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    Re: Automatic deviation calculation by electronic compasses
    From: Richard B. Langley
    Date: 2009 Dec 10, 10:38 -0400

    In  there's a brief
    discussion of electronic compass calibration and a reference to a Stanford University
    procedure for doing so.
    -- Richard Langley
    
    Quoting George Huxtable :
    
    > A few responses to Frank's posting.
    >
    > He writes, about orientation devices using Earth's magnetic field-
    > "...despite being technically off-topic for our traditional navigation group
    > (not a complaint, merely an observation),..."
    >
    > Does our group deliberately restrict itself to "traditional navigation",
    > whatever that may be? I would hate to think so. There are (in my view) good
    > reasons to limit discussion of GPS, as an enormous separate topic on its
    > own, which has supplanted other navigation and has its own forum. But we
    > happily deal, sometimes at length, with other applications of modern
    > technology; and why not, indeed?
    >
    > Determining orientation by the Earth's field is the one of the most-ancient
    > topics in the history of navigation, dating back to the 12th century. I do
    > not think we need to excuse ourselves for discussing modern instruments to
    > measure it; even if those might even be iPhones. Though it would be nice to
    > keep such discussion to navigational matters, rather than how a tourist
    > might identify the Taj Mahal.
    >
    > =========================
    >
    > Thanks to Frank for pointing to this information-
    > http://www.pnicorp.com/products/family/fieldforce/tcm-xb
    > which I hadn't seen before.
    >
    > It is indeed rather informative, in the details.
    >
    > Each component of magnetic field is measured, not by a fluxgate, but by a
    > magnetic core which is taken part-way towards magnetic saturation by a
    > biasing field, which is applied in alternate directions. This puts the core
    > into a region where the permeabilty, and so the overall inductance of a
    > coil, is very dependent on the total field. In one bias direction, the
    > component of the measured field adds to the bias; in the other direction, it
    > subtracts. The difference between those inductances is measured, and should,
    > over a certain range, be proportional to the measured field. I don't know
    > whether that has any technical advantages over a fluxgate; but I expect that
    > its novelty allows it to overcome existing fluxgate-using patents.
    >
    > Frank added- "Apparently, for a thousand bucks per chip, you can get a
    > pointing accuracy (after a certain calibration sequence) of 0.3 degrees
    > r.m.s. If you browse this site, there's lots of interesting talk about
    > calibration in various conditions and it seems at least somewhat clear to me
    > that they do all of this by detecting field gradients." Frank has proposed
    > that same gradient mechanism in other circumstances, although it later
    > became clear that it wasn't the case. In this case I can state, with
    > confidence and certainty, having looked at the information provided, that
    > there is no measurement of field gradient. The datasheet spells out what
    > sensors are held on board and there is a single sensor for each of the three
    > field components. Therefore, it has no ability to detect magnetic field
    > gradients.
    >
    > Frank wrote "... a big part of the problem in these discussions. NO ONE on
    > NavList knows how these devices work." However, where information is
    > provided, it's possible to deduce quite a lot from it.
    >
    > George.
    >
    > contact George Huxtable, at  george{at}hux.me.uk
    > or at +44 1865 820222 (from UK, 01865 820222)
    > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > ----- Original Message -----
    >
    > From: 
    > To: 
    > Sent: Thursday, December 10, 2009 6:51 AM
    > Subject: Re: [NavList 11059] Automatic deviation calculation by electronic
    > compasses
    >
    >
    > Digital compasses today certainly include the ability to generate their own
    > internal calibration tables. I've seen this in action on a new iPhone. When
    > it needs to re-calibrate, a little notice comes up with a graphic telling
    > the user to wave the phone around in a figure-eight pattern. And no, I have
    > no idea how that works, and I think that's a big part of the problem in
    > these discussions. NO ONE on NavList knows how these devices work. But if
    > you would like to wrap your brains around this issue a little more, despite
    > being technically off-topic for our traditional navigation group (not a
    > complaint, merely an observation), have a look at the technical specs on
    > this rather pricey little digital compass chip from PNI:
    > http://www.pnicorp.com/products/family/fieldforce/tcm-xb
    > Apparently, for a thousand bucks per chip, you can get a pointing accuracy
    > (after a certain calibration sequence) of 0.3 degrees r.m.s. If you browse
    > this site, there's lots of interesting talk about calibration in various
    > conditions and it seems at least somewhat clear to me that they do all of
    > this by detecting field gradients, however the details usually end with the
    > phrase "proprietary algorithms".
    >
    > -FER
    >
    > --
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    >
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    >
    
    
    ===============================================================================
     Richard B. Langley                            E-mail: lang---.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.city.fredericton.nb.ca/
    ===============================================================================
    
    
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