Non-Uniform in Time State Estimation of Dynamical Systems
  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. 
Iasson Karafyllis and Costas Kravaris