Indoor Navigation - A Use For That Magnetometer
Written by Harry Fairhead   
Sunday, 27 August 2017

Nearly all smart phones have a magnetometer which works like a 3D compass - but the question is what to do with it? Microsoft Research has an app that uses it to navigate through buildings - who needs GPS!


The sensor array in most mobile phones includes an accelerometer and a magnetometer and most of the time the magnetometer is ignored. It measures the ambient magnetic field in three directions and it is a marvel of engineering that goes well beyond the familiar compass needle. But what use is it in these days of GPS, and even WiFi location fixing? Who needs a hi-tech compass when you can fix your position to a few meters?

The answer is you might well need something more than GPS if you are inside a building. GPS relies on overhead satellites that transmit timing messages that allow the phone to fix your position relative to the satellites. The problem is that the building acts as a shield to radio waves with the result that GPS location fixes are unreliable, if you can get one at all.

This makes the problem of navigating inside a building a more difficult problem.The traditional solution is to try to use WiFi signal strengths with known WiFi access points to work out a position, but this is complicated because you have to know where the WiFi points are.

You need to know where you are before you can navigate to somewhere else and this is difficult because, as well as location, you also need a map of the building. Microsoft Research Asia has turned the problem on its head and done away with the need for locations and maps.The idea is that you simply record the path taken by a person who knows the way and then let someone else follow it. But how to record the path?



The answer is to use the magnetometer. Buildings distort the magnetic field of the earth, but in fairly stable ways that don't change too much over time. What this means is that by recording the signal from the magnetometer it might not be possible to work out where the user is, but it is possible to get them to follow the recording of a pathfinder's magnetic variations and so navigate from A to B.

"Experiments have determined that the indoor geomagnetic field is disturbed by building structures, and that it is relatively stable inside buildings. This gave us the idea of creating an indoor navigation system based on the magnetic sensor data gathered from different locations, while leveraging the other phone sensors to support real time navigation instructions."

So. while it is mainly the magnetometer. other sensors play a role.

The app, Path Guide, is available for Android (5.0 and above) in the Play store for free. You can use it to record and share paths from one location to another. The paths you record are uploaded to an Azure server and can be made available to other people. The paths are viewable in a web browser and you can make them available as a link in a webpage.You could also use it to avoid getting lost by recording your path and then replaying it or to find where you parked your car using the same approach.




The recording and playback include directions about the number of steps taken and where to turn. The pathfinder can also add annotations at various points to help the user. Another nice touch is the ability to aggregate multiple recordings of paths to improve accuracy.

There are problems with the approach and the biggest is accuracy. Telling the user how many steps to take is an approximation that might not be good enough in some situations.


More Information

Path Guide Website

Path Guide On Play Store

Path Guide: Plug-and-play Indoor Navigation

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Last Updated ( Sunday, 27 August 2017 )