The bow motions are measured by an accelerometer which is of the low-frequency type and placed in the bow section.
The motion sensor measures the vertical acceleration and velocity is calculated from acceleration by integration.
The vertical motion is found from velocity by a second integration.
The logging and the calculations are performed continuously by the software connected to the automation system.
In order to compensate for low frequency drift, two first order filters calculates and subtracts the mean value continuously.
The best compensation was found with filter constants at 24 seconds and 60 seconds.
In order to capture the motions correctly the sampling period is normally considered to have a value as low as 0.2 seconds (Marin 2011).