As we know that the angle from 'pitch, roll, yaw' is measured from first reference frame . Even though, we have subsequent frame. For example, if we rotate 90 degree around x axis, then rotate 90 degree around y axis. we noticed from the last couple that it is rotate 90 around y axis of the first frame ( not -90 degree around z axis in the secound frame ) We can use rotation formula, like R = RyRx However, in some case, if we want to use the the rotated frame as a reference frame, ( for example, -90 degree around z axis in the second frame, in this case ), we can do that and the formula to convert from rotated frame to the first reference frame is BRB -1 or BRB T ( because the rotation matrix is orthonormal ) for example, from the example above we rotate 90 degree around x axis and then 90 degree around y axis, so R = RyRx octave:1> [0 0 1;0 1 0;-1 0 0]*[1 0 0;0 0 -1;0 1 0] ans = 0 1 0 0 0 -1 -1 0 0 if we want to rotate 90 around x axis and then -90 degree arou...