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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@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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===============================================================================
 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.city.fredericton.nb.ca/
===============================================================================


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