ntpviz

This is a stratum 1 server with a GPS reference clock with a PPS (pulse-per-second) output connect via the serial port. It also has hand-picked stratum 1 and 2 servers for fallback if the PPS fails.

This server supports NTS. With NTPsec, you can use it with:
server ntp2.wiktel.com nts iburst

This server is part of the NTP pool. The pool monitoring system produces graphs of the server's offset for both IPv4 and IPv6.