The GPS gives information about the speed of the ship.
The speed is measured above ground whereas the speed measured by the ship’s log is speed through water.
In cases where the ship is not subject to any set and drift caused by current, the two measured speeds should be alike.
The difference in speed has an overweight of incidents with positive figures which could be caused by:
– An overweight of incidents where the ship experiences a positive effect on the ship’s speed caused by the current
– A constant offset on the speed log resulting in a lower measured speed
Fouling of the speed log sensor could cause an error or a drift in the measured speed from the speed log.
By analysing incidents over one year in specific areas the comparison of the GPS speed and the speed from the log is used to indicate if there is any drift in the measured speed by the log and to find a bias in the speed measurements from the speed log.
The speed log values are considered to be accurate as regards estimation of a trend in the performance for the ship.