The vehicle is assumed to have planar motion. Three coordinates are required to describe the motion of the vehicle: X , Y and y . (X, Y) are inertial coordinates of the location of the c.g. of the vehicle while y describes the orientation of the vehicle. The velocity at the c.g. of the vehicle is denoted by V and makes an angle ,8 with the longitudinal axis of the vehicle. The angle ,8 is called the slip angle of the vehicle