Approaches for estimation of site motion beyond linear models are 
discussed. It is proposed to represent site motion in a sum of three functions:
a linear function, an expansion with the B-spline bases, a sum of principal 
components of the expansion with the Fourier basis. The VLBI dataset from 
1979 to 2005 was re-analyzed following this approach. It is concluded that 
this approach is adequate for describing the non-linear position evolution 
of VLBI stations.