In 1960, R. E. Kalman published his celebrated paper on recursive min- imum variance estimation in dynamical systems 14]. This paper, which introduced an algorithm that has since been known as the discrete Kalman filter, produced a virtual revolution in the field of systems engineering. Today, Kalman filters are used in such diverse areas as navigation, guid- ance, oil drilling, water and air quality, and geodetic surveys. In addition, Kalman's work led to a multitude of books and papers on minimum vari- ance estimation in dynamical systems, including one by Kalman and Bucy on continuous...