OA Formulation-Flexible Body Input File

Hi everybody!

I am writing a FEM Code in Technische Üniversitat München and I have to deal with rigid body motion of my model somehow I need to eliminate RBM from total deformation to calculate internal forces in my structure and during searching on this topic, I found RADIOSS_10.0_Reference_Guide2.pdf on the net. Under the title of 'Flexible Body Input File' , it is telling that how to get RBM for the each node in the structure but for that we need four points like L1,L2,L3,L4 which  can create an orthonormal local frame and do not coincide with the mesh of the structure.

Guide tells that these four points are chosen arbitrarily and the distance between them does not change during the analysis ,so it means they are rigid. However, I wonder that how they can find the new positions of these points in the next time steps. They are saying that they have UL1, ... , U_L4   which are the rigid body displacements of these nodes but my questions is how they are getting these values

P.s. : I also uploaded the guide that I found and the title is in  page 1124. 

Thanks in Advance

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