We have a requirement at work to synchronise our servers to within a micro-second. To achieve this we used a combination of a CDMA modem (can use a GPS device), PTPD, and NTPD.
We used the CDMA modem as a stratum 0 reference to NTPD. This means that NTPD would have control of adjusting the clock on the server. PTPD was then configured to run as the master on this server as follows:-
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