2. SHIP MOTION MODEL 2.1. Establishment and Transformation of Coordinate In order to describe ship motion at sea, two the righthand cartesian coordinates are needed: North-East-Down
coordinate frame O0 −x0y0z0 , it is an inertial frame for describing movement of ship; the other one is body-fixed kinematic frame G−xyz, this body-fixed frame is fixed to the ship with its origin at the center of gravity, the defination of these two frames [5]: