A Vertical Reference Unit (VRU) is comprised of an inertial measurement unit (IMU) and filtering to be able to provide Roll and Pitch. It uses vertical reference (gravity) to stabilize the IMU. The gyroscopes data is fused with the gravity measurement from the accelerometers in the Kalman filter to provide Roll and Pitch angles.
VRUs take the advantage of gyroscopes to provide accurate roll and pitch during low to medium dynamic maneuvers. It is also very easy to setup and use. However, the motion precision might degrade in a dynamic environment due to the impossibility to distinguish accelerations from the gravity (vertical) reference.
A Motion Reference Unit (MRU) is a VRU that can also provide ship motion (Heave, Surge and Sway) in addition to Roll and Pitch for marine applications. For more information on Heave, follow Ship Motion Measurements.