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    GPS & Magnetic Bearings
    From: Brooke Clarke
    Date: 2003 Feb 9, 11:39 -0800

    Hi:
    
    I know my old Motorola GPS uses the World Magnetic Model that is updated every 
    5 years, mine has the 1995 version.  This model will tell you the magnetic 
    deviation if you know the date and your location, both of which a GPS 
    receiver knows.
    
    GPS has no provision to determine direction. For example on a trip to Japan I 
    had a way point for my hotel in a big city.  Many blocks away I knew I needed 
    to head in a certain direction to get back, but the GPS has no compass 
    functionality.  The answer was to walk very quickly for a block and then see 
    which direction the GPS said I was going based on the change in my position.  
    Some GPS receivers now incorporate a fluxgate or other
    electronic compass into the receiver for this reason, but the GPS system 
    derives bearing from change in position.  For more on sensors and the WMM see 
    my web page: http://www.pacificsites.com/~brooke/Sensors.shtml#Earth's 
    Magnetic
    
    Have Fun,
    
    Brooke
    
    Jared Sherman wrote:
    
    > Fred-
    >  
    >
    > 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 can.
    >
    > 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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