I've been wanting to build a Raspberry Pi controlled Nixie clock - but instead of a Atomic Clock, I'm going just use a USB GPS dongle. Also set it up as a NTP server for machines on my home network.
I'm working on just that. The GSP NTP server is handled by a different Pi, but makes little difference. I used Chrony and it's working like a charm[1].
I'm settling for a regular RTC[2] rather than an atomic clock for the fallback though.
Designing the PCB's is taking a bit of time though, me being rather new and Nixie tubes being 170V does slow things down a bit.
I've had a Pi GPS clock running for 6 years now, it's fantastic. It's not only an NTP server for my house but it's been contributing to pool.ntp.org for the duration as well. I adore it.
Good luck with the project. Are you adding the GPS component as a hobby challenge? If it's going to be an NTP server, then it could also be an NTP client.
Its more of a hobby project. Have a spare Rpi 3b and Usb GPS dongle from another project. Plus, saw some cool projects on the nixie sub. Figured, it would make a nice looking retro clock in my home office. The USB dongle is a big finicky. So if I can't get that to work - then I might just make it into a NTP client and let it sync up with a public server. Nothing on my LAN or homelab is mission critical where I need that level of time accuracy.
