Hi everybody, I add a link of my book, that defines an "undercarriage coordinate system", BUT I don't get it..!

"The Undercarriage coordinate system has its origin at the road level at the middle of the perpendicular projection of the rear axle. The Undercarriage coordinates system differs from COG coordinate system only in the Roll and Pitch angles."

If you can draw and show me where this coordinate system is, I will be so thankful.

Title of book : Automotive Control Systems: For Engine, Driveline, and Vehicle