A system and method are disclosed for determining relative motion between ship-based combat system elements using accelerometers. Relative motion between combat system elements can introduce error into the targeting information provided to the weapons system, and thus the system facilitates compensation for such relative motion. The system includes accelerometers mounted on radar systems, inertial navigation system (INS) sensors, and weapons systems. An algorithm is disclosed in which the raw accelerometer signals are filtered, then combined with ship INS attitude signals in a displacement calculation module (DCM). Within the DCM, the signals are manipulated to calculate, for each combat system element, the translational and rotational displacements due to hull modal vibration and the translational and rotational displacements due to force vibration. The sum of these values represent the movement of each of the affected combat system elements. Relative motion between elements is calculated simply as the difference between the calculated movement values for each element.