Reading ntp.org’s reference (the birthplace of NTP is just a few miles from where I sit) I stand corrected! I forgot (or possibly never noticed) that GPS receivers are considered valid reference clocks. And apparently nowadays you can get rack-mountable atomic clocks on the open market anyway.
When I said canonical stratum zero, I meant the devices that replaced the old Atomichrons at the Naval Surface Warfare Center. I guess that’s no longer a useful distinction, though.
