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    Re: GPS & Magnetic Bearings
    From: Brooke Clarke
    Date: 2003 Feb 9, 15:25 -0800

    Hi Rino
    
    Yes, there are "attitude sensing" GPS receivers that use multiple antennas, 
    but it's more comples than just using two antennas.
    "GPS-based attitude determination is just an extension of differential carrier 
    phase position determination." quoted from:
    http://einstein.stanford.edu/gps/PDF/att_for_aircraft_rch1998.pdf
    an interesting article.
    
    Brooke
    
    Rino wrote:
    
    > There are some models of GPS receivers that can give you a heading:  They
    > do it by using two antennas, and then calculating the heading between
    > them.
    >
    > Rino
    >
    > -----Original Message-----
    > From: Navigation Mailing List
    > [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM]On Behalf Of Robert Eno
    > Sent: Sunday, February 09, 2003 17:11
    > To: NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM
    > Subject: Re: GPS & Magnetic Bearings
    >
    > Just a quick note on the magnetic variation: I was told by an authority on
    > the subject that after 5 years, the accuracy of the magnetic variation
    > information contained within your GPS software starts to drop off. The
    > reason being, is that the calculations for determining the variation is
    > based on a set rate for a particular epoch. As we all know, the rate of
    > change in variation is generally steady over a short period of time,
    > however, over the long haul, that rate starts to change. So for example,
    > if
    > in 1995, the rate of change for a certain area was 12 minutes eastward per
    > year, that rate of change might be, say 20 minutes eastward per year in
    > 2003.
    >
    > That is my understanding but I could have misinterpreted the message.
    >
    > Comments?
    >
    > Robert Eno
    >
    > ----- Original Message -----
    > From: Brooke Clarke 
    > To: 
    > Sent: Sunday, February 09, 2003 2:39 PM
    > Subject: GPS & Magnetic Bearings
    >
    > > 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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