What is a motion reference unit (MRU)?

A Motion Reference Unit, commonly known as an MRU, is a high-precision instrument crucial for marine and subsea operations. Its primary function is to accurately measure the motion of the vessel or platform upon which it is mounted. It provides real-time data on angular rates (roll, pitch, yaw) and linear accelerations (heave, surge, sway). Essentially, it quantifies how the platform is moving, tilting, and turning in all six degrees of freedom (6DOF). At the heart of an MRU are sophisticated inertial sensors, typically including gyroscopes and accelerometers, which detect these subtle or significant movements even in challenging sea conditions.

The data generated by an MRU is vital for ensuring safety, stability, and accuracy in numerous offshore and subsea applications. For example, the measurements are used to actively compensate for vessel motion, allowing equipment like offshore cranes, launch and recovery systems (LARS), survey sonars, and personnel access gangways to operate effectively and safely despite wave action. MRUs are also integral components in Dynamic Positioning (DP) systems, providing essential feedback to maintain a vessel's position, and are critical for determining the precise orientation of subsea vehicles like ROVs and AUVs during complex underwater tasks. By providing reliable motion data, MRUs enable more efficient operations and higher quality data acquisition in the dynamic ocean environment.