NavList:

   

Reply

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.
>

   

Reply