

Tapping Earth's magnetic field for indoor navigation - SlipperySlope
http://news.cnet.com/8301-11386_3-57472649-76/tapping-earths-magnetic-field-for-indoor-navigation/

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SlipperySlope
Essentially ...

"Modern buildings with reinforced concrete and steel structures produce have
unique, spatially-varying ambient magnetic fields that can be used for
positioning, in much the same way (albeit on a much smaller spatial scale) as
animals use the Earth's magnetic field," the paper's authors say. "In
principle, a non-uniform ambient magnetic field produces different magnetic
observations, depending on the path taken through it."

