Modeling question

[hildasteffes]

Hello everyone, I am having trouble modeling a wheel bipedal robot like ascento, handle or diablo,... If my mobile robot has a floating base placed on its body, how can I determine the rotation matrix and position of the floating base relative to the global fixed frame? Because I see that the floating base, along with the robot, can move freely in space and has no direct connection to the global fixed coordinate system.
geometry dash breeze