NPL has developed a Kalman-filter-based clock algorithm for combining measurements from its three active hydrogen masers. The algorithm is designed to produce a near optimal composite when the dominant noise process present is flicker frequency modulation (FFM). The FFM is modelled approximately by a linear combination of Markov noise processes. Each Markov process is included in the Kalman filter and contributes an additional component to its state vector. Both the validity of the model and the effectiveness of adding these additional components to the state vector are examined. The performance of the new algorithm is examined when applied to simulated measurements and also to measurements obtained from NPL's hydrogen masers.

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