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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Learn the stars, by phone
Date: 2009 May 14, 14:27 -0400

```Thanks Lu, I wasn't aware of that advance.  Makes perfect sense

-----Original Message-----
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Lu Abel
Sent: Thursday, May 14, 2009 2:16 PM
To: NavList@fer3.com
Subject: [NavList 8272] Re: Learn the stars, by phone

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

> 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
>
> -----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.

>
>
>
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> >
>

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