Discrete event systems undergo perturbations, such as failures, that disrupt the control system and reduce the anticipation capacities of the future evolution of the process. Using the (max,+) algebra, processes modelled by a timed event graph may be represented by a linear model. The knowledge of the model and of the initial conditions enable one to characterize the state vector with a state equation iteration, but perturbations may generate a misappreciation of the state vector. This paper estimates the unknown state in order to predict the output trajectory and to calculate the control when the state vector is unknown.