NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Learn the stars, by phone
From: Lu Abel
Date: 2009 May 14, 11:15 -0700
From: Lu Abel
Date: 2009 May 14, 11:15 -0700
Brad: There two different ways to obtain direction from a GPS. One is by comparing current position with a previous position. Works fine if you're moving but, as you note, if one is standing still one will get a random "direction" resulting from dither in the signal. As I keep reminding my navigation students, all that a standard GPS receiver provides is one's geographic position at a particular point in time; everything else a GPS displays is derived. But there are also explicit GPS "compasses" that use multiple receivers and compare the phase of the received signal at two or more receivers. These are highly accurate and work even if the vehicle (be it boat, aircraft, or tank) is standing still. Here's an article about them by Chuck Husick, a noted writer about marine technology: http://www.boatus.com/husick/techno09_02.asp And here's a brochure about one of the units made by Furuno http://www.furunousa.com/Furuno/Doc/0/JFM9TDHLTIPKVAPI58G2CFI2C9/sc50_usa.pdf Brad Morris wrote: > 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 > Brad > > -----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 > > > George: > > 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. > > > > "Confidentiality and Privilege Notice > The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you." > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---