[time-nuts] NMEA Time

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Wed Nov 25 21:41:57 UTC 2009


> You aren't going to get nanosecond accuracy out of a ntp server running over
> an ethernet[1].  On the other hand, sub-ms isn't hard with a good OS and/or
> good software.
> 
> It's fairly easy to get a reasonable sanity check on a (s)ntp server.  Just
> setup a known good ntp system and have it monitor the DUT.  The reference
> implementation for ntp (http://www.ntp.org/) has lots of support for
> collecting data.
> 
> The key step in making a PC keep good time is tweaking the clock frequency.
> This is the software equivalent of the EFC on an oscillator.  ntpd calls it
> drift.  You can use it as a thermometer.

I don't think the oscillator quality in the typical PC is good enough to ever get nanoseconds, even with tons of tweaks and temperature compensation, etc.  The short term variability/phase noise is too high.  

Microseconds, I think you could do.




> 
> ----------
> 
> 1) There is a group working on that level of accuracy.  It takes special
> hardware that can put a time-stamp on a packet as it leaves or arrives.
>   http://en.wikipedia.org/wiki/Precision_Time_Protocol
> 
> 

More than just a casual group.  You can buy IEEE 1588 products from, among others, Symmetricom.



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