Precise cascade synchronization of two digitally tuned space clocks to UTC (GPS)

We report the results of a three-clock, cascade synchronization experiment, in which a crystal oscillator clock is slaved to a Rb atomic clock while the Rb clock is independently slaved to a Cs reference clock. The Cs clock in the experiment is calibrated against a GPS receiver. With such a cascade scheme and a real-time linear regression algorithm, we are able to precisely synchronize the digitally-tuned space clock with the Cs clock within 50 ns with an averaged residual frequency offset of 5 × 10^-13. This cascade synchronization technique has also significantly reduced the two space clocks´ long-term frequency drift to 1.7 × 10^-14 per day and 2.5 × 10^-13 per day for the Rb clock and the crystal oscillator clock, respectively.