I have a Linux server which has its time synchronized to a GPS-based NTP appliance located close by. Ping times from the server to the appliance are circa 1ms, with very low jitter: --- x.x.x.x ping ...
I've come across a handful of GPS NTP servers, as well as some inexpensive solutions using off-the-shelf receivers and software. Right now I'm just using NTP with a list of servers over the Internet. ...