Pure IMU-based Positional Tracking is a No-go

A brief explanation why absolute positional tracking, the kind that's needed for proper VR, can not be achieved using an inertial measurement unit (IMU) without an external 3D reference frame. When tracking position based on accelerometer data using dead reckoning, drift accumulates quadratically, meaning that the speed of drift increases proportionally to time passed.

This is different for orientational tracking; linear (not quadratic) drift in the orientation estimate from integrating gyroscope readings can be corrected by nudging the estimate towards the fixed reference frame established by the gravity ("up") direction, and the direction to magnetic north measured by the magnetometer.