Robust Ensemble Time Onboard a Satellite

Abstract

In the current versions of Global Navigation Satellite Systems as GPS and Galileo each satellite is launched with a set of redundant clocks e.g. each Galileo vehicle has two Rubidium clocks and two passive Hydrogen masers. Onboard each satellite only one clock signal is used as base for the navigation signals while the others serve as backup. Typically, one clock serves as a hot redundancy, the other two as cold redundancies [1]. Anomalies on the clocks have to be detected and handled by the ground monitor segment. We propose to use the ensembling approach of Brown [2] onboard each satellite to generate an ensemble time for each satellite. This approach includes a continuous measurement between the ensemble clocks providing the opportunity to monitor the clock directly onboard. In our Timing Laboratory we have set up a hardware demonstrator with three Rubidium clocks and a Micro Phase Stepper (MPS). A server is continuously receiving measurement data and processes it with a Kalman filter to obtain estimates of the single clocks in the ensemble. A second filter determines control values to be applied on the MPS. As a result, the MPS serves as a realization of the ensemble time generated from the Rubidium clocks. In addition, artificial jumps can be generated on the signal of one ensemble clock. The algorithms are augmented by jump detection and handling routines. The performance of the IEM realization and the ensemble clocks is analyzed. As a result, outages and anomalies can be detected and handled faster which leads to an enhanced robustness of the clock signal used onboard the satellite. The performance of the demonstrator is analyzed by a measurement against an Active Hydrogen Maser. The maser outperforms the ensemble clocks, therefore the behavior of the ensemble clocks and the IEM realization in values of Allan deviation can be observed from the measurement.