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    Re: Learn the stars, by phone
    From: Lu Abel
    Date: 2009 May 14, 11:15 -0700

    Brad:
    
    There two different ways to obtain direction from a GPS.  One is by 
    comparing current position with a previous position.  Works fine if 
    you're moving but, as you note, if one is standing still one will get a 
    random "direction" resulting from dither in the signal.   As I keep 
    reminding my navigation students, all that a standard GPS receiver 
    provides is one's geographic position at a particular point in time; 
    everything else a GPS displays is derived.
    
    But there are also explicit GPS "compasses" that use multiple receivers 
    and compare the phase of the received signal at two or more receivers.   
    These are highly accurate and work even if the vehicle (be it boat, 
    aircraft, or tank) is standing still.   Here's an article about them by 
    Chuck Husick, a noted writer about marine technology:  
    http://www.boatus.com/husick/techno09_02.asp
    And here's a brochure about one of the units made by Furuno  
    http://www.furunousa.com/Furuno/Doc/0/JFM9TDHLTIPKVAPI58G2CFI2C9/sc50_usa.pdf
    
    Brad Morris wrote:
    > A bit of caution, if you don't move by a significant displacement, then the 
    heading displayed on a GPS is not to be trusted.
    >
    > The reason is pretty obvious.  The dither in position due to the quantizing 
    and sampling of the satellite signals will actually let your position wander 
    about some nominal mean.  In one experiment, I found this to be 150 feet, 
    peak to peak in both NS and EW, for my particular GPS. If you sit still, and 
    then turn on the GPS, the heading is just random, independent of actual 
    heading.  Once there is course made good, the GPS has two positions relative 
    to time and naturally, a course.
    >
    > Best Regards
    > Brad
    >
    > -----Original Message-----
    > From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of James N Wilson
    > Sent: Thursday, May 14, 2009 1:51 PM
    > To: NavList@fer3.com
    > Subject: [NavList 8270] Re: Learn the stars, by phone
    >
    >
    > George:
    >
    > Your discussion of problems with compasses reminded me of an ancient
    > experience--putting a compass in a tank. I headed a team to advise the
    > army, and they were looking for a more modern way than the existing one:
    > the tank commander stopped the tank, got out and walked fifty feet away
    > and read his hand compass. They had been sold on a scheme which had an
    > aircraft fluxgate compass on a fender, but it was awful. Deviation of up
    > to 300 degrees! I couldn't believe it. The only way it worked was with
    > the tank stopped on level ground pointing north. Not very utile.
    >
    > A general asked me could GPS be used to get a heading. I asked our JPL
    > navigators about that, and they came up with two schemes that could do
    > the job. Of course, that's common now, and a lot more.
    >
    > Jim Wilson
    > ____________________________________________________________
    > Digital Photography - Click Now.
     
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