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    Re: Da Lurk
    From: Rodney Myrvaagnes
    Date: 2003 Feb 9, 19:48 -0500

    Fred,
    
    I think you have this already from some other answers, but: A normal
    GPS receiver updates its fix once a second. It calculates the bearing
    from the last fix to the current one. It might do a rolling average as
    it surely will for speed, but that is less important for the track.
    
    There are now fancy GPS compasses that use multiple antennas and
    carrier-phase comparisons to show the actual attitude of the antenna
    array. The ones I have seen so far cost $4K and up, but that will
    change.
    
    As long as you are moving, the ordinary GPS track tells you where you
    are actually going, automatically correcting for set and drift. I find
    I can steer to a display that includes track and cross-track error
    quite easily, and follow a shorter course than I would by using the
    compass. It is particularly good for setting course in an autopilot.
    
    One of your posts suggests you now think the original problem wasn't
    variation, but set and drift. In Maine that is quite possible, although
    15 degrees would be quite a lot. Tidal currents flow along the coast
    from Fundy tides, as well as in and out of bays.
    
    There are nearly always enough lobster-pot buoys to show you the
    current. If you cruise Maine often, these will become second nature to
    you, especially when you are motoring. :-)
    
    On Sun, 9 Feb 2003 13:37:56 -0500, Fred Hebard wrote:
    
    >Rodney,
    >
    >How does a GPS unit sense the direction a boat is steering?  I have
    >no clue.  My impression, 5 years after 5 partial days aboard, is that
    >the unit on the boat I chartered needed to be swung to the boat.  My
    >analysis, 5 years ago, was that it was swung incorrectly.  I vaguely
    >recall an error of 15 degrees.  I don't know whether it was a 15
    >degree error or a 30 degree error, and definitely do not know the
    >direction.  I meant to say in my previous email that it was _not_ a
    >true/magnetic issue, but see that I don't have sufficient data now to
    >resolve that one way or the other.  If the unit needed to be swung to
    >the boat, nor do you.
    >
    >I am aware of magnetic/true issues since I usually plot with a
    >triangle and routinely have to convert course and sight bearings from
    >one to the other.  I vividly recall a splitting headache of many
    >years ago from figuring out a course for another guy's fishing boat
    >compass that was about 90 degrees out, but varying depending upon the
    >heading, working at night down below over a throbbing engine housing,
    >with too much alcohol on board, not having thought I'd be called upon
    >to plot a course.  Fortunately, we had about a mile visibility and
    >some lights for bearings.
    >
    >Fred
    
    
    
    Rodney Myrvaagnes         NYC                       J36 Gjo/a
    
    "WooWooism lives"  Anon grafitto on the base of the Cuttyhunk breakwater light
    
    
    

       
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