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Fig. 9. Validation results for a) yaw, b) surge and c) sway motion a) b)

Fig. 9. Validation results for a) yaw, b) surge and c) sway motion c)

4.2 Identification based on open-loop step response

Open loop identification methods are suitable for laboratory purposes. They are often time consuming since a great number of experiments should be run in order to obtain satisfactory results. On the other hand, these experiments give precise model parameters - the more experiments, the more precise results. Two cases are possible:

1. The response is constantly rising. This is the case with astatic systems, i.e. when the observed variables are "positions" like heading, depth etc. In this case the SISO system response has a form shown in Fig. 10a.

2. The response has a steady state. This is the case with static systems, i.e. when the observed variables are velocities like heading rate, heave speed etc. In this case the SISO system response has a form shown in Fig. 10b.

For both cases, the drag coefficient can either be constant or linear. From now on we will assume that yaw is the observed degree of freedom.

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