Exclude NTP Servers using GPS?


Is there a way to exclude sources that sync with certain things? For example, if I wanted all of my NTP servers to be free from GPS sources? Or is NTP completely reliant on GPS?


I don’t think there is a way to reject based on stratum or refid…

NTP is not reliant on GPS… GPS is just one of several primary sources of accurate time.

Why would you want to do something like that? Would you also verify upstream sources of every server you do sync to ensuring you weren’t syncing to GPS?


If NTP is not reliant on GPS, then how does one uncouple all GPS sources from their NTP server?

So you’re saying one would have to verify upstream sources for all NTP servers to detect if you’re getting GPS-sourced time? Why… Well there are many reasons - just trying to check if it is technically possible :slight_smile:


Well, for instance you could use the NIST NTP servers… Those are based on UTC(NIST) using local atomic clocks and not GPS. FYI, UTC(NIST) and UTC(USNO) are always kept in close sync with each other, typically to within 20 nanosecond (according to the NIST webpage).


If you want a radio source, you could use various commercial (or home-brew) hardware to lock onto NIST’s WWVB signal to supply your time to NTP…


Likewise depending on where you live in the world, there are also DCF77 over in europe, providing the same thing as WWVB does for the US.

If you don’t want to use GPS, there are alternate GNSS systems out there… Glonass, Galileo, Beidou, etc…

Finally you could also get time from cellular networks. Though their time is typically derived from GNSS systems.


In order not to depend on gps, use the ip addresses of servers that are synchronized from time standards, as a rule, this is a time services server, for example,, but when synchronizing from other standards, you will not have a status higher than stratum 2.