The method of virtual measurements (MOVM) will be described for designing nonlinear filters. The new nonlinear filter theory generalizes the Kalman filter, and in some important applications, the performance of the new filter is vastly superior to the extended Kalman filter (EKF). Unlike the EKF, the new theory does not use linearization. The new design approach, MOVM, can be applied to exact nonlinear filters as well as nonlinear approximate filters.
Frederick E. Daum, Frederick E. Daum,
"Optimal nonlinear filtering with the method of virtual measurements", Proc. SPIE 2759, Signal and Data Processing of Small Targets 1996, (31 May 1996); doi: 10.1117/12.241192; https://doi.org/10.1117/12.241192