NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
GPS & Magnetic Bearings
From: Brooke Clarke
Date: 2003 Feb 9, 11:39 -0800
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.