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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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