The Inertial Reference System (IRS) receives data from three Ring Laser Gyros (RLGs) to calculate angular movements, and three internal accelerometers measuring accelerations on all three aircraft’s body axes (longitudinal, lateral, and normal). The IRSs are of the strap down principle – they are attached to the aircraft fuselage close to the CG, so the acceleration measurements are coordinated with the aircraft’s body axes (pitch, roll, yaw).

On the other hand, the Inertial Navigation System (INS) is a stand-alone platform aligned with the local vertical (=gravity) where the accelerations are measured in an axis co-ordinate system which is free from the aircraft’s body axis coordinate system.