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    Re: Da Lurk
    From: William Allen
    Date: 2003 Feb 9, 14:13 -0800

    Just to be clear ...
    There is a new to the market type of GPS compass, which apparently
    measure the time of signals from two or more antenna at the receiving
    station in order to calculate the compass direction in which the antenna
    array (and presumably the ship) is facing.  This will not require any
    distance or course made good type of calculation.  These are, however,
    relatively expensive compared to standard compasses and GPS sets.
    Bill Allen
    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM] On Behalf Of Jared Sherman
    Sent: Sunday, February 09, 2003 11:08 AM
    Subject: Re: Da Lurk
    GPS knows nothing about steering directions, it only knows the actual
    course taken by the GPS antenna. If you are steering 340 but making 350
    due to leeway, the GPS will only tell you that you are making 350. Or,
    that the GPS antenna is making 350 so it presumes the rest of the boat
    is moving that way too. GPS literally "can't" be swung like a compass
    GPS knows nothing about true/magnetic, unless someone tells it to
    display one versus the other and gives it deviation information. I'm not
    sure if that is available from the GPS system (I don't think so) but the
    deviation data for the world can be stored in ROM and then "generated"
    by the GPS unit running the current date against that to extrapolate the
    correct information for the time and position that the GPS system has
    given it.
    There was some discussion...here?...some time ago about one brand of GPS
    that indeed had a firmware error and was apparently calculating
    magnetic/true incorrectly. French or English I think--a brand that
    doesn't appear here on the US market.

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