Friday, December 7, 2012

On Earth's magnetic field in navigation indoors

Today I'm posting a little piece of news regarding some research I've conducted recently to find out if it's possible to make use of Earth's magnetic field for the indoor navigation task.

According to some sources magnetic field structure is unique in every point and could be used to identify location. Despite Earth's natural magnetic field is slowly changing, its pattern remains similar at the same place, if - and this is a big if - some artificial electromagnetic sources around do not interfere (such as switched on and off microwave oven, for example).

It would be great to have a permanent "reference signal" instead of WiFi hotspots. This would allow for navigation even in places where WiFi coverage is low.

There are even reports about existing such an application for Android, though the app is experimental and not available for public testing.

So, I decided to test this approach. To write it shortly, the result is negative, at least so far.

Currently available magnetic sensors of mobile devices produce in the same physical point too different measurements, if taken at separated moments. A couple of minutes is sufficient for measurements to change in a factor of 1.5 in the same axis.

Apart from instability of overall magnetic field value, another difficulty arises from the fact that mobile's changing orientation in space adds uncertainty in measurements, because 3 magnetic sensors that laid in 3 dimensions, do not work the same. For example, if you take a value 15 micro-Tesla along X axe of a mobile, then rotate it on 90 degrees and measure the same physical direction along Y axe, you'll never get similar values. This means that every device requires some sort of calibration before measurements of magnetic field from all 3 sensors can be used. Alternatively, we could leave amplitude only, but this dimension reduction (from 3 to 1) makes the resulting information insufficient for unique location estimation.

If a hypothetical application driven by magnetic field does exist, it should have been using a know-how, adding the most of the value to the whole approach. In other words, there is a great field for further research.

If you have an idea, please let me know.

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