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    Re: Learn the stars, by phone
    From: George Huxtable
    Date: 2009 May 15, 08:51 +0100

    I had written, about the claim that handheld magnetic devices could achieve
    pointing precision better than 1 degree,-
    
    " but Frank has taken it further, by writing- "it can determine where you're
    pointing in the sky from anywhere on Earth at any date and time ... with an
    accuracy of about 0.5 degrees." , but he still doesn't state where this
    figure comes from, what instrument it refers to, and under what conditions
    it applies."
    
    and Frank Reed replied-
    
    "It refers to the SkyScout, which I have tried out myself. That half-degree
    claim comes from the official specs"
    
    I had looked into the Skyscout website that Frank had pointed us to, at
    http://www.celestron.com/skyscout/
    and found the page of "specs" at-
    http://www.celestron.com/skyscout/skyscout_page.php?page_name=skyscout_specs&page_id=3
    but could find no claim of pointing precision there. That was why I asked
    where Frank's figure came from. I ask again, where can I find that claim?
    
    He continued- "It's a believable claim, based on performance, but the exact
    level of accuracy (whether it's 0.5 or 0.75 or 1.0 degrees) is not critical
    in any way to the device's use."
    
    Well, I wasn't wishing to investigate the Skyscout in particular, but
    following up the general claim made in Frank's original posting, when
    introducing this topic-
    "These compasses, aided, of course, by software models of the Earth's
    magnetic field, can determine true direction in three dimensions in most
    parts of the world to an accuracy of one degree or better." If it's true, it
    could open up many useful applications. That is why I've been trying to find
    out-
    1. Who makes such a claim?
    2. Under what environmental circumstances is it claimed to apply? Such as
    magnetic latitude, acceleration, sensitivity to local magnetic deviation,
    temperature range?
    3. What are the snags, if any?
    
    Frank responded to my inquiry, about using such a sensor on a boat, "Oh, I
    don't think it (the SkyScout or similar technology) would work on a boat...
    at least not very well. There's a way to make one work better under
    conditions with complex accelerations, but I don't think anyone would
    optimize these consumer devices for such applications. There's just no need
    for it."
    
    What is that way to make one work better under accelerations, that he
    casually refers to? We have discussed time-filtering to achieve that
    purpose, in referring to strapdown field sensors on this list some years
    ago. Whether Frank judges that "there's no need for it", is quite beside the
    point. The magnetoresistive sensors that are involved are very cheap and
    rather simple to operate, and my interest may well be in cobbling together a
    device that uses them. Hence my request for details about the claims he
    makes.
    
    The question about local magnetic deviation is a serious one. To meet the
    specs that Frank has quoted, more is needed than just the ability to detect
    some local field distortion; it would need to detect it at a level at which
    the effective horizontal field direction was changed by less than half a
    degree. To do so by detecting non-uniformity in the field would call for
    what is effectively a 2-axis magnetic gradiometer, by installing an array of
    differential magnetic sensors of remarkable sensitivity. I doubt if that
    speculation would be feasible. It seems very unlikely.
    
    Frank ends, wich this acidity- "PS: If you feel that I am "belittling" a
    problem then you are taking yourself waaaaaay too seriously."
    On the contrary; I suspect that I am taking Frank's pronouncements too
    seriously, in trying to discover the details on which they are based, and
    failing. Perhaps I should know better, by now.
    
    
    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.
    
    
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