[KenWittlief]

conventional inertail navigation systems used gyroscopes on gimbal bearings that allowed the system to determine how far it has turned on each axis

you can now get solid state yaw rate sensors that put out a signal proportional to the rate at which its being turned on one axis - if you wanted to make a 3D system you would use 3 of them

search the forum for 'yaw rate' sensor or 'gyro' - there lots of threads already - more than I could type out in a hour - that explain where to get them and how to use them

but in essence - their output is in degrees/second - so by simply accumulating the output (repeatedly adding it to a large variable) you end up with degrees - compass heading. the ADXRS150EB (or something like that) is the most common one to use - you can get em for $50