Prototype of the DLR operational composite clock: methods and test cases

The operational implementation of the ensemble Kalman filter Composite Clock (CC developed by K. R. Brown), is subject of the paper. Although the mathematical background of the CC is well-known, its autonomous and robust operation in a clock laboratory requires further modifications of the CC. The suitable initialization of the first ensemble estimate is discussed. Due to the fact, that only difference measurements are available, there is a dimensional degree of freedom in the first as in the following ensemble estimates. Furthermore, to guarantee a robust, consistent, and autonomous estimation of the ensemble clocks, a consistency check is described. It outputs a so-called consistency matrix which describes the active and nonactive clocks. Since the identified active clocks can change from measurement to measurement, the corresponding Kalman filter (KF) parameters are adapted in a consistent way. Finally, the non-active clocks are estimated based on the KF output. The performance of the OCC using an ensemble of five DLR laboratory clocks, measured over a period of one year, is outlined. The clock ensemble consists of three high-performance Cesiumclocks (HP5071A), an active H-Maser (CH1-75) and a GPS disciplined Rb clock. Although miscellaneous anomalies like phase steps, outliers and clock exclusions arise, the OCC autonomously processes the five ensemble clocks and establishes a robust system time. Analysis results of this ensemble with different types of clocks when applying the OCC are presented