So called 9 axis inertia guidance units exist (3 linear accelerometers, 3 axis gyroscope, and 3 axis magnetometer). The magnetometers are particularly useful fo

So called 9 axis inertia guidance units exist (3 linear accelerometers, 3 axis gyroscope, and 3 axis magnetometer). The magnetometers are particularly useful for dealing with gyroscope drift. Once you reject stuff like mains hum, in most typical situations, human scale magnetic fields tend to be pretty constant for many timescales in most human situations.