locked Re: GPS bug may cause PC time to revert to March 2002 on Oct 24th #Timesync #Timesync


Jim Pennino
 

Windows with Meinberg ntp does NOT use gpsd and the official version of gpsd from github does not support Windows at all.

While *nix like systems can use gpsd with a serial attached GPS, there is nothing that requires one to do so with ntp.

So what is gpsd?

Gpsd is a utility that attempts to provide a unified interface for GPS data for many different types of GPS receivers through a shared memory interface. It provides all the data available from a GPS and the package includes various GPS utilities that allow one to do things such as display the current GPS constellation in a graphical window. 

Ntp only requires the NMEA sentence data and a PPS signal to operate and everything else from gpsd is ignored.

Because of the extra processing done to provide the shared memory interface, gpsd degrades the time accuracy for highly accurate ntp servers, i.e, those with a high accuracy source down in the microsecond level, in determining the exact time of the PPS signal.

As an example, my moderately accurate ntp server went from a long term average error of about 5 microseconds when using gpsd to a bit over 1 microsecond when configured to use the serial interface directly.

This makes it unlikely that a professional stratum 1 server, which often use sources with an accuracy in nanoseconds, is using gpsd.

So what is the bottom line here?

If you are using windows with Meinberg ntp, with or without backup network servers, you are good to go. If some of the network servers have this bug, ntp will simply consider those servers broken and ignore them.

If you are just using network servers on any OS, use several, i.e 6 or more, such that any that do have the bug will be in the minority and ntp will consider those servers to be broken and ignore them.

If you are using gpsd on your *nix system, it would behoove you to make sure you are using the latest version, or if you have a serial GPS and no need for speed and position data for your computer, consider configuring to use the serial interface.

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