A relatively high accuracy and reliability of the Differential GPS system makes the receivers of its signal a very good sensor of a ship position and velocity. Among the input from the gyrocompass, a complete, self-contained and portable ship manoeuvring trial system is obtained. However, the ship instant velocity vector is not measured directly but estimated through the application of the Kalman filter (one of its functions). In case of the ship manoeuvring, this leads to a systematic deviation of such the estimate as compared to the real motion. The magnitude of this bias is being thoroughly investigated for different ship manoeuvres. It appears essential in ship manoeuvring mathematical model identification and/or validation based on full-scale trials.