[time-nuts] NMEA Time

Hal Murray hmurray at megapathdsl.net
Wed Nov 25 20:57:55 UTC 2009


SAIDJACK at aol.com said:
> thanks for the info. I am wondering how accurate their SNTP server
> with 1PPS RS-232 input is. Not sure how one would test that without
> specialized equipment..

What sort of accuracy do you want?

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.

----------

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


-- 
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