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    Re: GPS out by miles
    From: Lu Abel
    Date: 2016 Nov 2, 01:15 +0000
    My first question is how this person knew "where he was" -- offshore -- to prove "GPS was off by miles"

    Traditional navigation, whether by compass bearings when shoreside features are in view or by celestial, is - let's face it - way less accurate than GPS.  So if the "my GPS was off by five miles claim is based solely on traditional navigation methods, I'd say the person's position derived NOT using GPS was likely in error.

    Another possibility was that he asked a passing ship for a position report -- and he misunderstood their words.

    It would also be interesting to learn when this took place and what model of GPS was reportedly incorrect.  If I recall correctly, there was a software bug discovered in several models of GPS receivers (especially older ones) a year or two ago.   The Air Force had launched a new satellite that had not been brought on line yet.   But the AF turned its transmitter on -- transmitting a legitimate but never-before-used message "don't believe the positional information I'm transmitting."  Unfortunately, the affected GPS units did not recognize the message and used the positional information the satellite was transmitting.

    And as always, the good navigator uses ALL available information from ALL available sources.

    From: Frank Reed <NoReply_FrankReed@fer3.com>
    To: luabel@ymail.com
    Sent: Tuesday, November 1, 2016 12:03 PM
    Subject: [NavList] GPS out by miles

    I recently heard a story about GPS fallibility. A sailor related his enthusiasm for celestial navigation. He said that he didn't trust GPS because "sometimes it's wrong by miles". He described sailing off the coast of North Carolina and he knew where he was, but the GPS placed him five miles from that location. I think there's a lesson here... What might that be? Of course without interrogating him in detail, which would have been rude, we can't really be sure what was happening here. What are some of the possibilities??
    Frank Reed

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