Smartphone System Combines Gait Measurements, Magnets for Indoor Navigation

An indoor navigation system that tracks your gait relies on magnetic beacons instead of Wi-Fi for sub-meter accuracy

2 min read

Evan Ackerman is IEEE Spectrum’s robotics editor.

Smartphone System Combines Gait Measurements, Magnets for Indoor Navigation
Photo: Getty Images

Global Positioning Systems have spoiled us. With a glance at our smartphones we expect to immediately know not only exactly where we are but also where we’re going and how to get there. But GPS doesn’t work indoors, which has resulted in what I assume are millions of people getting hopelessly lost in shopping malls worldwide, as they try and figure out how to get from Brookstone to Hot Dog on a Stick.

Researchers at Fraunhofer Portugal think that they have this problem licked, with an innovative cell phone-based indoor localization system that combines magnetic beacons with inertial tracking of your gait to pinpoint your location with sub-meter accuracy.

Without GPS, there’s a limited amount of dead reckoning that a mobile device can do to keep track of where you are. The inertial measurement unit (IMU) inside a cell phone can attempt to measure your movements, but it’s not really designed for accurate localization, and errors and drift rapidly start to become problematic. 

Fraunhofer Portugal has substantially improved this dead reckoning approach by recognizing that a human exhibits consistent cyclical motions while walking. The IMU in a cell phone can measure these patterns to correlate gait (and individual steps) with distance, while constantly correcting for direction based on models of human motion.

Even with this new gait detection algorithm, the cumulative errors inherent in any dead reckoning method means that you also need some source of absolute position, both to correct your location, and also to feed back into the dead reckoning algorithms. One way to do this is with radio, triangulating a position based on the relative strength of nearby WiFi networks or using a more complicated time-of-flight or phase analysis.

In 2012, Fraunhofer patented a beacon system that uses magnets instead of radio, leveraging the compass inside your phone to pick up on ultra low-frequency modulated magnetic fields. The idea is that these relatively inexpensive magnetic beacons could be placed around an indoor area, providing a way for IMUs to get the needed periodic position fixes. And since your phone likely has a compass in it already, the hardware requirements are minimal:

By integrating their new gait-based dead reckoning algorithms with these magnetic beacons, Fraunhofer says that they’ve been able to achieve sub-meter accuracy in real-world situations, whether your phone is in use or in your pocket. Some minimal infrastructure investment is required because beacons are involved. But for large and complex indoor environments (like museums, or more importantly, shopping malls), this system could have substantial value for end users: imagine going into a grocery store with a shopping list, and having your phone provide you with an optimized route to each and every shelf. I’m sold.

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