Using an MTi to estimate ship motion (Heave, Surge, Sway)

Xsens industrial Motion Trackers (MTi) are primarily designed as orientation sensors. The VRU, AHRS and GNSS/INS models are able to deliver a drift-free orientation output, represented as Roll/Pitch/Yaw angles, Quaternions or a Rotation Matrix. Although the MTi does not directly output ship motion parameters such as heave, surge and sway, MTi’s have been used in various maritime applications for exactly that purpose. Instead of providing a direct ship motion output that is generalized for all types/sizes of vessels and maritime environments, users can take the existing outputs of the MTi and use those to accurately estimate ship motion parameters by applying a ship motion measurement model that is tailored to their application.

In addition to estimates of orientation, MTi products can produce a Free Acceleration output. The Free Acceleration output is in the local Earth-fixed frame (by default East-North-Up) and represents the acceleration with the gravitational component (g ≈ 9.81 m/s2) subtracted:


The third component, FreeAccUp.png, is a key parameter when it comes to computing heave motion. It always presents the global upward acceleration, irrespective of the inclination of the ship. If needed the Free Acceleration vector can also be transformed to a sensor-fixed (X-Y-Z) frame, by multiplying it with the Rotation Matrix output of the MTi. 


Please note that Xsens supplies the Free Acceleration and Orientation outputs as is. The further processing of MTi output data and the development of ship motion measurement models is outside of the scope of Xsens' technical support, and we cannot provide example code or calculations for this purpose. 

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