English: The ST-124-M3 has three gimbals, of which the outermost is attached to the Instrument Unit and the innermost is fixed in inertial space. As the vehicle rolls, pitches and yaws during flight and orbit the inner gimbal remains oriented toward the same point in space at which it was pointed at the moment of launch. The inner gimbal carries accelerometers which measure vehicle accelerations in this fixed, inertial coordinate system. Three gyros also attached to the inner gimbal sense any rotations of this gimbal and send signals to feedback circuits which drive servotorque motors that counter the rotations by nulling the gyro outputs. The resolvers measure the angles of the three gimbals, providing the attitude data of the vehicle to the navigation system. The prisms are part of the system for aligning the inner gimbal just before launch.