INS True Heading Measurment
The first step in the alignment process is to level the platform. The platform is "torqued" by electric motors until the X (fore and aft) and the Y (left and right) sensing accelerometers sense zero acceleration, and the Z (up and down) accelerometer senses a maximum acceleration. This acceleration is due to gravity. When the platform is level, the system uses gyros on the platform to measure any drift of level, and uses the torque motors to restore the platform to level. The clever bit is that the earth rotates, and as it dose so the platform, which is being held level in space, appears to drift towards the east. Once you know which direction true east lies in, you know the true heading of your aircraft.