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Kalman Filtering techniques are combined with a semianalytical orbit generator to develoo two sequential orbit determination algorithms, analogous to the standard Linearized Kalman Filter (LKF) and Extended Kalman Filter (EKF), respectively. The new.__algorithms are called the Semianalytical Kalman. Filter (SKF) and- the Extended Semianalytical Kalman Filter ( ESKF). The design implications of the interaction between the filter and the perturbations theory are discussed. The results of numerical tests conducted to verify design assumptions are presented. The new algorithms were imlemented in the testbed provided by the Research and Development version of the Goddard Trajectory Determination System (Ri) GTDS). They ar