Internal to the box, yes, but tied to the aircraft. In simple terms Nav depends on the integration of acceleration of the c of g along each axis to establish velocity and then the time at that velocity to establish where the INS isn't (as Idle Thrust says
), so sideslip would not affect, since the motion would still be correctly sensed, just as movement of wind in any direction accelerates the c of g. In your sideslip case the platform would detect motion along the roll AND pitch axes and wiggly amps and straight volts would decide where the c of g had really gone (or hadn't gone.....)