strongest skill.  Once we broke cloud, it was reassuring to look up and see the condensation trails of the jets navigating the airway above us.  It was follow that plane for a few minutes.  Dave


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