A general formulation of the moving horizon estimator is presented. An
algorithm with a fired-size estimation window and constraints on stat
es, disturbances, and measurement noise is developed and a probabilist
ic interpretation is given. The moving horizon formulation requires on
ly one more tuning parameter (horizon size) than many well-known appro
ximate nonlinear filters such as extended Kalman filter filter (EFK),
iterated EKF, Gaussian second-order filter, and statistically lineariz
ed filter. The choice of horizon size allows the user to achieve a com
promise between the better performance of the batch least-squares solu
tion and the reduced computational requirements of the approximate non
linear filters. Specific issues relevant to linear and nonlinear syste
ms are discussed with comparisons made to the Kalman filter, EKF, and
other recursive and optimization-based estimation schemes.