Robust performance of dynamic positioning of surface vessels is consid
ered. The structured singular value, mu, is used as a robustness measu
re. A tight uncertainty description of the wave disturbance model and
performance weight matrices based on physical insight is presented. Tw
o controllers are analysed: a decentralized PD controller with notch f
ilters and an LQG based controller. Performance of the control systems
is shown to be sensitive with respect to changes in the dominating wa
ve frequency, omega(0).