I am using Trimble Thunderbolt as my reference GPSDO. GPSDO is basically an ovenized crystal that is steered/disciplined to GPS time. The GPS time itself is steered to UTC(USNO). In short, GPSDO produces accurate frequency reference with zero long term frequency drift.
The video below shows that the 10 MHz frequency signal from the Rubidium Frequency Standard is virtually identical to the GPSDO signal.
Later I will perform a more in-depth comparison between the two clocks.