  This paper presents a new observer design approach for linear time invariant
multivariable systems subject to unknown inputs. The design is based on a
transformation to the so-called special coordinate basis. This form reveals
important system properties like invertability or the finite and infinite zero
structure. Depending on the system's strong observability properties, the
special coordinate basis allows for a straightforward unknown input observer
design utilizing linear or nonlinear observers design techniques. The chosen
observer design technique does not only depend on the system properties, but
also on the desired convergence behavior of the observer. Hence, the proposed
design procedure can be seen as a unifying framework for unknown input observer
design.
