A neural state estimator is described, acting on discrete-time nonlinear sy
stems with noisy measurement channels. A sliding-window quadratic estimatio
n cost function is considered and the measurement noise is assumed to be ad
ditive. No probabilistic assumptions are made on the measurement noise nor
on the initial state. Novel theoretical convergence results are developed f
or the error bounds of both the optimal and the neural approximate estimato
rs. To ensure the convergence properties of the neural estimator, a minimax
tuning technique is used. The approximate estimator can be designed off li
ne in such a may as to enable it to process on line any possible measure pa
ttern almost instantly.