BOAC is correct. The laser gyros only measure the aircraft displacement rates about the x, y and z axes. The displacement rates are integrated to obtain angular displacement from the starting attitude - when the aircraft was hopefully parked right side up somewhere upon the surface of the earth. When the operator enters the geographic coordinates into the system the IRS computes the acceleration rates and refines the raw aircraft attitude into a true hoizontal starting point. Once the system has an accurate starting point and horizontal reference, all motion along the x, y and z axes is resolved by mathematical integration of accelerometer outputs into velocity, direction and distance. The tricky bit with "strap down" laser gyro systems is that the three accelerometers are fixed to the aircraft axes, so the mathematical calculations become quite complex. In old "steam driven" systems the accelerometers were attached to a gyro stabilised level platform.