NavList:

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From: Richard B. Langley <lang@unb.ca>
To: NavList@fer3.com
Sent: Friday, April 6, 2012 11:24 AM
Subject: [NavList] Re: Horizontal Sextant angles plot.

Quoting Lu Abel <luabel@ymail.com>:

> 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).

Yes, they do exist for machine control in, e.g., agriculture, road building, gantry crane steering. A system for the latter application was developed at UNB. See here:

http://gge.unb.ca/Personnel/Kim/gssit/RTGC.html

> They actually have three GPS's arranged in a triangular antenna about 12 or 18 inches on a side.

Or just two if only the orientation angle in the horizontal plane is required and the system stays levelled. For our CASSIOPE satellite project, we will use three antennas with an additional fourth spare for determining the attitude of the spacecraft (launch has been delayed to -- hopefully -- later this year):

http://gge.unb.ca/Personnel/Kim/gssit/GAP.html

> As I understand it, they compare the slight differences in position to derive heading.

No. Again, you wouldn't get the required accuracy by comparing two positions. The way it is done is to compare the phases of the carrier waves received at the two (or three) antennas -- a technique known as interferometry (I did my Ph.D. on geodetic applications of interferometry using quasar radio signals but now work exclusively with GPS and the other global navigation satellite systems). This article describes the process:

http://gge.unb.ca/Resources/gpsworld.march01.pdf

Here is an example of a commercial unit:

http://www.hemispheregps.com/Portals/2/literature/V101_Data_Sheet_WEB_12.2011.pdf

-- Richard

>   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.
>>>
>>>
>>>
>>>
>>
>>
>>
>> -----------------------------------------------------------------------------
>> | Richard B. Langley                            E-mail: lang@unb.ca         |
>> | Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/ |
>> | Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142   |
>> | University of New Brunswick                   Fax:      +1 506 453-4943   |
>> | Fredericton, N.B., Canada  E3B 5A3                                        |
>> |        Fredericton?  Where's that?  See: http://www.fredericton.ca/       |
>> -----------------------------------------------------------------------------
>>
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>>
>>
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>>



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| Richard B. Langley                            E-mail: lang@unb.ca        |
| Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/ |
| Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142  |
| University of New Brunswick                  Fax:      +1 506 453-4943  |
| Fredericton, N.B., Canada  E3B 5A3                                        |
|        Fredericton?  Where's that?  See: http://www.fredericton.ca/      |
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