Session P5: GNSS Systems Timing Architectures and Capabilities

Performance Characterization of a GNSS RAFS Ensemble Timekeeping System
He Wang and Gabe H. Iyanu, The Aerospace Corporation
Location: Seaview A/B

An atomic clock ensemble consists of multiple independent atomic clocks working together to statistically reduce the system’s frequency noise and effectively improve the stability of the multiple clock system. In this paper, we present our recent experimental results 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 40% 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) algorithms 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 RAFS ensemble timekeeping system for future GNSS timekeeping.