Performance Characterization of a GNSS RAFS Ensemble Timekeeping System

In this paper, we report our recent experimental characterization of an atomic clock ensemble timekeeping system using four GNSS Rubidium Atomic Frequency Standards (RAFS) and the Equal Weight Frequency Averaging (EWFA) algorithm. The test was done with a multiple atomic clock flight simulation and test station in The Aerospace Corporation’s laboratory. Phases and frequencies of all four RAFS clocks are simultaneously measured relative to a cold Rb atomic reference clock and are processed in real-time to generate the ensemble phase and frequency outputs. In the experiment, we formed a 2-clock, a 3- clock, and a 4-clock ensemble with the four RAFS units and compared their short-term frequency stabilities and long-term time offsets with the one clock system. As an example, our results show that the four-RAFS ensemble under test with the EWFA algorithm effectively reduced the white frequency noise and improved the short-term frequency stability of the multiple clock system by about 41% over the single clock system. The ensemble’s long-term frequency stability is also improved with an averaged frequency drift rate over the individual clocks. The EWFA algorithm can also mitigate the impact of frequency jumps occurred in individual clocks by N times in a N-clock system. In addition, we also examined the Equal Weight Variance Inverse Sum (EWVIS) and the Equal Weight Variance Averaging (EWVA) equations with our experimental data and found that EWVIS and EWVA provide an estimated lower and upper bound (respectively) of the short-term frequency stability of the EWFA ensemble. Those lower and upper bounds can be estimated with the individual clock’s testing data prior to the ensemble operation. The real-time RAFS ensemble output will be used to steer a VCXO in a self-referenced RAFS ensemble timekeeping system for satellite timekeeping application.