NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: GPS & Magnetic Bearings
From: Rino
Date: 2003 Feb 9, 17:55 -0500
From: Rino
Date: 2003 Feb 9, 17:55 -0500
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@LISTSERV.WEBKAHUNA.COM]On Behalf Of Robert Eno Sent: Sunday, February 09, 2003 17:11 To: NAVIGATION-L@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 ClarkeTo: 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. >