  This work presents a notion of strong detectability for linear time varying
systems affected by unknown inputs. It is shown that this notion is equivalent
to detectability of an auxiliary system without unknown inputs. This allows a
straightforward observer design for dependable state estimation in the presence
of unknown inputs. The design reduces to a deterministic Kalman filter design
problem, where the observer gains can be obtained from the solution of a
differential Riccati equation. The efficacy of the proposed approach is
demonstrated by means of a numerical example.
