  In this paper it is showed that if a time-varying uncertain system is
robustly completely detectable then there exists an estimator for this system,
i.e. we can estimate asymptotically the state vector of the system. Moreover,
if a time-varying uncertain system is robustly completely observable then there
exists an estimator for this system that guarantees convergence of the
estimates with assignable rate of convergence. Finally, it is proved that under
the assumption of Robust Lipschitz complete observability, there is a global
solution of the observer problem.
