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    Re: Learn the stars, by phone
    From: George Huxtable
    Date: 2009 May 14, 11:04 +0100

    Frank Reed wrote-
    
    
    "Several companies have just begun adding tiny digital magnetic compasses to 
    smart mobile phones, and recently released tech data (see google news) 
    indicates that the next generation of the iPhone will also have a built-in 
    compass."
    
    Frank's directions, or perhaps my computer skills, were inadequate for 
    locating the information he was referring to. All I could find were stories 
    about an "Android" model, from a year back. So perhaps there's something new 
    come up that invalidates what I will say next, though I rather doubt it.
    
     "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. "
    
    That seems somewhat unlikely, and we should know enough to take such claims 
    with a pinch of salt. Who is making them?
    
    No problem about finding a rough, general direction, such as "you're facing 
    northeast". But "one degree or better"? We should look at such claims with 
    some scepticism, and question the hoop-la that goes with them. Under what 
    circumstances is such precision being claimed, and what are the limits to 
    which they apply?
    
    Even given accurate predictions of Earth's magnetic field, in place and 
    time, there are two problems about achieving such precision; both are 
    familiar to cruising navigators.
    
    One is deviation of the local magnetic field, due to the presence nearby of 
    ferrous metals. By the engine of a boat, by the metal of a car you are in or 
    by, by the steel framework of a building, the steel frame of your 
    spectacles, a belt-buckle, the battery case of the mobile phone or your 
    wristwatch. Under what circumstances can we presume that such effects are 
    less than 1�?
    
    But the biggest problem is that of dip, and tilt. Fluxgate compasses have 
    been available to boatowners for years, and all suffer from this problem. 
    The Earth's magnetic field points horizontally over a small part only of the 
    Earth's surface, and in many areas, such is mine and probably yours, it dips 
    very steeply. What is required is the direction of its horizontal component, 
    to define magnetic North. To do so, within a degree, requires the sensor to 
    be held level to the horizon, or its tilt to be measured; if the dip is 
    steeper than 45�, as is the case over much of the World, then that levelling 
    must be better than 1�. How is that to be done?
    
    On a needle-compass, this problem resolves itself, to some extent. You're 
    looking at the sensing element itself, with its card, which is pivoted 
    freely in such a way that it follows the horizontal component and, by 
    weighting and balancing, resists the dip to some extent, though some tilt 
    remains. And then the lubber-line arrangement, against which the card of a 
    steering-compass is read, is itself kept reasonably horizontal,  by 
    gimballing. Or a hand bearing-compass, in the best designs, reads off the 
    required point on the horizon, by optical means directly against the card. 
    Such a compass calls for damping of oscillations, and is really quite a 
    sophisticated arrangement despite its apparent simplicity.
    
    However, anyone using a marine fluxgate compass soon becomes aware of its 
    limitations. Where there's much dip, its readings are very sensitive to 
    tilt, in both directions. Tilt along its viewing axis is automatically 
    overcome, when you're taking the bearing of a point on the horizon, but 
    cross-tilt is another matter, especially at night on a heaving foredeck in 
    rough weather.
    
    Presumably, any other mobile handheld device will have similar difficulties 
    with tilt. Something could be done, at the expense of compactness, by 
    gimballing the sensor, or perhaps better by determining all three components 
    of magnetic field and introducing some set of damped gravity-sensors to 
    correct for tilt, as such sensors can now be minutely small. But then, any 
    such arrangement must give the device an unwanted sensitivity to 
    accelerations, which affect apparent gravity direction but not magnetic 
    field. It would be useful only when the device was held rather still, if a 
    1� precision is being required in high-dip areas; not on a boat in rough 
    seas, nor in a moving vehicle, nor by a walker.
    
    I'm suggesting that we shouldn't take for granted such claims for precision, 
    about hand-held mobile devices, without asking hard questions. However, 
    Franks's mailing also mentioned- "This sort of application has been 
    available for a few years in expensive stand-alone devices which have sold 
    well in a niche market...", and that's a rather different matter. If a 
    device is intended for a desk-top or a tripod environment, well and good; 
    many of the limitations I've discussed no longer apply.
    
    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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