NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Horizontal Sextant angles plot.
From: Lu Abel
Date: 2012 Apr 6, 10:54 -0700
From: Lu Abel
Date: 2012 Apr 6, 10:54 -0700
And one thing I forgot -- there are now available instruments that use GPS to determine direction, no movement required. These are typically used on larger yachts and ships (for maritime applications, at least, I am unaware of any terrestrial equivalents). They actually have three GPS's arranged in a triangular antenna about 12 or 18 inches on a side. As I understand it, they compare the slight differences in position to derive heading. Furuno is one of the makers. You hinted at these in your article.
Richard's article mentions the need for an electronic compass (say, a fluxgate
compass) in auto navigation systems. I remember when they were first appearing an acquaintance boasted that he was testing a prototype of a car navigation system. I asked him for a few technical details, he replied that it purely used GPS. I asked him how well it worked in tunnels. Then I asked him how the system knew whether the first command it needed to give for getting from the car's current location to the desire destination was "make a U-turn" He gulped and said he's bring those items up with the developers.....
From: Richard B. Langley <lang@unb.ca>
To: NavList@fer3.com
Sent: Friday, April 6, 2012 4:47 AM
Subject: [NavList] Re: Horizontal Sextant angles plot.
GPS receivers determine their velocity vector from Doppler shift measurements not a succession of positions. Quoting from my article "The Magnetic Compass and GPS":
"A GPS receiver can provide accurate information about its position and velocity, and from the velocity vector it can determine the direction in which it is moving - called the course or track. The accuracy with which a receiver can compute this direction depends on its speed (the velocity magnitude) but is usually better than one degree for speeds greater than about 10 kilometers per hour. The course is not necessarily the same as the heading or direction in which the GPS receiver, or the platform on which it is mounted, is pointing. A single-antenna GPS receiver cannot determine heading. However, a compass can provide this information and as mentioned earlier, some GPS receivers incorporate an electronic compass, usually a two-axis sensor. Some receivers have three-axis sensors that give relatively accurate bearings even if they are slightly tilted."
The article can be found here:
http://www.gpsworld.com/gps/the-magnetic-compass-and-gps-828
-- Richard Langley
Quoting Alexandre E Eremenko <eremenko@math.purdue.edu>:
>
> Lu,
>
>> The problem with "GPS as compass" is that GPSs can only determine position.
>> Everything else that a GPS might show (especially direction of movement) is derived from a succession of positions.
>
> This was exactly what I thought when I was shown this "GPS compass".
> One was supposed to turn the GPS device (like a compass) in certain
> direction and it told you the asimuth of this direction...
> with the error of sevral 10's of degrees:-)
>
> (I did not even need a compass to see this, an analog watch and the Sun was enough).
>
> I imagine some people who believe in technology, carry only GPS
> with them, and use this info without even looking at the Sun:-)
>
> Alex.
>
>
>
>
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