[blake8086]

The system you're describing (which is a real thing) would allow you to measure your absolute orientation.

The gyroscopes in iDevices only measure rotational acceleration, which is only the second derivative of your orientation, so you need to do a bit more work to extract an (slightly error-prone) orientation out of it.

[tgb]

I haven't specifically used an iDevice, but all the gyroscopes I'm familiar with provide the first derivative, not second.

I'm also curious why he doesn't mention any use of magnetometers, which can give absolute orientation (not just the derivatives). Are the changes in position/elevation too large for airplanes to rely upon the local magnetic field of the earth?