A Pitch-Depth Bottom Following Controller for Auvs Using Eigenstructure Assignment∗∗This Work Is Financed by the Erdf - European Regional Development Fund Through the Compete Programme (Operational Programme for Competitiveness) and by National Funds Through the Fct - Fundação Para a Ciência e a Tecnologia (Portuguese Foundation for Science and Technology) Within Project ↔Fcomp- 01-0124-Feder-037281.