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    Re: Learn the stars, by phone
    From: Brad Morris
    Date: 2009 May 14, 13:59 -0400

    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
    -----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
    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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