NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Learn the stars, by phone
From: George Huxtable
Date: 2009 May 14, 11:04 +0100
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@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---